Abstract: We study star formation histories (SFHs) of 500 dwarf galaxies (stellar mass M∗=10⁵−10⁹M⊙) from FIRE-2 cosmological zoom-in simulations. We compare dwarfs around individual Milky Way (MW)-mass galaxies, dwarfs in Local Group (LG)-like environments, and true field (i.e. isolated) dwarf galaxies. We reproduce observed trends wherein higher mass dwarfs quench later (if at all), regardless of environment. We also identify differences between the environments, both in terms of ‘satellite versus central’ and ‘LG versus individual MW versus isolated dwarf central.’ Around the individual MW-mass hosts, we recover the result expected from environmental quenching: central galaxies in the ‘near field’ have more extended SFHs than their satellite counterparts, with the former more closely resemble isolated (true field) dwarfs (though near-field centrals are still somewhat earlier forming). However, this difference is muted in the LG-like environments, where both near-field centrals and satellites have similar SFHs, which resemble satellites of single MW-mass hosts. This distinction is strongest for M* = 10⁶–10⁷M⊙ but exists at other masses. Our results suggest that the paired halo nature of the LG may regulate star formation in dwarf galaxies even beyond the virial radii of the MW and Andromeda. Caution is needed when comparing zoom-in simulations targeting isolated dwarf galaxies against observed dwarf galaxies in the LG.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 489 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20191025-090648233

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Abstract: We investigate the lowest-mass quiescent galaxies known to exist in isolated environments (M* = 10^(9.0-9.5) M⊙); 1.5 Mpc from a more massive galaxy). This population may represent the lowest stellar mass galaxies in which internal feedback quenches galaxy-wide star formation. We present a Keck/Echelle Spectrograph and Imager long-slit spectroscopy for 27 isolated galaxies in this regime (20 quiescent galaxies and 7 star-forming galaxies). We measure emission line strengths as a function of radius and place galaxies on the Baldwin–Phillips–Terlevich (BPT) diagram. Remarkably, 16 of 20 quiescent galaxies in our sample host central active galactic nucleus (AGN)-like line ratios. Only five of these quiescent galaxies were identified as AGN-like in the Sloan Digital Sky Survey due to a lower spatial resolution and signal-to-noise ratio. We find that many of the quiescent galaxies in our sample have spatially extended emission across the non-star-forming regions of BPT-space. While quenched galaxies in denser environments in this mass range often show no evidence for AGN activity, a significant fraction of quiescent galaxies in isolation host AGNs despite their overall passive appearances.

Publication: Astrophysical Journal Vol.: 884 No.: 2 ISSN: 1538-4357

ID: CaltechAUTHORS:20191023-160058903

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Abstract: We present a new set of high-resolution hydrodynamic cosmological zoom-in simulations that apply the Feedback In Realistic Environments physics to both Local Group (LG)-like and isolated Milky Way (MW)-like volumes (10 host systems in total with a baryonic particle mass ≃3500−7000M). We study the stellar mass functions, circular velocity or mass profiles, and velocity dispersions of the dwarf galaxy populations. The simulations reproduce the stellar mass function and central densities of MW satellite dwarfs for M∗≥10^(5.5) M⊙ and predict the existence of ∼3 unidentified galaxies with M∗∼10^5M⊙ within 300 kpc of the MW. Overall, we find no evidence for the classical missing satellites or too-big-to-fail (TBTF) problems for satellite galaxies in our sample. Among the satellites, TBTF is resolved primarily by subhalo disruption and overall mass-loss; central density profiles of subhaloes are of secondary importance. For non-satellite galaxies, our LG-like simulations predict as many as ∼10 as-of-yet unseen galaxies at distances 0.3−1Mpc from both hosts, with M∗≃10^(5−6) M⊙ (in haloes with V_(max) ∼ 20 km s^(−1)), albeit with large halo-to-halo variance. None of our simulations produces a compact, baryon-dominated, high-density dwarf elliptical-type galaxy (with V_(circ) ≳ 35 km s^(−1) at r < 1kpc), of which six may appear in the LG (but none in the MW). It may therefore remain a challenge to reproduce the full diversity of the dwarf population, including both the highest and lowest density systems.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 487 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20190206-105552297

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Abstract: We use hydrodynamic cosmological zoom-in simulations from the Feedback in Realistic Environments project to explore the morphologies and kinematics of 15 Milky Way (MW)-mass galaxies. Our sample ranges from compact, bulge-dominated systems with 90 per cent of their stellar mass within 2.5 kpc to well-ordered discs that reach ≳15 kpc. The gas in our galaxies always forms a thin, rotation-supported disc at z = 0, with sizes primarily determined by the gas mass. For stars, we quantify kinematics and morphology both via the fraction of stars on disc-like orbits and with the radial extent of the stellar disc. In this mass range, stellar morphology and kinematics are poorly correlated with the properties of the halo available from dark matter-only simulations (halo merger history, spin, or formation time). They more strongly correlate with the gaseous histories of the galaxies: those that maintain a high gas mass in the disc after z ∼ 1 develop well-ordered stellar discs. The best predictor of morphology we identify is the spin of the gas in the halo at the time the galaxy formed 1/2 of its stars (i.e. the gas that builds the galaxy). High-z mergers, before a hot halo emerges, produce some of the most massive bulges in the sample (from compact discs in gas-rich mergers), while later-forming bulges typically originate from internal processes, as satellites are stripped of gas before the galaxies merge. Moreover, most stars in z = 0 MW-mass galaxies (even z = 0 bulge stars) form in a disc: ≳60--90 per cent of stars begin their lives rotationally supported.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 481 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20181205-145100415

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Abstract: We use cosmological hydrodynamical simulations of Milky Way–mass galaxies from the FIRE project to evaluate various strategies for estimating the mass of a galaxy's stellar halo from deep, integrated-light images. We find good agreement with integrated-light observations if we mimic observational methods to measure the mass of the stellar halo by selecting regions of an image via projected radius relative to the disk scale length or by their surface density in stellar mass. However, these observational methods systematically underestimate the accreted stellar component, defined in our (and most) simulations as the mass of stars formed outside of the host galaxy, by up to a factor of 10, since the accreted component is centrally concentrated and therefore substantially obscured by the galactic disk. Furthermore, these observational methods introduce spurious dependencies of the estimated accreted stellar component on the stellar mass and size of galaxies that can obscure the trends in accreted stellar mass predicted by cosmological simulations, since we find that in our simulations, the size and shape of the central galaxy are not strongly correlated with the assembly history of the accreted stellar halo. This effect persists whether galaxies are viewed edge-on or face-on. We show that metallicity or color information may provide a way to more cleanly delineate in observations the regions dominated by accreted stars. Absent additional data, we caution that estimates of the mass of the accreted stellar component from single-band images alone should be taken as lower limits.

Publication: Astrophysical Journal Vol.: 869 No.: 1 ISSN: 1538-4357

ID: CaltechAUTHORS:20180109-173930013

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Abstract: We interpret the traversable wormhole in AdS/CFT in the context of quantum information theory. In particular, we investigate its properties as both a quantum channel and entanglement witness. We define protocols that allow either the bounding of the channel’s entanglement capacity or the determination of aspects of the entanglement structure between the two boundary CFTs. Such protocols and connections allow for the use of quantum channel techniques in the study of gravitational physics and vice versa. More generally, our results suggest a purely quantum information-theoretic criterion for recognizing when the product of two boundary theories has a classical bulk interpretation.

Publication: Journal of High Energy Physics Vol.: 2018 No.: 11 ISSN: 1029-8479

ID: CaltechAUTHORS:20180821-131134609

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Abstract: The Feedback In Realistic Environments (FIRE) project explores feedback in cosmological galaxy formation simulations. Previous FIRE simulations used an identical source code (“FIRE-1”) for consistency. Motivated by the development of more accurate numerics – including hydrodynamic solvers, gravitational softening, and supernova coupling algorithms – and exploration of new physics (e.g. magnetic fields), we introduce “FIRE-2”, an updated numerical implementation of FIRE physics for the GIZMO code. We run a suite of simulations and compare against FIRE-1: overall, FIRE-2 improvements do not qualitatively change galaxy-scale properties. We pursue an extensive study of numerics versus physics. Details of the star-formation algorithm, cooling physics, and chemistry have weak effects, provided that we include metal-line cooling and star formation occurs at higher-than-mean densities. We present new resolution criteria for high-resolution galaxy simulations. Most galaxy-scale properties are robust to numerics we test, provided: (1) Toomre masses are resolved; (2) feedback coupling ensures conservation, and (3) individual supernovae are time-resolved. Stellar masses and profiles are most robust to resolution, followed by metal abundances and morphologies, followed by properties of winds and circum-galactic media (CGM). Central (∼kpc) mass concentrations in massive (>L*) galaxies are sensitive to numerics (via trapping/recycling of winds in hot halos). Multiple feedback mechanisms play key roles: supernovae regulate stellar masses/winds; stellar mass-loss fuels late star formation; radiative feedback suppresses accretion onto dwarfs and instantaneous star formation in disks. We provide all initial conditions and numerical algorithms used.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 480 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20180723-104220389

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Abstract: We investigate the merger histories of isolated dwarf galaxies based on a suite of 15 high-resolution cosmological zoom-in simulations, all with masses of M_(halo) ≈ 10^(10) M⊙ (and M⋆∼10^5−10^7M⊙) at z = 0, from the Feedback in Realistic Environments project. The stellar populations of these dwarf galaxies at z = 0 are formed essentially entirely ‘in situ’: over 90 percent of the stellar mass is formed in the main progenitor in all but two cases, and all 15 of the galaxies have >70 percent of their stellar mass formed in situ. Virtually all galaxy mergers occur prior to z ∼ 3, meaning that accreted stellar populations are ancient. On average, our simulated dwarfs undergo five galaxy mergers in their lifetimes, with typical pre-merger galaxy mass ratios that are less than 1:10. This merger frequency is generally comparable to what has been found in dissipationless simulations when coupled with abundance matching. Two of the simulated dwarfs have a luminous satellite companion at z= 0. These ultra-faint dwarfs lie at or below current detectability thresholds but are intriguing targets for next-generation facilities. The small contribution of accreted stars makes it extremely difficult to discern the effects of mergers in the vast majority of dwarfs either photometrically or using resolved-star colour–magnitude diagrams (CMDs). The important implication for near-field cosmology is that star formation histories (SFHs) of comparably massive galaxies derived from resolved CMDs should trace the build-up of stellar mass in one main system across cosmic time as opposed to reflecting the contributions of many individual SFHs of merged dwarfs.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 479 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20180829-152027447

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Abstract: We measure how the properties of star-forming central galaxies correlate with large-scale environment, δ, measured on 10 h^(−1) Mpc scales. We use galaxy group catalogues to isolate a robust sample of central galaxies with high purity and completeness. The galaxy properties we investigate are star formation rate (SFR), exponential disc scale length R_(exp), and Sersic index of the galaxy light profile, n_S. We find that, at all stellar masses, there is an inverse correlation between SFR and δ, meaning that above-average star-forming centrals live in underdense regions. For n_S and R_(exp), there is no correlation with δ at M* ≲ 10^(10.5)M⊙, but at higher masses there are positive correlations; a weak correlation with R_(exp) and a strong correlation with n_S. These data are evidence of assembly bias within the star-forming population. The results for SFR are consistent with a model in which SFR correlates with present-day halo accretion rate, M_h. In this model, galaxies are assigned to haloes using the abundance-matching ansatz, which maps galaxy stellar mass onto halo mass. At fixed halo mass, SFR is then assigned to galaxies using the same approach, but M_h is used to map onto SFR. The best-fitting model requires some scatter in the M_h –SFR relation. The R_(exp) and n_S measurements are consistent with a model in which both of these quantities are correlated with the spin parameter of the halo, λ. Halo spin does not correlate with δ at low halo masses, but for higher mass haloes, high-spin haloes live in higher density environments at fixed M_h. Put together with the earlier instalments of this series, these data demonstrate that quenching processes have limited correlation with halo formation history, but the growth of active galaxies, as well as other detailed galaxies properties, are influenced by the details of halo assembly.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 478 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20180829-110132653

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Abstract: We test if the cosmological zoom-in simulations of isolated galaxies from the FIRE project reproduce the properties of ultra diffuse galaxies (UDGs). We show that outflows that dynamically heat galactic stars, together with a passively aging stellar population after imposed quenching, naturally reproduce the observed population of red UDGs, without the need for high spin halos, or dynamical influence from their host cluster. We reproduce the range of surface brightness, radius and absolute magnitude of the observed red UDGs by quenching simulated galaxies at a range of different times. They represent a mostly uniform population of dark matter-dominated dwarf galaxies with M* ∼ 10^8 M⊙, low metallicity and a broad range of ages; the more massive the UDGs, the older they are. The most massive red UDG in our sample (M* ∼ 3 × 10^8M⊙) requires quenching at z ∼ 3 when its halo reached M_h ∼ 10^(11) M⊙. Our simulated UDGs form with normal stellar-to-halo ratios and match the central enclosed masses and the velocity dispersions of the observed UDGs. Enclosed masses remain largely fixed across a broad range of quenching times because the central regions of their dark matter halos complete their growth early. If our simulated dwarfs are not quenched, they evolve into bluer low-surface brightness galaxies with M/L similar to observed field dwarfs. While our simulation sample covers a limited range of formation histories and halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy type around M* ∼ 10^8 M⊙, both in the field and in clusters.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 478 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20180517-101404866

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Abstract: We study the implementation of mechanical feedback from supernovae (SNe) and stellar mass loss in galaxy simulations, within the Feedback In Realistic Environments (FIRE) project. We present the FIRE-2 algorithm for coupling mechanical feedback, which can be applied to any hydrodynamics method (e.g. fixed-grid, moving-mesh, and mesh-less methods), and black hole as well as stellar feedback. This algorithm ensures manifest conservation of mass, energy, and momentum, and avoids imprinting ‘preferred directions’ on the ejecta. We show that it is critical to incorporate both momentum and thermal energy of mechanical ejecta in a self-consistent manner, accounting for SNe cooling radii when they are not resolved. Using idealized simulations of single SN explosions, we show that the FIRE-2 algorithm, independent of resolution, reproduces converged solutions in both energy and momentum. In contrast, common ‘fully thermal’ (energy-dump) or ‘fully kinetic’ (particle-kicking) schemes in the literature depend strongly on resolution: when applied at mass resolution ≳100 M⊙, they diverge by orders of magnitude from the converged solution. In galaxy-formation simulations, this divergence leads to orders-of-magnitude differences in galaxy properties, unless those models are adjusted in a resolution-dependent way. We show that all models that individually time-resolve SNe converge to the FIRE-2 solution at sufficiently high resolution (<100 M_⊙). However, in both idealized single-SN simulations and cosmological galaxy-formation simulations, the FIRE-2 algorithm converges much faster than other sub-grid models without re-tuning parameters.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 477 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:20180627-140420805

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Abstract: We discuss the branching structure of the quantum-gravitational wave function that describes the evaporation of a black hole. A global wave function which initially describes a classical Schwarzschild geometry is continually decohered into distinct semiclassical branches by the emission of Hawking radiation. The laws of quantum mechanics dictate that the wave function evolves unitarily, but this unitary evolution is only manifest when considering the global description of the wave function; it is not implemented by time evolution on a single semiclassical branch. Conversely, geometric notions like the position or smoothness of a horizon only make sense on the level of individual branches. We consider the implications of this picture for probes of black holes by classical observers in definite geometries, like those involved in the Almheiri-Marolf-Polchinski-Sully construction. We argue that individual branches can describe semiclassical geometries free of firewalls, even as the global wave function evolves unitarily. We show that the pointer states of infalling detectors that are robust under Hamiltonian evolution are distinct from, and incompatible with, those of exterior detectors stationary with respect to the black hole horizon, in the sense that the pointer bases are related to each other via nontrivial transformations that mix the system, apparatus, and environment. This result describes a Hilbert-space version of black hole complementarity.

Publication: Physical Review D Vol.: 97 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:20180109-165521863

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Abstract: Using group catalogues from the Sloan Digital Sky Survey (SDSS) Data Release 7, we measure galactic conformity in the local universe. We measure the quenched fraction of neighbour galaxies around isolated primary galaxies, dividing the isolated sample into star-forming and quiescent objects. We restrict our measurements to scales >1 Mpc to probe the correlations between halo formation histories. Over the stellar mass range 10^(9.7) ≤ M*/M⊙ ≤ 10^(10.9), we find minimal evidence for conformity. We further compare these data to predictions of the halo age-matching model, in which the oldest galaxies are associated with the oldest haloes. For models with strong correlations between halo and stellar age, the conformity is too large to be consistent with the data. Weaker implementations of the age-matching model would not produce a detectable signal in SDSS data. We reproduce the results of Kauffmann et al., in which the star formation rates of neighbour galaxies are reduced around primary galaxies when the primaries are low star formers. However, we find this result is mainly driven by contamination in the isolation criterion; when removing the small fraction of satellite galaxies in the sample, the conformity signal largely goes away. Lastly, we show that small conformity signals, i.e. 2–5 per cent differences in the quenched fractions of neighbour galaxies, can be produced by mechanisms other than halo assembly bias. For example, if passive galaxies occupy more massive haloes than star-forming galaxies of the same stellar mass, a conformity signal that is consistent with recent measurements from PRIMUS (Berti et al.) can be produced.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 477 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20180606-105527709

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Abstract: We identify subhalos in dark matter–only (DMO) zoom-in simulations that are likely to be disrupted due to baryonic effects by using a random forest classifier trained on two hydrodynamic simulations of Milky Way (MW)–mass host halos from the Latte suite of the Feedback in Realistic Environments (FIRE) project. We train our classifier using five properties of each disrupted and surviving subhalo: pericentric distance and scale factor at first pericentric passage after accretion and scale factor, virial mass, and maximum circular velocity at accretion. Our five-property classifier identifies disrupted subhalos in the FIRE simulations with an 85% out-of-bag classification score. We predict surviving subhalo populations in DMO simulations of the FIRE host halos, finding excellent agreement with the hydrodynamic results; in particular, our classifier outperforms DMO zoom-in simulations that include the gravitational potential of the central galactic disk in each hydrodynamic simulation, indicating that it captures both the dynamical effects of a central disk and additional baryonic physics. We also predict surviving subhalo populations for a suite of DMO zoom-in simulations of MW-mass host halos, finding that baryons impact each system consistently and that the predicted amount of subhalo disruption is larger than the host-to-host scatter among the subhalo populations. Although the small size and specific baryonic physics prescription of our training set limits the generality of our results, our work suggests that machine-learning classification algorithms trained on hydrodynamic zoom-in simulations can efficiently predict realistic subhalo populations.

Publication: Astrophysical Journal Vol.: 859 No.: 2 ISSN: 1538-4357

ID: CaltechAUTHORS:20180601-112102390

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Abstract: Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, we investigate the formation of star clusters and examine one of the formation hypotheses of present-day metal-poor globular clusters. We find that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce long-lasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of ≳ 10^(5–6) M⊙ collectively to high density, at which point it rapidly turns into stars before stellar feedback can stop star formation. The high dynamic range of the reported simulation is critical in realizing such dense star-forming clouds with a small dynamical time-scale, t_(ff)≲ 3 Myr, shorter than most stellar feedback time-scales. Our simulation then allows us to trace how clusters could become virialized and tightly bound to survive for up to ∼420 Myr till the end of the simulation. Because the cluster's tightly bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 474 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20180221-135659103

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Abstract: A longstanding issue in attempts to understand the Everett (many-worlds) approach to quantum mechanics is the origin of the Born rule: why is the probability given by the square of the amplitude? Following Vaidman, we note that observers are in a position of self-locating uncertainty during the period between the branches of the wave function splitting via decoherence and the observer registering the outcome of the measurement. In this period, it is tempting to regard each branch as equiprobable, but we argue that the temptation should be resisted. Applying lessons from this analysis, we demonstrate (using methods similar to those of Zurek’s envariance-based derivation) that the Born rule is the uniquely rational way of apportioning credence in Everettian quantum mechanics. In doing so, we rely on a single key principle: changes to the environment alone do not affect the probabilities one ought to assign to measurement outcomes in a local subsystem. We arrive at a method for assigning probabilities in cases that involve both classical and quantum self-locating uncertainty. This method provides unique answers to quantum Sleeping Beauty problems, as well as a well-defined procedure for calculating probabilities in quantum cosmological multiverses with multiple similar observers.

Publication: British Journal for the Philosophy of Science Vol.: 69 No.: 1 ISSN: 0007-0882

ID: CaltechAUTHORS:20141216-201958532

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Abstract: We prove, for any state in a conformal field theory defined on a set of boundary manifolds with corresponding classical holographic bulk geometry, that for any bipartition of the boundary into two non-clopen sets, the density matrix cannot be a tensor product of the reduced density matrices on each region of the bipartition. In particular, there must be entanglement across the bipartition surface. We extend this no-go theorem to general, arbitrary partitions of the boundary manifolds into non-clopen parts, proving that the density matrix cannot be a tensor product. This result gives a necessary condition for states to potentially correspond to holographic duals.

Publication: Europhysics Letters Vol.: 121 No.: 6 ISSN: 0295-5075

ID: CaltechAUTHORS:20170314-133758800

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Abstract: We investigate stellar metallicity distribution functions (MDFs), including Fe and α-element abundances, in dwarf galaxies from the Feedback in Realistic Environment (FIRE) project. We examine both isolated dwarf galaxies and those that are satellites of a Milky Way-mass galaxy. In particular, we study the effects of including a sub-grid turbulent model for the diffusion of metals in gas. Simulations that include diffusion have narrower MDFs and abundance ratio distributions, because diffusion drives individual gas and star particles towards the average metallicity. This effect provides significantly better agreement with observed abundance distributions in dwarf galaxies in the Local Group, including small intrinsic scatter in [α/Fe] versus [Fe/H] of ≲0.1 dex. This small intrinsic scatter arises in our simulations because the interstellar medium in dwarf galaxies is well mixed at nearly all cosmic times, such that stars that form at a given time have similar abundances to ≲0.1 dex. Thus, most of the scatter in abundances at z = 0 arises from redshift evolution and not from instantaneous scatter in the ISM. We find similar MDF widths and intrinsic scatter for satellite and isolated dwarf galaxies, which suggests that environmental effects play a minor role compared with internal chemical evolution in our simulations. Overall, with the inclusion of metal diffusion, our simulations reproduce abundance distribution widths of observed low-mass galaxies, enabling detailed studies of chemical evolution in galaxy formation.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 474 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:20171031-190653134

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Abstract: In a wide class of cosmological models, a positive cosmological constant drives cosmological evolution toward an asymptotically de Sitter phase. Here we connect this behavior to the increase of entropy over time, based on the idea that de Sitter spacetime is a maximum-entropy state. We prove a cosmic no-hair theorem for Robertson-Walker and Bianchi I spacetimes that admit a Q-screen (“quantum” holographic screen) with certain entropic properties: If generalized entropy, in the sense of the cosmological version of the generalized second law conjectured by Bousso and Engelhardt, increases up to a finite maximum value along the screen, then the spacetime is asymptotically de Sitter in the future. Moreover, the limiting value of generalized entropy coincides with the de Sitter horizon entropy. We do not use the Einstein field equations in our proof, nor do we assume the existence of a positive cosmological constant. As such, asymptotic relaxation to a de Sitter phase can, in a precise sense, be thought of as cosmological equilibration.

Publication: Physical Review D Vol.: 97 No.: 4 ISSN: 2470-0010

ID: CaltechAUTHORS:20170330-143452297

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Abstract: We study the z = 0 gas kinematics, morphology and angular momentum content of isolated galaxies in a suite of cosmological zoom-in simulations from the FIRE project spanning M_(star) = 10^(6–11) M_⊙. Gas becomes increasingly rotationally supported with increasing galaxy mass. In the lowest mass galaxies (M_(star) < 10^8 M_⊙), gas fails to form a morphological disc and is primarily dispersion and pressure supported. At intermediate masses (M_(star) = 10^(8–10) M_⊙), galaxies display a wide range of gas kinematics and morphologies, from thin, rotating discs to irregular spheroids with negligible net rotation. All the high-mass (M_(star) = 10^(10–11) M_⊙) galaxies form rotationally supported gas discs. Many of the haloes whose galaxies fail to form discs harbour high angular momentum gas in their circumgalactic medium. The ratio of the specific angular momentum of gas in the central galaxy to that of the dark matter halo increases significantly with galaxy mass, from 〈j_(gas)〉/〈j_(DM)〉 ∼ 0.1 at M_(star) = 10^(6-7) M_⊙ to 〈j_(gas)〉/〈j_(DM)〉 ∼ 2 at M_(star) = 10^(10–11) M_⊙. The reduced rotational support in the lowest mass galaxies owes to (a) stellar feedback and the UV background suppressing the accretion of high angular momentum gas at late times, and (b) stellar feedback driving large non-circular gas motions. We broadly reproduce the observed scaling relations between galaxy mass, gas rotation velocity, size and angular momentum, but may somewhat underpredict the incidence of disky, high angular momentum galaxies at the lowest observed masses (M_(star) = (10^6–2 × 10^7) M_⊙). Stars form preferentially from low angular momentum gas near the galactic centre and are less rotationally supported than gas. The common assumption that stars follow the same rotation curve as gas thus substantially overestimates the simulated galaxies’ stellar angular momentum, particularly at low masses.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 473 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:20180216-074830560

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Abstract: We introduce massive black holes (BHs) in the Feedback In Realistic Environments (FIRE) project and perform high-resolution cosmological hydrodynamic simulations of quasar-mass haloes [Mhalo(z = 2) ≈ 10^(12.5) M_⊙] down to z = 1. These simulations model stellar feedback by supernovae, stellar winds and radiation, and BH growth using a gravitational torque-based prescription tied to the resolved properties of galactic nuclei. We do not include BH feedback. We show that early BH growth occurs through short (≲1 Myr) accretion episodes that can reach or even exceed the Eddington rate. In this regime, BH growth is limited by bursty stellar feedback continuously evacuating gas from galactic nuclei, and BHs remain undermassive in low-mass galaxies relative to the local M_(BH)–M_(bulge) relation. BH growth is more efficient at later times, when the nuclear stellar potential retains a significant gas reservoir, star formation becomes less bursty and galaxies settle into a more ordered state. BHs rapidly converge on to the observed scaling relations when the host reaches Mbulge ∼ 10^(10) M_⊙. We show that resolving the effects of stellar feedback on the gas supply in the inner ∼100 pc of galaxies is necessary to accurately capture the growth of central BHs. Our simulations imply that bursty stellar feedback has important implications for BH–galaxy relations, AGN demographics and time variability, the formation of early quasars and massive BH mergers.

Publication: Monthly Notices of the Royal Astronomical Society: Letters Vol.: 472 No.: 1 ISSN: 1745-3925

ID: CaltechAUTHORS:20180726-143711819

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Abstract: We present a suite of 15 cosmological zoom-in simulations of isolated dark matter haloes, all with masses of M_(halo) ≈ 10^(10) M_⊙ at z = 0, in order to understand the relationship among halo assembly, galaxy formation and feedback's effects on the central density structure in dwarf galaxies. These simulations are part of the Feedback in Realistic Environments (FIRE) project and are performed at extremely high resolution (m_(baryon) = 500 M_⊙, m_(dm) = 2500 M_⊙). The resultant galaxies have stellar masses that are consistent with rough abundance matching estimates, coinciding with the faintest galaxies that can be seen beyond the virial radius of the Milky Way (M_*/M_⊙ ≈ 10^5 − 10^7). This non-negligible spread in stellar mass at z = 0 in haloes within a narrow range of virial masses is strongly correlated with central halo density or maximum circular velocity V_(max), both of which are tightly linked to halo formation time. Much of this dependence of M_* on a second parameter (beyond M_(halo)) is a direct consequence of the M_(halo) ∼ 10^(10) M_⊙ mass scale coinciding with the threshold for strong reionization suppression: the densest, earliest-forming haloes remain above the UV-suppression scale throughout their histories while late-forming systems fall below the UV-suppression scale over longer periods and form fewer stars as a result. In fact, the latest-forming, lowest-concentration halo in our suite fails to form any stars. Haloes that form galaxies with M_⋆ ≳ 2 × 10^6 M_⊙ have reduced central densities relative to dark-matter-only simulations, and the radial extent of the density modifications is well-approximated by the galaxy half-mass radius r_(1/2). Lower-mass galaxies do not modify their host dark matter haloes at the mass scale studied here. This apparent stellar mass threshold of M_⋆ ≈ 2 × 10^6 − 2 × 10^(−4) M_(halo) is broadly consistent with previous work and provides a testable prediction of FIRE feedback models in Λcold dark matter.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 471 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20170921-142943803

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Abstract: We consider a dark sector consisting of dark matter that is a Dirac fermion and a scalar mediator. This model has been extensively studied in the past. If the scalar couples to the dark matter in a parity conserving manner then dark matter annihilation to two mediators is dominated by the P-wave channel and hence is suppressed at very low momentum. The indirect detection constraint from the anisotropy of the Cosmic Microwave Background is usually thought to be absent in the model because of this suppression. In this letter we show that dark matter annihilation via bound state formation occurs through the S-wave and hence there is a constraint on the parameter space of the model from the Cosmic Microwave Background.

Publication: Physics Letters B Vol.: 773ISSN: 0370-2693

ID: CaltechAUTHORS:20160615-155545845

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Abstract: Among the most important goals in cosmology is detecting and quantifying small (M_(halo)≃10^(6−9) M⊙) dark matter (DM) subhaloes. Current probes around the Milky Way (MW) are most sensitive to such substructure within ∼20 kpc of the halo centre, where the galaxy contributes significantly to the potential. We explore the effects of baryons on subhalo populations in ΛCDM using cosmological zoom-in baryonic simulations of MW-mass haloes from the Latte simulation suite, part of the Feedback In Realistic Environments (FIRE) project. Specifically, we compare simulations of the same two haloes run using (1) DM-only (DMO), (2) full baryonic physics and (3) DM with an embedded disc potential grown to match the FIRE simulation. Relative to baryonic simulations, DMO simulations contain ∼2 × as many subhaloes within 100 kpc of the halo centre; this excess is ≳5 × within 25 kpc. At z = 0, the baryonic simulations are completely devoid of subhaloes down to 3×10^6M⊙ within 15 kpc of the MW-mass galaxy, and fewer than 20 surviving subhaloes have orbital pericentres <20 kpc. Despite the complexities of baryonic physics, the simple addition of an embedded central disc potential to DMO simulations reproduces this subhalo depletion, including trends with radius, remarkably well. Thus, the additional tidal field from the central galaxy is the primary cause of subhalo depletion. Subhaloes on radial orbits that pass close to the central galaxy are preferentially destroyed, causing the surviving population to have tangentially biased orbits compared to DMO predictions. Our method of embedding a potential in DMO simulations provides a fast and accurate alternative to full baryonic simulations, thus enabling suites of cosmological simulations that can provide accurate and statistical predictions of substructure populations.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 471 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:20170921-133516143

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Abstract: Dwarf galaxies are known to have remarkably low star formation efficiency due to strong feedback. Adopting the dwarf galaxies of the Milky Way (MW) as a laboratory, we explore a flexible semi-analytic galaxy formation model to understand how the feedback processes shape the satellite galaxies of the MW. Using Markov Chain Monte Carlo, we exhaustively search a large parameter space of the model and rigorously show that the general wisdom of strong outflows as the primary feedback mechanism cannot simultaneously explain the stellar mass function and the mass–metallicity relation of the MW satellites. An extended model that assumes that a fraction of baryons is prevented from collapsing into low-mass halos in the first place can be accurately constrained to simultaneously reproduce those observations. The inference suggests that two different physical mechanisms are needed to explain the two different data sets. In particular, moderate outflows with weak halo mass dependence are needed to explain the mass–metallicity relation, and prevention of baryons falling into shallow gravitational potentials of low-mass halos (e.g., "pre-heating") is needed to explain the low stellar mass fraction for a given subhalo mass.

Publication: Astrophysical Journal Vol.: 846 No.: 1 ISSN: 1538-4357

ID: CaltechAUTHORS:20170905-132641863

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Abstract: With the upcoming release of the Gaia catalog and the many multiplexed spectroscopic surveys on the horizon, we are rapidly moving into a new data-driven era in the study of the Milky Way’s stellar halo. When combined, these data sets will give us a many-dimensional view of stars in accreted structures in the halo that includes both dynamical information about their orbits and chemical information about their formation histories. Using simulated data from the state-of-the-art Latte simulations of Milky-Way-like galaxies, which include hydrodynamics, feedback, and chemical evolution in a cosmological setting, we demonstrate that while dynamical information alone can be used to constrain models of the Galactic mass distribution in the halo, including the extra dimensions provided by chemical abundances can improve these constraints as well as assist in untangling different accreted components.

Publication: Galaxies Vol.: 5 No.: 3 ISSN: 2075-4434

ID: CaltechAUTHORS:20190130-121936352

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Abstract: We use the first Gaia data release, combined with the RAVE and APOGEE spectroscopic surveys, to investigate the origin of halo stars within ≾3 kpc from the Sun. We identify halo stars kinematically as moving at a relative speed of at least 220 km s^(−1) with respect to the local standard of rest. These stars are generally less metal-rich than the disk, but surprisingly, half of our halo sample is comprised of stars with [Fe/H] > -1. The orbital directions of these metal-rich halo stars are preferentially aligned with the disk rotation, in sharp contrast with the intrinsically isotropic orbital distribution of the metal-poor halo stars. We find similar properties in the Latte cosmological zoom-in simulation of a Milky Way-like galaxy from the FIRE project. In Latte, metal-rich halo stars formed primarily inside of the solar circle, whereas lower-metallicity halo stars preferentially formed at larger distances (extending beyond the virial radius). This suggests that metal-rich halo stars in the solar neighborhood actually formed in situ within the Galactic disk, rather than having been accreted from satellite systems. These stars, currently on halo-like orbits, therefore have likely undergone substantial radial migration/heating.

Publication: Astrophysical Journal Vol.: 845 No.: 2 ISSN: 1538-4357

ID: CaltechAUTHORS:20170908-093448095

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Abstract: Slow-roll inflation can become eternal if the quantum variance of the inflaton field around its slowly rolling classical trajectory is converted into a distribution of classical spacetimes inflating at different rates, and if the variance is large enough compared to the rate of classical rolling that the probability of an increased rate of expansion is sufficiently high. Both of these criteria depend sensitively on whether and how perturbation modes of the inflaton interact and decohere. Decoherence is inevitable as a result of gravitationally sourced interactions whose strength are proportional to the slow-roll parameters. However, the weakness of these interactions means that decoherence is typically delayed until several Hubble times after modes grow beyond the Hubble scale. We present perturbative evidence that decoherence of long-wavelength inflaton modes indeed leads to an ensemble of classical spacetimes with differing cosmological evolutions. We introduce the notion of per-branch observables—expectation values with respect to the different decohered branches of the wave function—and show that the evolution of modes on individual branches varies from branch to branch. Thus, single-field slow-roll inflation fulfills the quantum-mechanical criteria required for the validity of the standard picture of eternal inflation. For a given potential, the delayed decoherence can lead to slight quantitative adjustments to the regime in which the inflaton undergoes eternal inflation.

Publication: Physical Review D Vol.: 96 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:20170728-110444626

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Abstract: We propose a new table-top experimental configuration for the direct detection of dark matter QCD axions in the traditional open mass window 10^(-6) eV ≲ m_a ≲ 10^(-2) eV using nonperturbative effects in a system with nontrivial spatial topology. Different from most experimental setups found in literature on direct dark matter axion detection, which relies on ˙θ or ⃗∇θ, we found that our system is in principle sensitive to a static θ ≥ 10^(-14) and can also be used to set limit on the fundamental constant θ_(QED) which becomes the fundamental observable parameter of the Maxwell system if some conditions are met. Furthermore, the proposed experiments can probe entire open mass window 10^(-6) eV ≲ m_a ≲10^(-2) eV with the same design, which should be contrasted with conventional cavity-type experiments being sensitive to a specific axion mass. Connection with Witten effect when the induced electric charge e′ is proportional to θ and the magnetic monopole becomes the dyon with nonvanishing e′ = -eθ/2π is also discussed.

Publication: Physical Review D Vol.: 96 No.: 1 ISSN: 2470-0010

ID: CaltechAUTHORS:20170209-150222935

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Abstract: We construct perturbative classical solutions of the Yang-Mills equations coupled to dynamical point particles carrying color charge. By applying a set of color to kinematics replacement rules first introduced by Bern, Carrasco and Johansson, these are shown to generate solutions of d-dimensional dilaton gravity, which we also explicitly construct. Agreement between the gravity result and the gauge theory double copy implies a correspondence between non-Abelian particles and gravitating sources with dilaton charge. When the color sources are highly relativistic, dilaton exchange decouples, and the solutions we obtain match those of pure gravity. We comment on possible implications of our findings to the calculation of gravitational waveforms in astrophysical black hole collisions, directly from computationally simpler gluon radiation in Yang-Mills theory.

Publication: Physical Review D Vol.: 95 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:20170630-102344509

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Abstract: Central galaxies make up the majority of the galaxy population, including the majority of the quiescent population at M_* > 10^(10)M_☉. Thus, the mechanism(s) responsible for quenching central galaxies play a crucial role in galaxy evolution as whole. We combine a high-resolution cosmological N-body simulation with observed evolutionary trends of the "star formation main sequence," quiescent fraction, and stellar mass function at z < 1 to construct a model that statistically tracks the star formation histories and quenching of central galaxies. Comparing this model to the distribution of central galaxy star formation rates in a group catalog of the SDSS Data Release 7, we constrain the timescales over which physical processes cease star formation in central galaxies. Over the stellar mass range 10^(9.5)-10^(11)M_☉ we infer quenching e-folding times that span 1.5–0.5 Gyr with more massive central galaxies quenching faster. For M_* = 10^(10.5)M_☉, this implies a total migration time of ~4 Gyr from the star formation main sequence to quiescence. Compared to satellites, central galaxies take ~2 Gyr longer to quench their star formation, suggesting that different mechanisms are responsible for quenching centrals versus satellites. Finally, the central galaxy quenching timescale we infer provides key constraints for proposed star formation quenching mechanisms. Our timescale is generally consistent with gas depletion timescales predicted by quenching through strangulation. However, the exact physical mechanism(s) responsible for this remain unclear.

Publication: Astrophysical Journal Vol.: 841 No.: 1 ISSN: 1538-4357

ID: CaltechAUTHORS:20170516-100219721

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Abstract: We study the structure, age and metallicity gradients, and dynamical evolution using a cosmological zoom-in simulation of a Milky Way-mass galaxy from the Feedback in Realistic Environments project. In the simulation, stars older than 6 Gyr were formed in a chaotic, bursty mode and have the largest vertical scaleheights (1.5–2.5 kpc) by z = 0, while stars younger than 6 Gyr were formed in a relatively calm, stable disc. The vertical scaleheight increases with stellar age at all radii, because (1) stars that formed earlier were thicker ‘at birth’, and (2) stars were kinematically heated to an even thicker distribution after formation. Stars of the same age are thicker in the outer disc than in the inner disc (flaring). These lead to positive vertical age gradients and negative radial age gradients. The radial metallicity gradient is negative at the mid-plane, flattens at larger disc height |Z|, and turns positive above |Z| ∼ 1.5 kpc. The vertical metallicity gradient is negative at all radii, but is steeper at smaller radii. These trends broadly agree with observations in the Milky Way and can be naturally understood from the age gradients. The vertical stellar density profile can be well described by two components, with scaleheights 200–500 pc and 1–1.5 kpc, respectively. The thick component is a mix of stars older than 4 Gyr, which formed through a combination of several mechanisms. Our results also demonstrate that it is possible to form a thin disc in cosmological simulations even with a strong stellar feedback.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 467 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:20161107-114246249

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Abstract: We extend recently developed mesh-free Lagrangian methods for numerical magnetohydrodynamics (MHD) to arbitrary anisotropic diffusion equations, including: passive scalar diffusion, Spitzer–Braginskii conduction and viscosity, cosmic ray diffusion/streaming, anisotropic radiation transport, non-ideal MHD (Ohmic resistivity, ambipolar diffusion, the Hall effect) and turbulent ‘eddy diffusion’. We study these as implemented in the code gizmo for both new meshless finite-volume Godunov schemes (MFM/MFV). We show that the MFM/MFV methods are accurate and stable even with noisy fields and irregular particle arrangements, and recover the correct behaviour even in arbitrarily anisotropic cases. They are competitive with state-of-the-art AMR/moving-mesh methods, and can correctly treat anisotropic diffusion-driven instabilities (e.g. the MTI and HBI, Hall MRI). We also develop a new scheme for stabilizing anisotropic tensor-valued fluxes with high-order gradient estimators and non-linear flux limiters, which is trivially generalized to AMR/moving-mesh codes. We also present applications of some of these improvements for SPH, in the form of a new integral-Godunov SPH formulation that adopts a moving-least squares gradient estimator and introduces a flux-limited Riemann problem between particles.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 466 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20160317-141936363

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Abstract: We consider cosmological evolution from the perspective of quantum information. We present a quantum circuit model for the expansion of a comoving region of space, in which initially-unentangled ancilla qubits become entangled as expansion proceeds. We apply this model to the comoving region that now coincides with our Hubble volume, taking the number of entangled degrees of freedom in this region to be proportional to the de Sitter entropy. The quantum circuit model is applicable for at most 140 e-folds of inflationary and post-inflationary expansion: we argue that no geometric description was possible before the time t_1 when our comoving region was one Planck length across, and contained one pair of entangled degrees of freedom. This approach could provide a framework for modeling the initial state of inflationary perturbations.

Publication: arXiv
ID: CaltechAUTHORS:20170228-191741490

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Abstract: Some modern cosmological models predict the appearance of Boltzmann Brains: observers who randomly fluctuate out of a thermal bath rather than naturally evolving from a low-entropy Big Bang. A theory in which most observers are of the Boltzmann Brain type is generally thought to be unacceptable, although opinions differ. I argue that such theories are indeed unacceptable: the real problem is with fluctuations into observers who are locally identical to ordinary observers, and their existence cannot be swept under the rug by a choice of probability distributions over observers. The issue is not that the existence of such observers is ruled out by data, but that the theories that predict them are cognitively unstable: they cannot simultaneously be true and justifiably believed.

Publication: arXiv
ID: CaltechAUTHORS:20170209-151733308

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Abstract: In low-mass galaxies, stellar feedback can drive gas outflows that generate non-equilibrium fluctuations in the gravitational potential. Using cosmological zoom-in baryonic simulations from the Feedback in Realistic Environments project, we investigate how these fluctuations affect stellar kinematics and the reliability of Jeans dynamical modeling in low-mass galaxies. We find that stellar velocity dispersion and anisotropy profiles fluctuate significantly over the course of galaxies' starburst cycles. We therefore predict an observable correlation between star formation rate and stellar kinematics: dwarf galaxies with higher recent star formation rates should have systemically higher stellar velocity dispersions. This prediction provides an observational test of the role of stellar feedback in regulating both stellar and dark-matter densities in dwarf galaxies. We find that Jeans modeling, which treats galaxies as virialized systems in dynamical equilibrium, overestimates a galaxy's dynamical mass during periods of post-starburst gas outflow and underestimates it during periods of net inflow. Short-timescale potential fluctuations lead to typical errors of ~20% in dynamical mass estimates, even if full three-dimensional stellar kinematics—including the orbital anisotropy—are known exactly. When orbital anisotropy is not known a priori, typical mass errors arising from non-equilibrium fluctuations in the potential are larger than those arising from the mass-anisotropy degeneracy. However, Jeans modeling alone cannot reliably constrain the orbital anisotropy, and problematically, it often favors anisotropy models that do not reflect the true profile. If galaxies completely lose their gas and cease forming stars, fluctuations in the potential subside, and Jeans modeling becomes much more reliable.

Publication: Astrophysical Journal Vol.: 835 No.: 2 ISSN: 1538-4357

ID: CaltechAUTHORS:20170616-094907367

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Abstract: We examine how to construct a spatial manifold and its geometry from the entanglement structure of an abstract quantum state in Hilbert space. Given a decomposition of Hilbert space H into a tensor product of factors, we consider a class of “redundancy-constrained states” in H that generalize the area-law behavior for entanglement entropy usually found in condensed-matter systems with gapped local Hamiltonians. Using mutual information to define a distance measure on the graph, we employ classical multidimensional scaling to extract the best-fit spatial dimensionality of the emergent geometry. We then show that entanglement perturbations on such emergent geometries naturally give rise to local modifications of spatial curvature which obey a (spatial) analog of Einstein’s equation. The Hilbert space corresponding to a region of flat space is finite-dimensional and scales as the volume, though the entropy (and the maximum change thereof) scales like the area of the boundary. A version of the ER=EPR conjecture is recovered, in that perturbations that entangle distant parts of the emergent geometry generate a configuration that may be considered as a highly quantum wormhole.

Publication: Physical Review D Vol.: 95 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:20160704-200753575

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Abstract: Using an isolated Milky Way-mass galaxy simulation, we compare results from nine state-of-the-art gravito-hydrodynamics codes widely used in the numerical community. We utilize the infrastructure we have built for the AGORA High-resolution Galaxy Simulations Comparison Project. This includes the common disk initial conditions, common physics models (e.g., radiative cooling and UV background by the standardized package Grackle) and common analysis toolkit yt, all of which are publicly available. Subgrid physics models such as Jeans pressure floor, star formation, supernova feedback energy, and metal production are carefully constrained across code platforms. With numerical accuracy that resolves the disk scale height, we find that the codes overall agree well with one another in many dimensions including: gas and stellar surface densities, rotation curves, velocity dispersions, density and temperature distribution functions, disk vertical heights, stellar clumps, star formation rates, and Kennicutt–Schmidt relations. Quantities such as velocity dispersions are very robust (agreement within a few tens of percent at all radii) while measures like newly formed stellar clump mass functions show more significant variation (difference by up to a factor of ~3). Systematic differences exist, for example, between mesh-based and particle-based codes in the low-density region, and between more diffusive and less diffusive schemes in the high-density tail of the density distribution. Yet intrinsic code differences are generally small compared to the variations in numerical implementations of the common subgrid physics such as supernova feedback. Our experiment reassures that, if adequately designed in accordance with our proposed common parameters, results of a modern high-resolution galaxy formation simulation are more sensitive to input physics than to intrinsic differences in numerical schemes.

Publication: Astrophysical Journal Vol.: 833 No.: 2 ISSN: 0004-637X

ID: CaltechAUTHORS:20161219-102104799

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Abstract: The AMPS paradox challenges black hole complementarity by apparently constructing a way for an observer to bring information from the outside of the black hole into its interior if there is no drama at its horizon, making manifest a violation of monogamy of entanglement. We propose a new resolution to the paradox: this violation cannot be explicitly checked by an infalling observer in the finite proper time they have to live after crossing the horizon. Our resolution depends on a weak relaxation of the no-drama condition (we call it “little-drama”) which is the “complementarity dual” of scrambling of information on the stretched horizon. When translated to the description of the black hole interior, this implies that the fine-grained quantum information of infalling matter is rapidly diffused across the entire interior while classical observables and coarse-grained geometry remain unaffected. Under the assumption that information has diffused throughout the interior, we consider the difficulty of the information-theoretic task that an observer must perform after crossing the event horizon of a Schwarzschild black hole in order to verify a violation of monogamy of entanglement. We find that the time required to complete a necessary subroutine of this task, namely the decoding of Bell pairs from the interior and the late radiation, takes longer than the maximum amount of time that an observer can spend inside the black hole before hitting the singularity. Therefore, an infalling observer cannot observe monogamy violation before encountering the singularity.

Publication: Journal of High Energy Physics Vol.: 2016 No.: 12 ISSN: 1029-8479

ID: CaltechAUTHORS:20170113-125141759

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Abstract: We present a constructive algorithm for the determination of Ryu-Takayanagi surfaces in AdS_3/CFT_2 which exploits previously noted connections between holographic entanglement entropy and max-flow/min-cut. We then characterize its complexity as a polynomial time algorithm.

Publication: Journal of High Energy Physics Vol.: 2016 No.: 11 ISSN: 1126-6708

ID: CaltechAUTHORS:20161024-193659528

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Abstract: Many properties of the Milky Way's (MW) dark matter halo, including its mass-assembly history, concentration, and subhalo population, remain poorly constrained. We explore the connection between these properties of the MW and its satellite galaxy population, especially the implication of the presence of the Magellanic Clouds for the properties of the MW halo. Using a suite of high-resolution N-body simulations of MW-mass halos with a fixed final M_(vir) ~ 10^(12.1),M_⊙, we find that the presence of Magellanic Cloud-like satellites strongly correlates with the assembly history, concentration, and subhalo population of the host halo, such that MW-mass systems with Magellanic Clouds have lower concentration, more rapid recent accretion, and more massive subhalos than typical halos of the same mass. Using a flexible semi-analytic galaxy formation model that is tuned to reproduce the stellar mass function of the classical dwarf galaxies of the MW with Markov-Chain Monte-Carlo, we show that adopting host halos with different mass-assembly histories and concentrations can lead to different best-fit models for galaxy-formation physics, especially for the strength of feedback. These biases arise because the presence of the Magellanic Clouds boosts the overall population of high-mass subhalos, thus requiring a different stellar-mass-to-halo-mass ratio to match the data. These biases also lead to significant differences in the mass–metallicity relation, the kinematics of low-mass satellites, the number counts of small satellites associated with the Magellanic Clouds, and the stellar mass of MW itself. Observations of these galaxy properties can thus provide useful constraints on the properties of the MW halo.

Publication: Astrophysical Journal Vol.: 830 No.: 2 ISSN: 0004-637X

ID: CaltechAUTHORS:20161012-082135875

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Abstract: In numerical magnetohydrodynamics (MHD), a major challenge is maintaining ∇⋅B=0. Constrained transport (CT) schemes achieve this but have been restricted to specific methods. For more general (meshless, moving-mesh, ALE) methods, ‘divergence-cleaning’ schemes reduce the ∇⋅B errors; however they can still be significant and can lead to systematic errors which converge away slowly. We propose a new constrained gradient (CG) scheme which augments these with a projection step, and can be applied to any numerical scheme with a reconstruction. This iteratively approximates the least-squares minimizing, globally divergence-free reconstruction of the fluid. Unlike ‘locally divergence free’ methods, this actually minimizes the numerically unstable ∇⋅B terms, without affecting the convergence order of the method. We implement this in the mesh-free code gizmo and compare various test problems. Compared to cleaning schemes, our CG method reduces the maximum ∇⋅B errors by ∼1–3 orders of magnitude (∼2–5 dex below typical errors if no ∇⋅B cleaning is used). By preventing large ∇⋅B at discontinuities, this eliminates systematic errors at jumps. Our CG results are comparable to CT methods; for practical purposes, the ∇⋅B errors are eliminated. The cost is modest, ∼30 per cent of the hydro algorithm, and the CG correction can be implemented in a range of numerical MHD methods. While for many problems, we find Dedner-type cleaning schemes are sufficient for good results, we identify a range of problems where using only Powell or ‘8-wave’ cleaning can produce order-of-magnitude errors.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 462 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20151022-135013652

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Abstract: In a weakly coupled gravity theory in the anti-de Sitter space, local states in the bulk are linear superpositions of Ishibashi states for a crosscap in the dual conformal field theory. The superposition structure can be constrained either by the microscopic causality in the bulk gravity or the bootstrap condition in the boundary conformal field theory. We show, contrary to some expectation, that these two conditions are not compatible to each other in the weak gravity regime. We also present an evidence to show that bulk local states in three dimensions are not organized by the Virasoro symmetry.

Publication: Journal of High Energy Physics Vol.: 2016 No.: 10 ISSN: 1126-6708

ID: CaltechAUTHORS:20160509-164600548

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Abstract: Low-mass "dwarf" galaxies represent the most significant challenges to the cold dark matter (CDM) model of cosmological structure formation. Because these faint galaxies are (best) observed within the Local Group (LG) of the Milky Way (MW) and Andromeda (M31), understanding their formation in such an environment is critical. We present first results from the Latte Project: the Milky Way on Feedback in Realistic Environments (FIRE). This simulation models the formation of an MW-mass galaxy to z = 0 within ΛCDM cosmology, including dark matter, gas, and stars at unprecedented resolution: baryon particle mass of 7070 M_⊙ with gas kernel/softening that adapts down to 1 pc (with a median of 2-60 pc at z = 0). Latte was simulated using the GIZMO code with a mesh-free method for accurate hydrodynamics and the FIRE-2 model for star formation and explicit feedback within a multi-phase interstellar medium. For the first time, Latte self-consistently resolves the spatial scales corresponding to half-light radii of dwarf galaxies that form around an MW-mass host down to M_(star) ≳ 10^5 M_⊙. Latte's population of dwarf galaxies agrees with the LG across a broad range of properties: (1) distributions of stellar masses and stellar velocity dispersions (dynamical masses), including their joint relation; (2) the mass–metallicity relation; and (3) diverse range of star formation histories, including their mass dependence. Thus, Latte produces a realistic population of dwarf galaxies at M_(star) ≳ 10^5 M_⊙ that does not suffer from the "missing satellites" or "too big to fail" problems of small-scale structure formation. We conclude that baryonic physics can reconcile observed dwarf galaxies with standard ΛCDM cosmology.

Publication: Astrophysical Journal Letters Vol.: 827 No.: 2 ISSN: 2041-8205

ID: CaltechAUTHORS:20160906-220609945

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Abstract: We discuss the physical interpretation of stress-energy tensors that source static spherically symmetric Kerr-Schild metrics. We find that the sources of such metrics with no curvature singularities or horizons do not simultaneously satisfy the weak and strong energy conditions. Sensible stress-energy tensors usually satisfy both of them. Under most circumstances, these sources are not perfect fluids and contain shear stresses. We show that for these systems the classical double copy associates the electric charge density to the Komar energy density. In addition, we demonstrate that the stress-energy tensors are determined by the electric charge density and their conservation equations.

Publication: Physical Review D Vol.: 94 No.: 4 ISSN: 2470-0010

ID: CaltechAUTHORS:20160811-110734508

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Abstract: Loop-level scattering amplitudes for massless particles have singularities in regions where tree amplitudes are perfectly smooth. For example, a 2 → 4 gluon scattering process has a singularity in which each incoming gluon splits into a pair of gluons, followed by a pair of 2 → 2 collisions between the gluon pairs. This singularity mimics double parton scattering because it occurs when the transverse momentum of a pair of outgoing gluons vanishes. The singularity is logarithmic at fixed order in perturbation theory. We exploit the duality between scattering amplitudes and polygonal Wilson loops to study six-point amplitudes in this limit to high loop order in planar N = 4 super-Yang-Mills theory. The singular configuration corresponds to the limit in which a hexagonal Wilson loop develops a self-crossing. The singular terms are governed by an evolution equation, in which the hexagon mixes into a pair of boxes; the mixing back is suppressed in the planar (large N_c) limit. Because the kinematic dependence of the box Wilson loops is dictated by (dual) conformal invariance, the complete kinematic dependence of the singular terms for the self-crossing hexagon on the one nonsingular variable is determined to all loop orders. The complete logarithmic dependence on the singular variable can be obtained through nine loops, up to a couple of constants, using a correspondence with the multi-Regge limit. As a byproduct, we obtain a simple formula for the leading logs to all loop orders. We also show that, although the MHV six-gluon amplitude is singular, remarkably, the transcendental functions entering the non-MHV amplitude are finite in the same limit, at least through four loops.

Publication: Journal of High Energy Physics Vol.: 2016 No.: 7 ISSN: 1126-6708

ID: CaltechAUTHORS:20160210-144009886

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Abstract: We consider the question of entanglement conservation in the context of the ER=EPR correspondence equating quantum entanglement with wormholes. In quantum mechanics, the entanglement between a system and its complement is conserved under unitary operations that act independently on each; ER=EPR suggests that an analogous statement should hold for wormholes. We accordingly prove a new area theorem in general relativity: for a collection of dynamical wormholes and black holes in a spacetime satisfying the null curvature condition, the maximin area for a subset of the horizons (giving the largest area attained by the minimal cross section of the multi-wormhole throat separating the subset from its complement) is invariant under classical time evolution along the outermost apparent horizons. The evolution can be completely general, including horizon mergers and the addition of classical matter satisfying the null energy condition. This theorem is the gravitational dual of entanglement conservation and thus constitutes an explicit characterization of the ER=EPR duality in the classical limit.

Publication: Journal of High Energy Physics Vol.: 2016 No.: 7 ISSN: 1126-6708

ID: CaltechAUTHORS:20160622-105352946

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Abstract: We investigate theories in which gravity arises as a consequence of entropy. We distinguish between two approaches to this idea: holographic gravity, in which Einstein’s equation arises from keeping entropy stationary in equilibrium under variations of the geometry and quantum state of a small region, and thermodynamic gravity, in which Einstein’s equation emerges as a local equation of state from constraints on the area of a dynamical light sheet in a fixed spacetime background. Examining holographic gravity, we argue that its underlying assumptions can be justified in part using recent results on the form of the modular energy in quantum field theory. For thermodynamic gravity, on the other hand, we find that it is difficult to formulate a self-consistent definition of the entropy, which represents an obstacle for this approach. This investigation points the way forward in understanding the connections between gravity and entanglement.

Publication: Physical Review D Vol.: 93 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:20160210-142850363

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Abstract: Turbulence is thought to be a primary driving force behind the early stages of star formation. In this framework large, self-gravitating, turbulent clouds fragment into smaller clouds which in turn fragment into even smaller ones. At the end of this cascade we find the clouds which collapse into protostars. Following this process is extremely challenging numerically due to the large dynamical range, so in this paper we propose a semi-analytic framework which is able to model star formation from the largest, giant molecular cloud scale, to the final protostellar size scale. Because of the simplicity of the framework it is ideal for theoretical experimentation to explore the principal processes behind different aspects of star formation, at the cost of introducing strong assumptions about the collapse process. The basic version of the model discussed in this paper only contains turbulence, gravity and crude assumptions about feedback; nevertheless it can reproduce the observed core mass function and provide the protostellar system mass function (PSMF), which shows a striking resemblance to the observed initial mass function (IMF), if a non-negligible fraction of gravitational energy goes into turbulence. Furthermore we find that to produce a universal IMF protostellar feedback must be taken into account otherwise the PSMF peak shows a strong dependence on the background temperature.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 459 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20150812-101357981

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Abstract: We study the impact of bound state formation on dark matter annihilation rates in models where dark matter interacts via a light mediator, the dark photon. We derive the general cross section for radiative capture into all possible bound states, and point out its nontrivial dependence on the dark matter velocity and the dark photon mass. For indirect detection, our result shows that dark matter annihilation inside bound states can play an important role in enhancing signal rates over the rate for direct dark matter annihilation with Sommerfeld enhancement. The effects are strongest for large dark gauge coupling and when the dark photon mass is smaller than the typical momentum of dark matter in the Galaxy. As an example, we show that for thermal dark matter the Fermi gamma ray constraint is substantially increased once bound state effects are taken into account. We also find that bound state effects are not important for dark matter annihilation during the freeze-out and recombination epochs.

Publication: Physical Review D Vol.: 93 No.: 11 ISSN: 2470-0010

ID: CaltechAUTHORS:20160411-175600335

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Abstract: We investigate the possibility that the dark matter candidate is from a pure non-Abelian gauge theory of the hidden sector, motivated in large part by its elegance and simplicity. The dark matter is the lightest bound state made of the confined gauge fields, the hidden glueball. We point out that this simple setup is capable of providing rich and novel phenomena in the dark sector, especially in the parameter space of large N. They include self-interacting and warm dark matter scenarios, Bose-Einstein condensation leading to massive dark stars possibly millions of times heavier than our sun giving rise to gravitational lensing effects, and indirect detections through higher dimensional operators as well as interesting collider signatures.

Publication: Physical Review D Vol.: 93 No.: 11 ISSN: 2470-0010

ID: CaltechAUTHORS:20160210-143427671

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Abstract: We argue that, under certain plausible assumptions, de Sitter space settles into a quiescent vacuum in which there are no dynamical quantum fluctuations. Such fluctuations require either an evolving microstate, or time-dependent histories of out-of-equilibrium recording devices, which we argue are absent in stationary states. For a massive scalar field in a fixed de Sitter background, the cosmic no-hair theorem implies that the state of the patch approaches the vacuum, where there are no fluctuations. We argue that an analogous conclusion holds whenever a patch of de Sitter is embedded in a larger theory with an infinite-dimensional Hilbert space, including semiclassical quantum gravity with false vacua or complementarity in theories with at least one Minkowski vacuum. This reasoning provides an escape from the Boltzmann brain problem in such theories. It also implies that vacuum states do not uptunnel to higher-energy vacua and that perturbations do not decohere while slow-roll inflation occurs, suggesting that eternal inflation is much less common than often supposed. On the other hand, if a de Sitter patch is a closed system with a finite-dimensional Hilbert space, there will be Poincaré recurrences and dynamical Boltzmann fluctuations into lower-entropy states. Our analysis does not alter the conventional understanding of the origin of density fluctuations from primordial inflation, since reheating naturally generates a high-entropy environment and leads to decoherence, nor does it affect the existence of non-dynamical vacuum fluctuations such as those that give rise to the Casimir effect.

Publication: Foundations of Physics Vol.: 46 No.: 6 ISSN: 0015-9018

ID: CaltechAUTHORS:20150316-131744979

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Abstract: We study the interaction of feedback from active galactic nuclei (AGN) and a multiphase interstellar medium (ISM), in simulations including explicit stellar feedback, multiphase cooling, accretion-disc winds, and Compton heating. We examine radii ∼0.1–100 pc around a black hole (BH), where the accretion rate on to the BH is determined and where AGN-powered winds and radiation couple to the ISM. We conclude: (1) the BH accretion rate is determined by exchange of angular momentum between gas and stars in gravitational instabilities. This produces accretion rates ∼0.03–1 M_⊙ yr^(−1), sufficient to power luminous AGN. (2) The gas disc in the galactic nucleus undergoes an initial burst of star formation followed by several million years where stellar feedback suppresses the star formation rate (SFR). (3) AGN winds injected at small radii with momentum fluxes ∼L_(AGN)/c couple efficiently to the ISM and have dramatic effects on ISM properties within ∼100 pc. AGN winds suppress the nuclear SFR by factors ∼10–30 and BH accretion rate by factors ∼3–30. They increase the outflow rate from the nucleus by factors ∼10, consistent with observational evidence for galaxy-scale AGN-driven outflows. (4) With AGN feedback, the predicted column density distribution to the BH is consistent with observations. Absent AGN feedback, the BH is isotropically obscured and there are not enough optically thin sightlines to explain type-I AGN. A ‘torus-like’ geometry arises self-consistently as AGN feedback evacuates gas in polar regions.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 458 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20150422-112435004

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Abstract: A model of dark sector where O(few GeV) mass dark matter particles χ are supplied by a lighter dark force mediator V, m_V ≪ m_χ, is motivated by the recently discovered mismatch between simulated and observed shapes of galactic haloes. Such models, in general, provide a challenge for direct detection efforts and collider searches. We show that for a large range of coupling constants and masses, the production and decay of the bound states of χ, such as 0^(−+) and 1^(−−) states, η_D and Υ_D, is an important search channel. We show that e+e− → η_D + V or Υ_D + γ production at B-factories for α_D > 0.1 is sufficiently strong to result in multiple pairs of charged leptons and pions via η_D → 2V → 2(l^+l^−) and Υ_D → 3V → 3(l+l−) (l = e, µ, π). The absence of such final states in the existing searches performed at BABAR and Belle sets new constraints on the parameter space of the model. We also show that a search for multiple bremsstrahlung of dark force mediators, e^+e^− → χχ¯ + nV , resulting in missing energy and multiple leptons, will further improve the sensitivity to self-interacting dark matter.

Publication: Physical Review Letters Vol.: 116 No.: 15 ISSN: 0031-9007

ID: CaltechAUTHORS:20151022-123702814

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Abstract: We examine the effects of stellar feedback and bursty star formation on low-mass galaxies (M_(star) = 2 × 10^6 − 5 × 10^(10) M_⊙) using the Feedback in Realistic Environments (FIRE) simulations. While previous studies emphasized the impact of feedback on dark matter profiles, we investigate the impact on the stellar component: kinematics, radial migration, size evolution, and population gradients. Feedback-driven outflows/inflows drive significant radial stellar migration over both short and long timescales via two processes: (1) outflowing/infalling gas can remain star-forming, producing young stars that migrate ~1 kpc within their first 100 Myr, and (2) gas outflows/inflows drive strong fluctuations in the global potential, transferring energy to all stars. These processes produce several dramatic effects. First, galaxies' effective radii can fluctuate by factors of >2 over ~200 Myr, and these rapid size fluctuations can account for much of the observed scatter in the radius at fixed M_(star). Second, the cumulative effects of many outflow/infall episodes steadily heat stellar orbits, causing old stars to migrate outward most strongly. This age-dependent radial migration mixes—and even inverts—intrinsic age and metallicity gradients. Thus, the galactic-archaeology approach of calculating radial star formation histories from stellar populations at z = 0 can be severely biased. These effects are strongest at M_(star) ≈ 10^(7–9.6) M_⊙, the same regime where feedback most efficiently cores galaxies. Thus, detailed measurements of stellar kinematics in low-mass galaxies can strongly constrain feedback models and test baryonic solutions to small-scale problems in ΛCDM.

Publication: Astrophysical Journal Vol.: 820 No.: 2 ISSN: 0004-637X

ID: CaltechAUTHORS:20160510-102001148

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Abstract: We use modular invariance and crossing symmetry of conformal field theory to reveal approximate reflection symmetries in the spectral decompositions of the partition function in two dimensions in the limit of large central charge and of the four-point function in any dimension in the limit of large scaling dimensions Δ0 of external operators. We use these symmetries to motivate universal upper bounds on the spectrum and the operator product expansion coefficients, which we then derive by independent techniques. Some of the bounds for four-point functions are valid for finite Δ0 as well as for large Δ0. We discuss a similar symmetry in a large spacetime dimension limit. Finally, we comment on the analogue of the Cardy formula and sparse light spectrum condition for the four-point function.

Publication: Journal of High Energy Physics Vol.: 2016 No.: 4 ISSN: 1126-6708

ID: CaltechAUTHORS:20151104-124350087

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Abstract: We study novel types of contributions to the partition function of the Maxwell system defined on a small compact manifold. These contributions, often not addressed in the perturbative treatment with physical photons, emerge as a result of tunneling transitions between topologically distinct but physically identical vacuum winding states. These new terms give an extra contribution to the Casimir pressure, yet to be measured. We argue that this effect is highly sensitive to a small external electric field, which should be contrasted with the conventional Casimir effect, where the vacuum photons are essentially unaffected by any external field. Furthermore, photons will be emitted from the vacuum in response to a time-dependent electric field, similar to the dynamical Casimir effect in which real particles are radiated from the vacuum due to the time-dependent boundary conditions. We also propose an experimental setup using a quantum LC circuit to detect this novel effect. We expect physical electric charges to appear on the capacitor plates when the system dimension is such that coherent Aharonov-Bohm phases can be maintained over macroscopically large distances.

Publication: Physical Review D Vol.: 93 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20160426-103346757

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Abstract: We study the behaviour of large dust grains in turbulent molecular clouds (MCs). In primarily neutral regions, dust grains move as aerodynamic particles, not necessarily with the gas. We therefore directly simulate, for the first time, the behaviour of aerodynamic grains in highly supersonic, magnetohydrodynamic turbulence typical of MCs. We show that, under these conditions, grains with sizes a ≳ 0.01 micron exhibit dramatic (exceeding factor ∼1000) fluctuations in the local dust-to-gas ratio (implying large small-scale variations in abundances, dust cooling rates, and dynamics). The dust can form highly filamentary structures (which would be observed in both dust emission and extinction), which can be much thinner than the characteristic width of gas filaments. Sometimes, the dust and gas filaments are not even in the same location. The ‘clumping factor’ ⟨n^2_(dust)⟩/⟨n_(dust)⟩^2 of the dust (critical for dust growth/coagulation/shattering) can reach ∼100, for grains in the ideal size range. The dust clustering is maximized around scales ∼ 0.2 pc (a/μm) (n_(gas)/100 cm^(−3))^(−1), and is ‘averaged out’ on larger scales. However, because the density varies widely in supersonic turbulence, the dynamic range of scales (and interesting grain sizes) for these fluctuations is much broader than in the subsonic case. Our results are applicable to MCs of essentially all sizes and densities, but we note how Lorentz forces and other physics (neglected here) may change them in some regimes. We discuss the potentially dramatic consequences for star formation, dust growth and destruction, and dust-based observations of MCs.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 456 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20151022-134506351

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Abstract: This paper investigates the decays from heavy Higgsino-like weak doublets into Z, h bosons and missing particles. When pair-produced at the LHC, the subsequent Z, h→ℓℓ, bb¯ decays in the doublet decay cascade can yield 4ℓ, 2ℓ2b and 4b+E_T+j(s) final states. Mutual observation of any two of these channels would provide information on the associated doublets’ decay branching fractions into a Z or h, thereby probing the Goldstone equivalence relation, shedding additional light on the Higgs sector of beyond the Standard Model theories and facilitating the discrimination of various contending models, in turn. We compare the Z/h decay ratio expected in the minimal supersymmetric model, the next-to-minimal supersymmetric model (NMSSM)and a minimal singlet-doublet dark matter model. Additionally, we conduct a full Monte Carlo analysis of the prospects for detecting the targeted final states during 14 TeV running of the LHC in the context of a representative NMSSM benchmark model.

Publication: Physical Review D Vol.: 93 No.: 5 ISSN: 2470-0010

ID: CaltechAUTHORS:20151022-120109719

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Abstract: We use high-resolution cosmological zoom-in simulations from the Feedback in Realistic Environment (FIRE) project to study the galaxy mass–metallicity relations (MZR) from z=0–6. These simulations include explicit models of the multi-phase ISM, star formation, and stellar feedback. The simulations cover halo masses M_(halo) = 10^9–10^(13) M_☉ and stellar masses M_* = 10^4–10^(11) M_☉ at z = 0 and have been shown to produce many observed galaxy properties from z = 0–6. For the first time, our simulations agree reasonably well with the observed mass–metallicity relations at z = 0–3 for a broad range of galaxy masses. We predict the evolution of the MZR from z = 0–6, as log(Z_(gas)/Z_☉) = 12+log(O/H)-9.0 = 0.35 [log(M_*/M_☉) - 10] + 0.93exp(-0.43z) - 1.05 and log(Z_*/Z_☉) = [Fe=H] + 0.2 = 0.40 [log(M_*/M_☉)-10]+0.67exp(-0.50z)-1.04, for gas-phase and stellar metallicity, respectively. Our simulations suggest that the evolution of MZR is associated with the evolution of stellar/gas mass fractions at different redshifts, indicating the existence of a universal metallicity relation between stellar mass, gas mass, and metallicities. In our simulations, galaxies above M_* = 10^6 M_☉ are able to retain a large fraction of their metals inside the halo, because metal-rich winds fail to escape completely and are recycled into the galaxy. This resolves a long-standing discrepancy between “sub-grid” wind models (and semi-analytic models) and observations, where common sub-grid models cannot simultaneously reproduce the MZR and the stellar mass functions.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 456 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:20150423-081343792

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Abstract: Recently, we explored new meshless finite-volume Lagrangian methods for hydrodynamics: the ‘meshless finite mass’ (MFM) and ‘meshless finite volume’ (MFV) methods; these capture advantages of both smoothed particle hydrodynamics (SPH) and adaptive mesh refinement (AMR) schemes. We extend these to include ideal magnetohydrodynamics (MHD). The MHD equations are second-order consistent and conservative. We augment these with a divergence-cleaning scheme, which maintains ∇⋅B≈0∇⋅B≈0. We implement these in the code GIZMO, together with state-of-the-art SPH MHD. We consider a large test suite, and show that on all problems the new methods are competitive with AMR using constrained transport (CT) to ensure ∇⋅B=0∇⋅B=0. They correctly capture the growth/structure of the magnetorotational instability, MHD turbulence, and launching of magnetic jets, in some cases converging more rapidly than state-of-the-art AMR. Compared to SPH, the MFM/MFV methods exhibit convergence at fixed neighbour number, sharp shock-capturing, and dramatically reduced noise, divergence errors, and diffusion. Still, ‘modern’ SPH can handle most test problems, at the cost of larger kernels and ‘by hand’ adjustment of artificial diffusion. Compared to non-moving meshes, the new methods exhibit enhanced ‘grid noise’ but reduced advection errors and diffusion, easily include self-gravity, and feature velocity-independent errors and superior angular momentum conservation. They converge more slowly on some problems (smooth, slow-moving flows), but more rapidly on others (involving advection/rotation). In all cases, we show divergence control beyond the Powell 8-wave approach is necessary, or all methods can converge to unphysical answers even at high resolution.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 455 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20150615-061712828

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Abstract: We use the hexagon function bootstrap to compute the ratio function which characterizes the next-to-maximally-helicity-violating (NMHV) six-point amplitude in planar N=4 super-Yang-Mills theory at four loops. A powerful constraint comes from dual superconformal invariance, in the form of a Q differential equation, which heavily constrains the first derivatives of the transcendental functions entering the ratio function. At four loops, it leaves only a 34-parameter space of functions. Constraints from the collinear limits, and from the multi-Regge limit at the leading-logarithmic (LL) and next-to-leading-logarithmic (NLL) order, suffice to fix these parameters and obtain a unique result. We test the result against multi-Regge predictions at NNLL and N^3LL, and against predictions from the operator product expansion involving one and two flux-tube excitations; all cross-checks are satisfied. We study the analytical and numerical behavior of the parity-even and parity-odd parts on various lines and surfaces traversing the three-dimensional space of cross ratios. As part of this program, we characterize all irreducible hexagon functions through weight eight in terms of their coproduct. We also provide representations of the ratio function in particular kinematic regions in terms of multiple polylogarithms.

Publication: Journal of High Energy Physics Vol.: 2016 No.: 1 ISSN: 1126-6708

ID: CaltechAUTHORS:20151022-122311732

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Abstract: We demonstrate an algorithm for the retrieval of a qubit, encoded in spin angular momentum, that has been dropped into a no-firewall black hole. Retrieval is achieved analogously to quantum teleportation by collecting Hawking radiation and performing measurements on the black hole. Importantly, these methods require only the ability to perform measurements from outside the event horizon.

Publication: Physical Review Letters Vol.: 115 No.: 26 ISSN: 0031-9007

ID: CaltechAUTHORS:20150731-184935946

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Abstract: ATLAS and CMS have each reported a modest diphoton excess consistent with the decay of a broad resonance at ~ 750 GeV. We show how this signal can arise in a weakly coupled theory comprised solely of narrow width particles. In particular, if the decaying particle is produced off-shell, then the associated diphoton resonance will have a broad, adjustable width. We present simplified models which explain the diphoton excess through the three-body decay of a scalar or fermion. Our minimal ultraviolet completion is a weakly coupled and renormalizable theory of a singlet scalar plus a heavy vector-like quark and lepton. The smoking gun of this mechanism is an asymmetric diphoton peak recoiling against missing transverse energy, jets, or leptons.

Publication: arXiv
ID: CaltechAUTHORS:20160113-132530546

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Abstract: We present an analysis of the galaxy-scale gaseous outflows from the Feedback in Realistic Environments (FIRE) simulations. This suite of hydrodynamic cosmological zoom simulations resolves formation of star-forming giant molecular clouds to z = 0, and features an explicit stellar feedback model on small scales. Our simulations reveal that high-redshift galaxies undergo bursts of star formation followed by powerful gusts of galactic outflows that eject much of the interstellar medium and temporarily suppress star formation. At low redshift, however, sufficiently massive galaxies corresponding to L* progenitors develop stable discs and switch into a continuous and quiescent mode of star formation that does not drive outflows far into the halo. Mass-loading factors for winds in L* progenitors are η ≈ 10 at high redshift, but decrease to η ≪ 1 at low redshift. Although lower values of η are expected as haloes grow in mass over time, we show that the strong suppression of outflows with decreasing redshift cannot be explained by mass evolution alone. Circumgalactic outflow velocities are variable and broadly distributed, but typically range between one and three times the circular velocity of the halo. Much of the ejected material builds a reservoir of enriched gas within the circumgalactic medium, some of which could be later recycled to fuel further star formation. However, a fraction of the gas that leaves the virial radius through galactic winds is never regained, causing most haloes with mass M_h ≤ 10^(12) M_⊙ to be deficient in baryons compared to the cosmic mean by z = 0.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 454 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20160205-132422140

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Abstract: Evanescent operators such as the Gauss-Bonnet term have vanishing perturbative matrix elements in exactly D=4 dimensions. Similarly, evanescent fields do not propagate in D=4; a three-form field is in this class, since it is dual to a cosmological-constant contribution. In this Letter, we show that evanescent operators and fields modify the leading ultraviolet divergence in pure gravity. To analyze the divergence, we compute the two-loop identical-helicity four-graviton amplitude and determine the coefficient of the associated (nonevanescent) R^3 counterterm studied long ago by Goroff and Sagnotti. We compare two pairs of theories that are dual in D=4: gravity coupled to nothing or to three-form matter, and gravity coupled to zero-form or to two-form matter. Duff and van Nieuwenhuizen showed that, curiously, the one-loop trace anomaly—the coefficient of the Gauss-Bonnet operator—changes under p-form duality transformations. We concur and also find that the leading R^3 divergence changes under duality transformations. Nevertheless, in both cases, the physical renormalized two-loop identical-helicity four-graviton amplitude can be chosen to respect duality. In particular, its renormalization-scale dependence is unaltered.

Publication: Physical Review Letters Vol.: 115 No.: 21 ISSN: 0031-9007

ID: CaltechAUTHORS:20150731-185747920

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Abstract: Motivated by the recent discovery of several dwarfs near the Large Magellanic Cloud (LMC), we study the accretion of massive satellites onto Milky Way (MW)/M31-like haloes using the ELVIS suite of N-body simulations. We identify 25 surviving LMC-mass subhaloes, and investigate the lower-mass satellites that were associated with these subhaloes before they fell into the MW/M31 haloes. Typically, 7 per cent of the overall z = 0 satellite population of MW/M31 haloes were in a surviving LMC-group before falling into the MW/M31 halo. This fraction can vary between 1 and 25 per cent, being higher for groups with higher mass and/or more recent infall times. Groups of satellites disperse rapidly in phase space after infall, and their distances and velocities relative to the group centre become statistically similar to the overall satellite population after 4–8 Gyr. We quantify the likelihood that satellites were associated with an LMC-mass group as a function of both distance and velocity relative to the LMC at z = 0. The close proximity in distance of the nine Dark Energy Survey candidate dwarf galaxies to the LMC suggest that ∼2–4 are likely associated with the LMC. Furthermore, if several of these dwarfs are genuine members, then the LMC-group probably fell into the MW very recently, ≲2 Gyr ago. If the connection with the LMC is established with follow-up velocity measurements, these ‘satellites of satellites’ represent prime candidates to study the effects of group pre-processing on lower mass dwarfs.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 453 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20150422-111317757

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Abstract: We analyze the no-cloning theorem in quantum mechanics through the lens of the proposed ER=EPR (Einstein-Rosen = Einstein-Podolsky-Rosen) duality between entanglement and wormholes. In particular, we find that the no-cloning theorem is dual on the gravity side to the no-go theorem for topology change, violating the axioms of which allows for wormhole stabilization and causality violation. Such a duality between important no-go theorems elucidates the proposed connection between spacetime geometry and quantum entanglement.

Publication: Fortschritte der Physik Vol.: 63 No.: 11-12 ISSN: 0015-8208

ID: CaltechAUTHORS:20150720-090635834

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Abstract: The recently proposed ER=EPR correspondence postulates the existence of wormholes (Einstein-Rosen bridges) between entangled states (such as EPR pairs). Entanglement is famously known to be unobservable in quantum mechanics, in that there exists no observable (or, equivalently, projector) that can accurately pick out whether a generic state is entangled. Many features of the geometry of spacetime, however, are observables, so one might worry that the presence or absence of a wormhole could identify an entangled state in ER=EPR, violating quantum mechanics, specifically, the property of state-independence of observables. In this note, we establish that this cannot occur: there is no measurement in general relativity that unambiguously detects the presence of a generic wormhole geometry. This statement is the ER=EPR dual of the undetectability of entanglement.

Publication: Journal of High Energy Physics Vol.: 2015 No.: 11 ISSN: 1126-6708

ID: CaltechAUTHORS:20151022-121803520

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Abstract: We present Feedback in Realistic Environment (FIRE)/GIZMO hydrodynamic zoom-in simulations of isolated dark matter haloes, two each at the mass of classical dwarf galaxies (M_(vir) ≃ 10^(10) M_⊙) and ultra-faint galaxies (M_(vir) ≃ 10^9 M_⊙), and with two feedback implementations. The resulting central galaxies lie on an extrapolated abundance matching relation from M_★ ≃ 10^6 to 10^4 M_⊙ without a break. Every host is filled with subhaloes, many of which form stars. Each of our dwarfs with M_★ ≃ 10^6 M_⊙ has 1–2 well-resolved satellites with M_★ = 3-200 × 10^3 M_⊙. Even our isolated ultra-faint galaxies have star-forming subhaloes. If this is representative, dwarf galaxies throughout the Universe should commonly host tiny satellite galaxies of their own. We combine our results with the Exploring the Local Volume in Simulations (ELVIS) simulations to show that targeting ∼ 50 kpc regions around nearby isolated dwarfs could increase the chances of discovering ultra-faint galaxies by ∼35 per cent compared to random pointings, and specifically identify the region around the Phoenix dwarf galaxy as a good potential target. The well-resolved ultra-faint galaxies in our simulations (M_★ ≃ 3-30 × 10^3 M_⊙) form within M_(peak) ≃ 0.5-3 × 10^9 M_⊙ haloes. Each has a uniformly ancient stellar population ( > 10 Gyr) owing to reionization-related quenching. More massive systems, in contrast, all have late-time star formation. Our results suggest that M_(halo) ≃ 5 × 10^9 M_⊙ is a probable dividing line between haloes hosting reionization ‘fossils’ and those hosting dwarfs that can continue to form stars in isolation after reionization.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 453 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:20150423-090135687

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Abstract: We formulate a minimum requirement for CFT operators to be localized in the dual AdS. In any spacetime dimensions, we show that a general solution to the requirement is a linear superposition of operators creating spherical boundaries in CFT, with the dilatation by the imaginary unit from their centers. This generalizes the recent proposal by Miyaji et al. for bulk local operators in the three dimensional AdS. We show that Ishibashi states for the global conformal symmetry in any dimensions and with the imaginary di-latation obey free field equations in AdS and that incorporating bulk interactions require their superpositions. We also comment on the recent proposals by Kabat et al., and by H. Verlinde.

Publication: Journal of High Energy Physics Vol.: 2015 No.: 10 ISSN: 1126-6708

ID: CaltechAUTHORS:20150812-140634734

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Abstract: We present a series of high-resolution (20–2000 M⊙, 0.1–4 pc) cosmological zoom-in simulations at z ≳ 6 from the Feedback In Realistic Environment (FIRE) project. These simulations cover halo masses 10^9–10^(11) M⊙ and rest-frame ultraviolet magnitude M_(UV) = −9 to −19. These simulations include explicit models of the multi-phase ISM, star formation, and stellar feedback, which produce reasonable galaxy properties at z = 0–6. We post-process the snapshots with a radiative transfer code to evaluate the escape fraction (f_(esc)) of hydrogen ionizing photons. We find that the instantaneous f_(esc) has large time variability (0.01–20 per cent), while the time-averaged f_(esc) over long time-scales generally remains ≲ 5 per cent, considerably lower than the estimate in many reionization models. We find no strong dependence of f_(esc) on galaxy mass or redshift. In our simulations, the intrinsic ionizing photon budgets are dominated by stellar populations younger than 3 Myr, which tend to be buried in dense birth clouds. The escaping photons mostly come from populations between 3 and 10 Myr, whose birth clouds have been largely cleared by stellar feedback. However, these populations only contribute a small fraction of intrinsic ionizing photon budgets according to standard stellar population models. We show that f_(esc) can be boosted to high values, if stellar populations older than 3 Myr produce more ionizing photons than standard stellar population models (as motivated by, e.g. models including binaries). By contrast, runaway stars with velocities suggested by observations can enhance f_(esc) by only a small fraction. We show that ‘sub-grid’ star formation models, which do not explicitly resolve star formation in dense clouds with n ≫ 1 cm^(−3), will dramatically overpredict f_(esc).

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 453 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20150330-163106550

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Abstract: We study renormalizable extensions of the standard model that contain vector-like fermions in a (single) complex representation of the standard model gauge group. There are 11 models where the vector-like fermions Yukawa couple to the standard model fermions via the Higgs field. These models do not introduce additional fine-tunings. They can lead to, and are constrained by, a number of different flavor-changing processes involving leptons and quarks, as well as direct searches. An interesting feature of the models with strongly interacting vector-like fermions is that constraints from neutral meson mixings (apart from CP violation in K^0–K^0 mixing) are not sensitive to higher scales than other flavor-changing neutral-current processes. We identify order 1/(4πM)^2 (where M is the vector-like fermion mass) one-loop contributions to the coefficients of the four-quark operators for meson mixing, that are not suppressed by standard model quark masses and/or mixing angles.

Publication: Journal of High Energy Physics Vol.: 2015 No.: 10 ISSN: 1126-6708

ID: CaltechAUTHORS:20150812-194949578

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Abstract: We explore CP violating aspects in the Higgs sector of models where new vectorlike quarks carry Yukawa couplings mainly to the third generation quarks of the Standard Model. We point out that in the simplest model, Higgs CP violating interactions only exist in the hWW channel. At low energy, we find that rare B decays can place similarly strong constraints as those from electric dipole moments on the source of CP violation. These observations offer a new handle to discriminate from other Higgs CP violating scenarios such as scalar sector extensions of the Standard Model, and imply an interesting future interplay among limits from different experiments.

Publication: Physical Review D Vol.: 92 No.: 7 ISSN: 2470-0010

ID: CaltechAUTHORS:20150801-082110179

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Abstract: We extend our studies of holographic entropy inequalities to gapped phases of matter. For any number of regions, we determine the linear entropy inequalities satisfied by systems in which the entanglement entropy satisfies an exact area law. In particular, we find that all holographic entropy inequalities are valid in such systems. In gapped systems with topological order, the “cyclic inequalities” derived recently for the holo-graphic entanglement entropy generalize the Kitaev-Preskill formula for the topological entanglement entropy. Finally, we propose a candidate linear inequality for general 4-party quantum states.

Publication: Journal of High Energy Physics Vol.: 2015 No.: 9 ISSN: 1126-6708

ID: CaltechAUTHORS:20150812-144213214

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Abstract: We derive a generalization of the second law of thermodynamics that uses Bayesian updates to explicitly incorporate the effects of a measurement of a system at some point in its evolution. By allowing an experimenter's knowledge to be updated by the measurement process, this formulation resolves a tension between the fact that the entropy of a statistical system can sometimes fluctuate downward and the information-theoretic idea that knowledge of a stochastically evolving system degrades over time. The Bayesian second law can be written as ΔH(ρ_m,ρ)+(2)F|M ⩾ 0, where ΔH(ρ_m,ρ) is the change in the cross entropy between the original phase-space probability distribution ρ and the measurement-updated distribution ρ_m and (2)F|m is the expectation value of a generalized heat flow out of the system. We also derive refined versions of the second law that bound the entropy increase from below by a non-negative number, as well as Bayesian versions of integral fluctuation theorems. We demonstrate the formalism using simple analytical and numerical examples.

Publication: Physical Review E Vol.: 94 No.: 2 ISSN: 2470-0045

ID: CaltechAUTHORS:20150812-142230808

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Abstract: The cosmic accretion of both dark matter and baryons into halos is typically measured using some evolving virial relation, but recent work suggests that most halo growth at late cosmic time (z ≾ 2) is not physical but is rather the by-product of an evolving virial radius ("pseudo-evolution"). Using Omega25, a suite of cosmological simulations that incorporate both dark matter and gas dynamics with differing treatments of gas cooling, star formation, and thermal feedback, we systematically explore the physics that governs cosmic accretion into halos and their galaxies. Physically meaningful cosmic accretion of both dark matter and baryons occurs at z ≳ 1 across our halo mass range: M_(200m) = 10^(11-14) M_⊙. However, dark matter, because it is dissipationless, is deposited (in a time-average sense) at ≳ R_(200m) (z) in a shell-like manner, such that dark matter mass and density experience little-to-no physical growth at any radius within a halo at z < 1. In contrast, gas, because it is able to cool radiatively, experiences significant accretion at all radii, at a rate that roughly tracks the accretion rate at R_(200m), at all redshifts. Infalling gas starts to decouple from dark matter at ≈2 R_(200m) and continues to accrete to smaller radii until the onset of strong angular-momentum support at ≈0.1 R_(200m). Thus, while the growth of dark matter is subject to pseudo-evolution, the growth of baryons is not. The fact that the accretion rate of gas on galactic scales tracks the accretion rate near R_(200m) provides insight into the tight relations between the masses/sizes of galaxies and those of their host halos across cosmic time.

Publication: Astrophysical Journal Vol.: 808 No.: 1 ISSN: 0004-637X

ID: CaltechAUTHORS:20150831-103016402

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Abstract: We study the Anastasiou–Bern–Dixon–Kosower relation using maximal cuts of one- and two-loop integrals with up to five external legs. We show how to find a special combination of integrals that allows the relation to exist, and how to reconstruct the terms with one-loop integrals squared. The reconstruction relies on the observation that integrals across different loop orders can have support on the same generalized unitarity cuts and can share global poles. We discuss the appearance of nonhomologous integration contours in multivariate residues. Their origin can be understood in simple terms, and their existence enables us to distinguish contributions from different integrals. Our analysis suggests that maximal and near-maximal cuts can be used to infer the existence of integral identities more generally.

Publication: Physical Review D Vol.: 92 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:20150424-114953441

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Abstract: It has been shown that fragmentation within self-gravitating, turbulent molecular clouds (‘turbulent fragmentation’) can naturally explain the observed properties of protostellar cores, including the core mass function (CMF). Here, we extend recently developed analytic models for turbulent fragmentation to follow the time-dependent hierarchical fragmentation of self-gravitating cores, until they reach effectively infinite density (and form stars). We show that turbulent fragmentation robustly predicts two key features of the initial mass function (IMF). First, a high-mass power-law scaling very close to the Salpeter slope, which is a generic consequence of the scale-free nature of turbulence and self-gravity. We predict the IMF slope (−2.3) is slightly steeper than the CMF slope (−2.1), owing to the slower collapse and easier fragmentation of large cores. Secondly, a turnover mass, which is set by a combination of the CMF turnover mass (a couple solar masses, determined by the ‘sonic scale’ of galactic turbulence, and so weakly dependent on galaxy properties), and the equation of state (EOS). A ‘soft’ EOS with polytropic index γ < 1.0 predicts that the IMF slope becomes ‘shallow’ below the sonic scale, but fails to produce the full turnover observed. An EOS, which becomes ‘stiff’ at sufficiently low surface densities Σgas ∼ 5000 M_⊙ pc^(−2), and/or models, where each collapsing core is able to heat and effectively stiffen the EOS of a modest mass (∼0.02 M_⊙) of surrounding gas, are able to reproduce the observed turnover. Such features are likely a consequence of more detailed chemistry and radiative feedback.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 450 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20151002-124114892

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Abstract: We study dark matter (DM) which is cosmologically long-lived because of standard model (SM) symmetries. In these models an approximate stabilizing symmetry emerges accidentally, in analogy with baryon and lepton number in the renormalizable SM. Adopting an effective theory approach, we classify DM models according to representations of $SU(3)_C\times SU(2)_L\times U(1)_Y \times U(1)_B\times U(1)_L$, allowing for all operators permitted by symmetry, with weak scale DM and a cutoff at or below the Planck scale. We identify representations containing a neutral long-lived state, thus excluding dimension four and five operators that mediate dangerously prompt DM decay into SM particles. The DM relic abundance is obtained via thermal freeze-out or, since effectively stable DM often carries baryon or lepton number, asymmetry sharing through the very operators that induce eventual DM decay. We also incorporate baryon and lepton number violation with a spurion that parameterizes hard breaking by arbitrary units. However, since proton stability precludes certain spurions, a residual symmetry persists, maintaining the cosmological stability of certain DM representations. Finally, we survey the phenomenology of effectively stable DM as manifested in probes of direct detection, indirect detection, and proton decay.

Publication: arXiv
ID: CaltechAUTHORS:20150731-184249348

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Abstract: We propose and investigate a novel, minimal, and experimentally testable framework for baryogenesis, dubbed dexiogenesis, using baryon number violating effective interactions of right-handed Majorana neutrinos responsible for the seesaw mechanism. The distinct LHC signature of our framework is same-sign top quark final states, possibly originating from displaced vertices. The region of parameters relevant for LHC phenomenology can also yield concomitant signals in nucleon decay experiments. We provide a simple ultraviolet origin for our effective operators, by adding a color-triplet scalar, which could ultimately arise from a grand unified theory.

Publication: Physical Review D Vol.: 92 No.: 1 ISSN: 2470-0010

ID: CaltechAUTHORS:20150601-133007174

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Abstract: We compute the interference between the resonant process pp→H(→γγ)+2 jets and the corresponding continuum background at leading order in QCD. For the Higgs signal, we include gluon fusion (GF) and vector boson fusion (VBF) production channels, while for the background we consider all tree-level contributions, including pure EW effects (O(α^4_(QED))) and QCD contributions (O(α^2_(QED) α^2_s)), plus the loop-induced gluon-initiated process. After convolution with the experimental mass resolution, the main effect of the interference is to shift the position of the mass peak, as in the inclusive GF case studied previously. The apparent mass shift is small in magnitude but strongly dependent on the Higgs width, potentially allowing for a measurement of, or bound on, the width itself. In the H(→γγ)+2 jets channel, the VBF and GF contributions generate shifts of opposite signs which largely cancel, depending on the sets of cuts used, to as little as 5 MeV (toward a lower Higgs mass). The small magnitude of the shift makes this channel a good reference mass for measuring the inclusive mass shift of around 60 MeV in the Standard Model.

Publication: Physical Review D Vol.: 92 No.: 1 ISSN: 2470-0010

ID: CaltechAUTHORS:20150603-073442246

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Abstract: In the Local Group (LG), almost all satellite dwarf galaxies that are within the virial radius of the Milky Way (MW) and Andromeda (M31) exhibit strong environmental influence. The orbital histories of these satellites provide the key to understanding the role of the MW/M31 halo, lower-mass groups, and cosmic reionization on the evolution of dwarf galaxies. We examine the virial-infall histories of satellites with M_star = 10^(3-9) M_⊙ using the Exploring the Local Volume in Simulations suite of cosmological zoom-in dissipationless simulations of 48 MW/M31-like halos. Satellites at z = 0 fell into the MW/M31 halos typically 5-8 Gyr ago at z = 0.5-1. However, they first fell into any host halo typically 7-10 Gyr ago at z = 0.7-1.5. This difference arises because many satellites experienced “group preprocessing” in another host halo, typically of M_vir ~ 10^(10-12) M_⊙, before falling into the MW/M31 halos. Satellites with lower mass and/or those closer to the MW/M31 fell in earlier and are more likely to have experienced group preprocessing; half of all satellites with M_star < 10^6 M_⊙ were preprocessed in a group. Infalling groups also drive most satellite–satellite mergers within the MW/M31 halos. Finally, none of the surviving satellites at z = 0 were within the virial radius of their MW/M31 halo during reionization (z > 6), and only <4% were satellites of any other host halo during reionization. Thus, effects of cosmic reionization versus host-halo environment on the formation histories of surviving dwarf galaxies in the LG occurred at distinct epochs, separated typically by 2-4 Gyr, so they are separable theoretically and, in principle, observationally.

Publication: Astrophysical Journal Vol.: 807 No.: 1 ISSN: 0004-637X

ID: CaltechAUTHORS:20150820-102154818

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Abstract: The multiscale entanglement renormalization ansatz (MERA) is a tensor network that provides an efficient way of variationally estimating the ground state of a critical quantum system. The network geometry resembles a discretization of spatial slices of an anti–de Sitter (AdS) spacetime and “geodesics” in the MERA reproduce the Ryu-Takayanagi formula for the entanglement entropy of a boundary region in terms of bulk properties. It has therefore been suggested that there could be an AdS/MERA correspondence, relating states in the Hilbert space of the boundary quantum system to ones defined on the bulk lattice. Here we investigate this proposal and derive necessary conditions for it to apply, using geometric features and entropy inequalities that we expect to hold in the bulk. We show that, perhaps unsurprisingly, the MERA lattice can only describe physics on length scales larger than the AdS radius. Further, using the covariant entropy bound in the bulk, we show that there are no conventional MERA parameters that completely reproduce bulk physics even on super-AdS scales. We suggest modifications or generalizations of this kind of tensor network that may be able to provide a more robust correspondence.

Publication: Physical Review D Vol.: 91 No.: 12 ISSN: 1550-7998

ID: CaltechAUTHORS:20150601-131525807

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Abstract: We present two new Lagrangian methods for hydrodynamics, in a systematic comparison with moving-mesh, smoothed particle hydrodynamics (SPH), and stationary (non-moving) grid methods. The new methods are designed to simultaneously capture advantages of both SPH and grid-based/adaptive mesh refinement (AMR) schemes. They are based on a kernel discretization of the volume coupled to a high-order matrix gradient estimator and a Riemann solver acting over the volume ‘overlap’. We implement and test a parallel, second-order version of the method with self-gravity and cosmological integration, in the code gizmo:1 this maintains exact mass, energy and momentum conservation; exhibits superior angular momentum conservation compared to all other methods we study; does not require ‘artificial diffusion’ terms; and allows the fluid elements to move with the flow, so resolution is automatically adaptive. We consider a large suite of test problems, and find that on all problems the new methods appear competitive with moving-mesh schemes, with some advantages (particularly in angular momentum conservation), at the cost of enhanced noise. The new methods have many advantages versus SPH: proper convergence, good capturing of fluid-mixing instabilities, dramatically reduced ‘particle noise’ and numerical viscosity, more accurate sub-sonic flow evolution, and sharp shock-capturing. Advantages versus non-moving meshes include: automatic adaptivity, dramatically reduced advection errors and numerical overmixing, velocity-independent errors, accurate coupling to gravity, good angular momentum conservation and elimination of ‘grid alignment’ effects. We can, for example, follow hundreds of orbits of gaseous discs, while AMR and SPH methods break down in a few orbits. However, fixed meshes minimize ‘grid noise’. These differences are important for a range of astrophysical problems.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 450 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20141202-094306473

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Abstract: We analyze the constraints on a CP-violating, flavor conserving, two Higgs doublet model from the measurements of Higgs properties and from the search for heavy Higgs bosons at LHC, and show that the stronger limits typically come from the heavy Higgs search channels. The limits on CP violation arising from the Higgs sector measurements are complementary to those from EDM measurements. Combining all current constraints from low energy to colliders, we set generic upper bounds on the CP violating angle which parametrizes the CP odd component in the 126 GeV Higgs boson.

Publication: Journal of High Energy Physics Vol.: 2015 No.: 6 ISSN: 1126-6708

ID: CaltechAUTHORS:20150324-083556306

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Abstract: Many modern cosmological scenarios feature large volumes of spacetime in a de Sitter vacuum phase. Such models are said to be faced with a "Boltzmann Brain problem" - the overwhelming majority of observers with fixed local conditions are random fluctuations in the de Sitter vacuum, rather than arising via thermodynamically sensible evolution from a low-entropy past. We argue that this worry can be straightforwardly avoided in the Many-Worlds (Everett) approach to quantum mechanics, as long as the underlying Hilbert space is infinite-dimensional. In that case, de Sitter settles into a truly stationary quantum vacuum state. While there would be a nonzero probability for observing Boltzmann-Brain-like fluctuations in such a state, "observation" refers to a specific kind of dynamical process that does not occur in the vacuum (which is, after all, time-independent). Observers are necessarily out-of-equilibrium physical systems, which are absent in the vacuum. Hence, the fact that projection operators corresponding to states with observers in them do not annihilate the vacuum does not imply that such observers actually come into existence. The Boltzmann Brain problem is therefore much less generic than has been supposed.

Publication: arXiv
ID: CaltechAUTHORS:20150814-091340697

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Abstract: We consider the cosmological consequences if a small fraction (f≲0.1) of the dark matter is ultra-strongly self-interacting, with an elastic self-interaction cross-section per unit mass σ≫1 cm^2/g. This possibility evades all current constraints that assume that the self-interacting component makes up the majority of the dark matter. Nevertheless, even a small fraction of ultra-strongly self-interacting dark matter (uSIDM) can have observable consequences on astrophysical scales. In particular, the uSIDM subcomponent can undergo gravothermal collapse and form seed black holes in the center of a halo. These seed black holes, which form within several hundred halo interaction times, contain a few percent of the total uSIDM mass in the halo. For reasonable values of σf, these black holes can form at high enough redshifts to grow to ∼10^9M_⊙ quasars by z≳6, alleviating tension within the standard ΛCDM cosmology. The ubiquitous formation of central black holes in halos could also create cores in dwarf galaxies by ejecting matter during binary black hole mergers, potentially resolving the "too big to fail" problem.

Publication: Astrophysical Journal Vol.: 804 No.: 2 ISSN: 0004-637X

ID: CaltechAUTHORS:20150316-133259467

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Abstract: We use high-resolution cosmological zoom-in simulations from the FIRE (Feedback in Realistic Environments) project to make predictions for the covering fractions of neutral hydrogen around galaxies at z = 2–4. These simulations resolve the interstellar medium of galaxies and explicitly implement a comprehensive set of stellar feedback mechanisms. Our simulation sample consists of 16 main haloes covering the mass range M_h ≈ 10^9–6 × 10^(12) M_⊙ at z = 2, including 12 haloes in the mass range M_h ∼ 10^(11)–10^(12) M_⊙ corresponding to Lyman break galaxies (LBGs). We process our simulations with a ray tracing method to compute the ionization state of the gas. Galactic winds increase the H i covering fractions in galaxy haloes by direct ejection of cool gas from galaxies and through interactions with gas inflowing from the intergalactic medium. Our simulations predict H i covering fractions for Lyman limit systems (LLSs) consistent with measurements around z ∼ 2–2.5 LBGs; these covering fractions are a factor ∼2 higher than our previous calculations without galactic winds. The fractions of H i absorbers arising in inflows and in outflows are on average ∼50 per cent but exhibit significant time variability, ranging from ∼10 to ∼90 per cent. For our most massive haloes, we find a factor ∼3 deficit in the LLS covering fraction relative to what is measured around quasars at z ∼ 2, suggesting that the presence of a quasar may affect the properties of halo gas on ∼100 kpc scales. The predicted covering fractions, which decrease with time, peak at M_h ∼ 10^(11)–10^(12) M_⊙, near the peak of the star formation efficiency in dark matter haloes. In our simulations, star formation and galactic outflows are highly time dependent; H i covering fractions are also time variable but less so because they represent averages over large areas.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 449 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20141217-095337831

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Abstract: We calculate the early universe evolution of perturbations in the dark matter energy density in the context of simple dark sector models containing a GeV scale light mediator. We consider the case that the mediator is long lived, with lifetime up to a second, and before decaying it temporarily dominates the energy density of the universe. We show that for primordial perturbations that enter the horizon around this period, the interplay between linear growth during matter domination and collisional damping can generically lead to a sharp peak in the spectrum of dark matter density perturbation. As a result, the population of the smallest DM halos gets enhanced. Possible implications of this scenario are discussed.

Publication: Journal of Cosmology and Astroparticle Physics Vol.: 2015 No.: 5 ISSN: 1475-7516

ID: CaltechAUTHORS:20150324-084840570

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Abstract: We consider the bound state problem for a field theory that contains a Dirac fermion x that Yukawa couples to a (light) scalar field ø. We are interested in bound states with a large number N of x particles. A Fermi gas model is used to numerically determine the dependence of the radius R of these bound states on N and also the dependence of the binding energy on N. Since scalar interactions with relativistic x's are suppressed two regimes emerge. For modest values of N the state is composed of non-relativistic x particles. In this regime as N increases R decreases. Eventually the core region becomes relativistic and the size of the state starts to increase as N increases. As a result, for fixed Yukawa coupling and x mass, there is a minimum sized state that occurs roughly at the value of N where the core region first becomes relativistic. As an application to dark matter, our analysis offers the possibility of having a supermassive thermal DM candidate. We also compute an elastic scattering form factor that can be relevant for direct detection if the dark matter is composed of such x particles.

Publication: Journal of High Energy Physics Vol.: 2015 No.: 2 ISSN: 1126-6708

ID: CaltechAUTHORS:20141116-071612082

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Abstract: Dust grains in neutral gas behave as aerodynamic particles, so they can develop large local density fluctuations entirely independent of gas density fluctuations. Specifically, gas turbulence can drive order-of-magnitude “resonant” fluctuations in the dust density on scales where the gas stopping/drag timescale is comparable to the turbulent eddy turnover time. Here we show that for large grains (size ≳ 0.1 µm, containing most grain mass) in sufficiently large molecular clouds (radii ≳ 1 - 10pc, masses ≳ 10^4M_⊙), this scale becomes longer than the characteristic sizes of pre-stellar cores (the sonic length), so large fluctuations in the dust-to-gas ratio are imprinted on cores. As a result, star clusters and protostellar disks formed in large clouds should exhibit substantial abundance spreads in the elements preferentially found in large grains (C, O). This naturally predicts populations of carbon-enhanced stars, certain highly unusual stellar populations observed in nearby open clusters, and may explain the “UV upturn” in early-type galaxies. It will also dramatically change planet formation in the resulting protostellar disks, by preferentially “seeding” disks with an enhancement in large carbonaceous or silicate grains. The relevant threshold for this behavior scales simply with cloud densities and temperatures, making straightforward predictions for clusters in starbursts and high-redshift galaxies. Because of the selective sorting by size, this process is not visible in extinction mapping. We also predict the shape of the abundance distribution – when these fluctuations occur, a small fraction of the cores are actually seeded with abundances Z ~ 100〈Z〉 such that they are almost “totally metal” (Z ~ 1)! Assuming the cores collapse, these totally metal stars would be rare (1 in ~ 10^4 in clusters where this occurs), but represent a fundamentally new stellar evolution channel.

Publication: Astrophysical Journal Vol.: 797 No.: 1 ISSN: 0004-637X

ID: CaltechAUTHORS:20140707-083959908

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Abstract: We present a series of high-resolution cosmological simulations of galaxy formation to z = 0, spanning halo masses ∼10^8–10^(13) M⊙, and stellar masses ∼10^4–10^(11) M⊙. Our simulations include fully explicit treatment of the multiphase interstellar medium and stellar feedback. The stellar feedback inputs (energy, momentum, mass, and metal fluxes) are taken directly from stellar population models. These sources of feedback, with zero adjusted parameters, reproduce the observed relation between stellar and halo mass up to M_(halo) ∼ 10^(12) M⊙. We predict weak redshift evolution in the M*–M_(halo) relation, consistent with current constraints to z > 6. We find that the M*–M_(halo) relation is insensitive to numerical details, but is sensitive to feedback physics. Simulations with only supernova feedback fail to reproduce observed stellar masses, particularly in dwarf and high-redshift galaxies: radiative feedback (photoheating and radiation pressure) is necessary to destroy giant molecular clouds and enable efficient coupling of later supernovae to the gas. Star formation rates (SFRs) agree well with the observed Kennicutt relation at all redshifts. The galaxy-averaged Kennicutt relation is very different from the numerically imposed law for converting gas into stars, and is determined by self-regulation via stellar feedback. Feedback reduces SFRs and produces reservoirs of gas that lead to rising late-time star formation histories, significantly different from halo accretion histories. Feedback also produces large short-time-scale variability in galactic SFRs, especially in dwarfs. These properties are not captured by common ‘sub-grid’ wind models.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 445 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20131125-073705692

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Abstract: We consider a simple supersymmetric hidden sector: pure SU(N) gauge theory. Dark matter is made up of hidden glueballinos with mass m_X and hidden glueballs with mass near the confinement scale Λ. For m_X ∼ 1 TeV and Λ ∼ 100 MeV, the glueballinos freeze out with the correct relic density and selfinteract through glueball exchange to resolve small-scale structure puzzles. An immediate consequence is that the glueballino spectrum has a hyperfine splitting of order Λ^2 = m_X ∼ 10 keV. We show that the radiative decays of the excited state can explain the observed 3.5 keV x-ray line signal from clusters of galaxies, Andromeda, and the Milky Way.

Publication: Physical Review D Vol.: 90 No.: 9 ISSN: 2470-0010

ID: CaltechAUTHORS:20140917-074642072

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Abstract: A light hidden gauge boson with kinetic mixing with the usual photon is a popular setup in theories of dark matter. The supernova cooling via radiating the hidden boson is known to put an important constraint on the mixing. I consider the possible role dark matter, which under reasonable assumptions naturally exists inside supernova, can play in the cooling picture. Because the interaction between the hidden gauge boson and DM is likely unsuppressed, even a small number of dark matter compared to protons inside the supernova could dramatically shorten the free streaming length of the hidden boson. A picture of a dark matter "smog" inside the supernova, which substantially relaxes the cooling constraint, is discussed in detail.

Publication: Journal of Cosmology and Astroparticle Physics Vol.: 2014 No.: 11 ISSN: 1475-7516

ID: CaltechAUTHORS:20140512-094346126

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Abstract: We explore models for the GeV Galactic Center Excess (GCE) observed by the Fermi Telescope, focusing on χχ→ƒƒ(bar) annihilation processes in the Z3 NMSSM. We begin by examining the requirements for a simplified model (parametrized by the couplings and masses of dark matter (DM) and mediator particles) to reproduce the GCE via χχ→ƒƒ^bar, while simultaneously thermally producing the observed relic abundance. We apply the results of our simplified model to the Z_3 NMSSM for Singlino/Higgsino~(S/H) or Bino/Higgsino~(B/H) DM. In the case of S/H DM, we find that the DM must be be very close to a pseudoscalar resonance to be viable, and large tan β and positive values of μ are preferred for evading direct detection constraints while simultaneously obtaining the observed Higgs mass. In the case of B/H DM, by contrast, the situation is much less tuned: annihilation generally occurs off-resonance, and for large tan β direct detection constraints are easily satisfied by choosing μ sufficiently large and negative. The B/H model generally has a light, largely MSSM-like pseudoscalar with no accompanying charged Higgs, which could be searched for at the LHC.

Publication: Physical Review D Vol.: 90 No.: 7 ISSN: 1550-7998

ID: CaltechAUTHORS:20140714-090925327

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Abstract: The simplest renormalizable effective field theories with asymmetric dark matter bound states contain two additional gauge singlet fields, one being the dark matter and the other a mediator particle that the dark matter annihilates into. We examine the physics of one such model with a Dirac fermion as the dark matter and a real scalar mediator. For a range of parameters the Yukawa coupling of the dark matter to the mediator gives rise to stable asymmetric dark matter bound states. We derive properties of the bound states including nuggets formed from N ≫ 1 dark matter particles. We also consider the formation of bound states in the early Universe and direct detection of dark matter bound states. Many of our results also hold for symmetric dark matter.

Publication: Physical Review D Vol.: 90 No.: 5 ISSN: 2470-0010

ID: CaltechAUTHORS:20140804-083111315

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Abstract: We address the issue of how many e-folds we would naturally expect if inflation occurred at an energy scale of order 10^(16) GeV. We use the canonical measure on trajectories in classical phase space, specialized to the case of flat universes with a single scalar field. While there is no exact analytic expression for the measure, we are able to derive conditions that determine its behavior. For a quadratic potential V(ϕ)=m^2ϕ^2/2 with m=2×10^(13) GeV and cutoff at M_(Pl)=2.4×10^(18) GeV, we find an expectation value of 2×1010 e-folds on the set of FRW trajectories. For cosine inflation V(ϕ)=Λ^4[1−cos(ϕ/f)] with f=1.5×10^(19) GeV, we find that the expected total number of e-folds is 50, which would just satisfy the observed requirements of our own Universe; if f is larger, more than 50 e-folds are generically attained. We conclude that one should expect a large amount of inflation in large-field models and more limited inflation in small-field (hilltop) scenarios.

Publication: Physical Review D Vol.: 90 No.: 6 ISSN: 1550-7998

ID: CaltechAUTHORS:20140529-121305448

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Abstract: We consider all the dimension 6 operators as well as some simple extensions of the standard model that give new contributions to neutrino interactions with matter. Such interactions are usually parametrized by ϵ_(αβ), where α and β are neutrino flavor indices taking the values e, μ and τ. In the simple models we consider the ϵ_(αβ)'s are much more constrained than in the operator-based model-independent approach. Typically the ϵ_(αβ)'s are restricted to be smaller in magnitude than around 10^(−3). In some of the leptoquark models, a specific pattern for the leptoquark Yukawa couplings allows the diagonal element ϵ_(ττ) to be as large as ∼0.1, or one of ϵ_(ee), ϵ_(μμ)∼0.01. We discuss the interplay between neutrino physics and leptoquark searches at the LHC.

Publication: Physical Review D Vol.: 90 No.: 5 ISSN: 2470-0010

ID: CaltechAUTHORS:20140505-131013279

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Abstract: We study entanglement in thermofield double states of strongly coupled conformal field theories by analyzing two-sided Reissner-Nordström solutions in anti–de Sitter space. The central object of study is the mutual information between a pair of regions, one on each asymptotic boundary of the black hole. For large regions the mutual information is positive and for small ones it vanishes; we compute the critical length scale, which goes to infinity for extremal black holes, of the transition. We also generalize the butterfly effect of Shenker and Stanford [J. High Energy Phys. 03 (2014) 067] to a wide class of charged black holes, showing that mutual information is disrupted upon perturbing the system and waiting for a time of order logE/δE in units of the temperature. We conjecture that the parametric form of this time scale is universal.

Publication: Physical Review D Vol.: 90 No.: 4 ISSN: 2470-0010

ID: CaltechAUTHORS:20140813-102938601

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Abstract: Effective field theory provides a perturbative framework to study the evolution of cosmological large-scale structure. We investigate the underpinnings of this approach, and suggest new ways to compute correlation functions of cosmological observables. We find that, in contrast with quantum field theory, the appropriate effective theory of classical cosmological perturbations involves interactions that are nonlocal in time. We describe an alternative to the usual approach of smoothing the perturbations, based on a path-integral formulation of the renormalization group equations. This technique allows for improved handling of short-distance modes that are perturbatively generated by long-distance interactions.

Publication: Physical Review D Vol.: 90 No.: 2 ISSN: 1550-7998

ID: CaltechAUTHORS:20141120-145800084

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Abstract: We analyze the constraints on CP-violating, flavor conserving two Higgs doublet models implied by measurements of Higgs boson properties at the Large Hadron Collider (LHC) and by the nonobservation of permanent electric dipole moments (EDMs) of molecules, atoms, and neutrons. We find that the LHC and EDM constraints are largely complementary, with the LHC studies constraining the mixing between the neutral CP-even states and the EDMs probing the effect of mixing between the CP-even and CP-odd scalars. Presently, the most stringent constraints are implied by the nonobservation of the ThO molecule EDM signal. Future improvements in the sensitivity of neutron and diamagnetic atom EDM searches could yield competitive or even more severe constraints. We analyze the quantitative impact of hadronic and nuclear theory uncertainties on the interpretation of the latter systems and conclude that these uncertainties cloud the impact of projected improvements in the corresponding experimental sensitivities.

Publication: Physical Review D Vol.: 89 No.: 11 ISSN: 1550-7998

ID: CaltechAUTHORS:20140807-085331165

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Abstract: We describe models in which dark matter is xenophobic, with significantly reduced signal strength in direct detection experiments using xenon as a target material. Such models alleviate tension between xenon-based constraints and possible signal at other direct detection experiments, and in particular regions of interest at CoGeNT and CDMS are largely below XENON100 bounds in the purely xenophobic limit. We also show the constraints from upcoming LUX results and orthogonal searches using Fermi-LAT results from line searches in dwarf spheroidals and monojet searches at CMS.

No.: 1604
ID: CaltechAUTHORS:20140707-080005855

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Abstract: There is strong evidence in favor of the idea that dark matter is self interacting, with the cross section-to-mass ratio σ/m∼1 cm^2/g∼1 barn/GeV. We show that viable models of dark matter with this large cross section are straightforwardly realized with non-Abelian hidden sectors. In the simplest of such models, the hidden sector is a pure gauge theory, and the dark matter is composed of hidden glueballs with a mass around 100 MeV. Alternatively, the hidden sector may be a supersymmetric pure gauge theory with a ∼10 TeV gluino thermal relic. In this case, the dark matter is largely composed of glueballinos that strongly self interact through the exchange of light glueballs. We present a unified framework that realizes both of these possibilities in anomaly-mediated supersymmetry breaking, where, depending on a few model parameters, the dark matter may be composed of hidden glueballinos, hidden glueballs, or a mixture of the two. These models provide simple examples of multicomponent dark matter, have interesting implications for particle physics and cosmology, and include cases where a subdominant component of dark matter may be extremely strongly self interacting, with interesting astrophysical consequences.

Publication: Physical Review D Vol.: 89 No.: 11 ISSN: 2470-0010

ID: CaltechAUTHORS:20140225-091243221

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Abstract: It is commonplace in discussions of modern cosmology to assert that the early universe began in a special state. Conventionally, cosmologists characterize this fine-tuning in terms of the horizon and flatness problems. I argue that the fine-tuning is real, but these problems aren't the best way to think about it: causal disconnection of separated regions isn't the real problem, and flatness isn't a problem at all. Fine-tuning is better understood in terms of a measure on the space of trajectories: given reasonable conditions in the late universe, the fraction of cosmological histories that were smooth at early times is incredibly tiny. This discussion helps clarify what is required by a complete theory of cosmological initial conditions.

Publication: arXiv
ID: CaltechAUTHORS:20140716-105154654

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Abstract: The Galactic Center is one of the most promising targets for indirect detection of dark matter with gamma rays. We investigate the sensitivity of the upcoming Cherenkov Telescope Array (CTA) to dark matter annihilation and decay in the Galactic Center. As the inner density profile of the Milky Way's dark matter halo is uncertain, we study the impact of the slope of the Galactic density profile, inwards of the Sun, on the prospects for detecting a dark matter signal with CTA. Adopting the Ring Method to define the signal and background regions in an ON-OFF analysis approach, we find that the sensitivity achieved by CTA to annihilation signals is strongly dependent on the inner profile slope, whereas the dependence is more mild in the case of dark matter decay. Surprisingly, we find that the optimal choice of signal and background regions is virtually independent of the assumed density profile. For the fiducial case of a Navarro-Frenk-White profile, we find that CTA will be able to probe annihilation cross-sections well below the canonical thermal relic value for dark matter masses from a few tens of GeV up to similar to 5 TeV for annihilation to T^+T^- and will achieve only a slightly weaker sensitivity for annihilation to bb or µ^+µ^-. CTA will improve significantly on current sensitivity to annihilation signals for dark matter masses above similar to 100 GeV, covering parameter space that is complementary to that probed by searches with the Fermi Large Area Telescope. The interpretation of apparent excesses in the measured cosmic-ray electron and positron spectra as signals of dark matter decay will also be testable with CTA observations of the Galactic Center. We demonstrate that both for annihilation and for decay, including spectral information for hard channels (such as µ^+µ^- and Ƭ^+T^- leads to enhanced sensitivity for dark matter masses above m_(DM) similar to 200 GeV.

Publication: Journal of Cosmology and Astroparticle Physics Vol.: 2014 No.: 6 ISSN: 1475-7516

ID: CaltechAUTHORS:20150109-111350090

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Abstract: Black holes emit high energy particles which induce a finite density potential for any scalar field ϕ coupling to the emitted quanta. Due to energetic considerations, ϕ evolves locally to minimize the effective masses of the outgoing states. In theories where ϕ resides at a metastable minimum, this effect can drive ϕ over its potential barrier and classically catalyze the decay of the vacuum. Because this is not a tunneling process, the decay rate is not exponentially suppressed and a single black hole in our past light cone may be sufficient to activate the decay. Moreover, decaying black holes radiate at ever higher temperatures, so they eventually probe the full spectrum of particles coupling to ϕ. We present a detailed analysis of vacuum decay catalyzed by a single particle, as well as by a black hole. The former is possible provided large couplings or a weak potential barrier. In contrast, the latter occurs much more easily and places new stringent limits on theories with hierarchical spectra. Finally, we comment on how these constraints apply to the standard model and its extensions, e.g. metastable supersymmetry breaking.

Publication: Physical Review D Vol.: 89 No.: 10 ISSN: 1550-7998

ID: CaltechAUTHORS:20130917-134949603

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Abstract: We investigate the feasibility of directly detecting a generation mechanism of the cosmic baryon asymmetry by repeating the same particle physics process inside the LHC. We propose a framework with R-parity and CP violating squark decays responsible for baryogenesis, which can be embedded in supersymmetric models and is partly motivated by naturalness. We argue that the baryon number generation here is closely related to lepton charge asymmetry on the resonance. We emphasize the importance of the single charged lepton plus multijet channel in the absence of significant missing energy in search of such a scenario.

Publication: Physical Review D Vol.: 89 No.: 7 ISSN: 2470-0010

ID: CaltechAUTHORS:20140521-090126540

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Abstract: We explore simplified models of mixed dark matter (DM), defined here to be a stable relic composed of a singlet and an electroweak charged state. Our setup describes a broad spectrum of thermal DM candidates that can naturally accommodate the observed DM abundance but are subject to substantial constraints from current and upcoming direct detection experiments. We identify "blind spots" at which the DM-Higgs coupling is identically zero, thus nullifying direct detection constraints on spin independent scattering. Furthermore, we characterize the fine-tuning in mixing angles, i.e. well-tempering, required for thermal freeze-out to accommodate the observed abundance. Present and projected limits from LUX and XENON1T force many thermal relic models into blind spot tuning, well-tempering, or both. This simplified model framework generalizes bino-Higgsino DM in the MSSM, singlino-Higgsino DM in the NMSSM, and scalar DM candidates that appear in models of extended Higgs sectors.

Publication: Journal of Cosmology and Astroparticle Physics Vol.: 2014 No.: 2 ISSN: 1475-7516

ID: CaltechAUTHORS:20131209-084406844

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Abstract: We provide a derivation of the Born Rule in the context of the Everett (Many-Worlds) approach to quantum mechanics. Our argument is based on the idea of self-locating uncertainty: in the period between the wave function branching via decoherence and an observer registering the outcome of the measurement, that observer can know the state of the universe precisely without knowing which branch they are on. We show that there is a uniquely rational way to apportion credence in such cases, which leads directly to the Born Rule. [Editors note: for a video of the talk given by Prof. Carroll at the Aharonov-80 conference in 2012 at Chapman University, see quantum.chapman.edu/talk-14.]

Publication: arXiv
ID: CaltechAUTHORS:20141216-203110170

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Abstract: We present a simple theory where baryon and lepton numbers are spontaneously broken at the supersymmetry scale. In this context R parity must be spontaneously broken but the theory still contains a stable field which can play the role of the cold dark matter of the Universe. We discuss the spectrum of the theory, the properties of the dark matter candidate and the predictions for direct detection experiments. This theory provides a concrete example of exotic supersymmetric signatures associated with having the simultaneous presence of R-parity-violating and missing energy signals at the Large Hadron Collider.

Publication: Physical Review D Vol.: 88 No.: 11 ISSN: 2470-0010

ID: CaltechAUTHORS:20131112-091815274

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Abstract: Models of cosmological scalar fields often feature “attractor solutions” to which the system evolves for a wide range of initial conditions. There is some tension between this well-known fact and another well-known fact: Liouville’s theorem forbids true attractor behavior in a Hamiltonian system. In universes with vanishing spatial curvature, the field variables ϕ and ϕ˙ specify the system completely, defining an effective phase space. We investigate whether one can define a unique conserved measure on this effective phase space, showing that it exists for m^2ϕ^2 potentials and deriving conditions for its existence in more general theories. We show that apparent attractors are places where this conserved measure diverges in the ϕ-ϕ˙ variables and suggest a physical understanding of attractor behavior that is compatible with Liouville’s theorem.

Publication: Physical Review D Vol.: 88 No.: 8 ISSN: 1550-7998

ID: CaltechAUTHORS:20131125-140730326

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Abstract: We investigate the possibility that quarks and leptons are unified at a low energy scale much smaller than the grand unified scale. A simple theory for quark-lepton unification based on the gauge group SU(4)_C⊗SU(2)_L⊗U(1)_R is proposed. This theory predicts the existence of scalar leptoquarks which could be produced at the Large Hadron Collider. In order to have light neutrinos without fine-tuning, their masses are generated through the inverse seesaw mechanism.

Publication: Physical Review D Vol.: 88 No.: 5 ISSN: 2470-0010

ID: CaltechAUTHORS:20131106-082506178

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Abstract: In supersymmetric models with minimal particle content and without left-right squark mixing, the conventional wisdom is that the 125.6 GeV Higgs boson mass implies top squark masses of O(10) TeV, far beyond the reach of colliders. This conclusion is subject to significant theoretical uncertainties, however, and we provide evidence that it may be far too pessimistic. We evaluate the Higgs boson mass, including the dominant three-loop terms at O(α_tα^2_s), in currently viable models. For multi-TeV top squarks, the three-loop corrections can increase the Higgs boson mass by as much as 3 GeV and lower the required top-squark masses to 3–4 TeV, greatly improving prospects for supersymmetry discovery at the upcoming run of the LHC and its high-luminosity upgrade.

Publication: Physical Review Letters Vol.: 111 No.: 13 ISSN: 0031-9007

ID: CaltechAUTHORS:20131105-095346189

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Abstract: We study the impact that a heavy generation of vectorlike leptons can have on the value of the electric dipole moment of the electron, and the rates for the flavor violating processes μ→eγ and μ→3e. The smallness of the charged lepton masses suggests that at least some of the Yukawa coupling constants of the vectorlike leptons to the ordinary leptons or amongst themselves are small, but even with such small couplings experiments trying to detect these quantities are sensitive to extra generation lepton masses up to about 100 TeV.

Publication: Physical Review D Vol.: 88 No.: 5 ISSN: 2470-0010

ID: CaltechAUTHORS:20131018-093200954

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Abstract: We propose a simple renormalizable model of baryogenesis and asymmetric dark matter generation at the electroweak phase transition. Our setup utilizes the two Higgs doublet model plus two complex gauge singlets, the lighter of which is stable dark matter. The dark matter is charged under a global symmetry that is broken in the early universe but restored during the electroweak phase transition. Because the ratio of baryon and dark matter asymmetries is controlled by model parameters, the dark matter need not be light. Thus, new force carriers are unnecessary and the symmetric dark matter abundance can be eliminated via Higgs portal interactions alone. Our model places a rough upper bound on the dark matter mass, and has implications for direct detection experiments and particle colliders.

Publication: Journal of High Energy Physics Vol.: 2013 No.: 9 ISSN: 1126-6708

ID: CaltechAUTHORS:20130708-081604840

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Abstract: We observe a generic connection between LHC Higgs data and electroweak baryogenesis: the particle that contributes to the CP-odd hgg or hγγ vertex would provide the CP-violating source during a first-order phase transition. It is illustrated in the two Higgs doublet model that a common complex phase controls the lightest Higgs properties at the LHC, electric dipole moments, and the CP-violating source for electroweak baryogenesis. We perform a general parametrization of Higgs effective couplings and a global fit to the LHC Higgs data. Current LHC measurements prefer a nonzero phase for tan β≲1 and electric dipole moment constraints still allow an order-one phase for tan β∼1, which gives sufficient room to generate the correct cosmic baryon asymmetry. We also give some prospects in the direct measurements of CP violation in the Higgs sector at the LHC.

Publication: Physical Review Letters Vol.: 111 No.: 9 ISSN: 0031-9007

ID: CaltechAUTHORS:20130924-111407015

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Abstract: The standard ΛCDM model provides an excellent fit to current cosmological observations but suffers from a potentially serious Boltzmann Brain problem. If the universe enters a de Sitter vacuum phase that is truly eternal, there will be a finite temperature in empty space and corresponding thermal fluctuations. Among these fluctuations will be intelligent observers, as well as configurations that reproduce any local region of the current universe to arbitrary precision. We discuss the possibility that the escape from this unacceptable situation may be found in known physics: vacuum instability induced by the Higgs field. Avoiding Boltzmann Brains in a measure-independent way requires a decay timescale of order the current age of the universe, which can be achieved if the top quark pole mass is approximately 178 GeV. Otherwise we must invoke new physics or a particular cosmological measure before we can consider ΛCDM to be an empirical success.

ID: CaltechAUTHORS:20130925-113239870

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Abstract: We study the simplest renormalizable scalar leptoquark models where the standard model is augmented only by one additional scalar representation of SU(3)×SU(2)×U(1). The requirement that there be no proton decay from renormalizable interactions singles out two such models, one of which exhibits an unusual top mass enhancement of the μ→eγ decay rate. We analyze the phenomenology of the model with the unusual top mass enhancement of loop level chirality changing charged lepton processes in the light of existing and upcoming experiments. Both of the models that do not allow proton decay from renormalizable interactions have dimension-5 operators that, even if suppressed by the Planck scale, can give rise to an unacceptably high level of baryon number violation. We discuss symmetries that can forbid these dimension-5 operators.

Publication: Physical Review D Vol.: 88 No.: 3 ISSN: 2470-0010

ID: CaltechAUTHORS:20130515-135302971

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Abstract: We determine the prospects for direct and indirect detection of thermal relic neutralinos in supersymmetric theories with multi-TeV squarks and sleptons. We consider the concrete example of the focus point region of minimal supergravity, but our results are generically valid for all models with decoupled scalars and mixed Bino-Higgsino or Higgsino-like dark matter. We determine the parameter space consistent with a 125 GeV Higgs boson including 3-loop corrections in the calculation of the Higgs mass. These corrections increase m_h by 1–3 GeV, lowering the preferred scalar mass scale and decreasing the fine-tuning measure in these scenarios. We then systematically examine prospects for dark matter direct and indirect detection. Direct detection constraints do not exclude these models, especially for μ<0. At the same time, the scenario generically predicts spin-independent signals just beyond current bounds. We also consider indirect detection with neutrinos, gamma rays, antiprotons, and antideuterons. Current IceCube neutrino constraints are competitive with direct detection, implying bright prospects for complementary searches with both direct and indirect detection.

Publication: Physical Review D Vol.: 88 No.: 1 ISSN: 2470-0010

ID: CaltechAUTHORS:20130826-132657561

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Abstract: We consider models of xenophobic dark matter, in which isospin-violating dark matter–nucleon interactions significantly degrade the response of xenon direct detection experiments. For models of near-maximal xenophobia, with neutron-to-proton coupling ratio f_n/f_p ≈ -0.64, and dark matter mass near 8 GeV, the regions of interest for CoGeNT and CDMS-Si and the region of interest identified by Collar and Fields in CDMS-Ge data can be brought into agreement. This model may be tested in future direct, indirect, and collider searches. Interestingly, because the natural isotope abundance of xenon implies that xenophobia has its limits, we find that this xenophobic model may be probed in the near future by xenon experiments. Near-future data from the LHC and Fermi-LAT may also provide interesting alternative probes of xenophobic dark matter.

Publication: Physical Review D Vol.: 88 No.: 1 ISSN: 2470-0010

ID: CaltechAUTHORS:20130815-080352449

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Abstract: We propose a simple model of baryogenesis comprised of the standard model coupled to a singlet X via higher dimension operators O. In the early universe, X is thermalized by O mediated scattering processes before it decouples relativistically and evolves into a sizable fraction of the total energy density. Eventually, X decays via O in an out of equilibrium, baryon number and CP violating process that releases entropy and achieves baryogenesis for a broad range of parameters. The decay can also produce a primordial abundance of dark matter. Because X may be as light as a TeV, viable regions of parameter space lie within reach of experimental probes of n-n oscillation, flavor physics, and proton decay.

Publication: Physical Review D Vol.: 88 No.: 1 ISSN: 1550-7998

ID: CaltechAUTHORS:20130415-085731062

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Abstract: In Lorentzian AdS/CFT there exists a mapping between local bulk operators and nonlocal conformal field theory (CFT) operators. In global anti-de Sitter (AdS) this mapping can be found through use of bulk equations of motion and allows the nonlocal CFT operator to be expressed as a local operator smeared over a range of positions and times. We argue that such a construction is not possible if there are bulk normal modes with exponentially small near boundary imprint. We show that the AdS-Schwarzschild background is such a case, with the horizon introducing modes with angular momentum much larger than frequency, causing them to be trapped by the centrifugal barrier. More generally, we argue that any barrier in the radial effective potential which prevents null geodesics from reaching the boundary will lead to modes with vanishingly small near boundary imprint, thereby obstructing the existence of a smearing function. While one may have thought the bulk-boundary dictionary for low curvature regions, such as the exterior of a black hole, should be as in empty AdS, our results demonstrate otherwise.

Publication: Physical Review D Vol.: 88 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:20130815-083401045

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Abstract: We briefly review the motivations and features of focus point supersymmetry and in particular the focus point region of the CMSSM. Applying the constraint that the neutralino is a thermal relic, we examine current and projected collider and dark matter constraints on the focus point region. We demonstrate that the focus point region is currently constrained by multiple dark matter experiments, and future sensitivy on multiple fronts will probe large portions of the parameter space.

ID: CaltechAUTHORS:20130708-081244977

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Abstract: Models where the baryon (B) and lepton (L) numbers are local gauge symmetries that are spontaneously broken at a low scale are revisited. We find new extensions of the standard model which predict the existence of fermions that carry both baryon and lepton numbers (i.e., leptoquarks). The local baryonic and leptonic symmetries can be broken at a scale close to the electroweak scale and we do not need to postulate the existence of a large desert to satisfy the experimental constraints on baryon number violating processes like proton decay.

Publication: Physical Review Letters Vol.: 110 No.: 23 ISSN: 0031-9007

ID: CaltechAUTHORS:20130718-102128997

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Abstract: Primordial non-Gaussianity of local type is predicted to lead to enhanced halo clustering on very large scales. Photometric quasars, which can be seen from cosmological redshifts z > 2 even in wide-shallow optical surveys, are promising tracers for constraining non-Gaussianity using this effect. However, large-scale systematics can also mimic this signature of non-Gaussianity. In order to assess the contribution of systematic effects, we cross-correlate overdensity maps of photometric quasars from the Sloan Digital Sky Survey (SDSS) Data Release 6 (DR6) in different redshift ranges. We find that the maps are significantly correlated on large scales, even though we expect the angular distributions of quasars at different redshifts to be uncorrelated. This implies that the quasar maps are contaminated with systematic errors. We investigate the use of external templates that provide information on the spatial dependence of potential systematic errors to reduce the level of spurious clustering in the quasar data. We find that templates associated with stellar density, the stellar color locus, airmass, and seeing are major contaminants of the quasar maps, with seeing having the largest effect. Using template projection, we are able to decrease the significance of the cross-correlation measurement on the largest scales from 9.2σ to 5.4σ. Although this is an improvement, the remaining cross-correlation suggests the contamination in this quasar sample is too great to allow a competitive constraint on f_NL by correlations internal to this sample. The SDSS quasar catalog exhibits spurious number density fluctuations of approximately 2% rms, and we need a contamination level less than 1% (0.6%) in order to measure values of f_NL less than 100 (10). Properly dealing with these systematics will be paramount for future large scale structure surveys that seek to constrain non-Gaussianity.

Publication: Publications of the Astronomical Society of the Pacific Vol.: 125 No.: 928 ISSN: 0004-6280

ID: CaltechAUTHORS:20130812-092104251

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Abstract: A possible connection between the cosmological baryon asymmetry, dark matter and vector-like fermions is investigated. In this scenario an asymmetry generated through baryogenesis or leptogenesis (in the vector-like matter sector) connects the baryon asymmetry to the dark matter density. We present explicit renormalizable models where this connection occurs. These models have asymmetric dark matter and a significant invisible Higgs decay width to dark matter particles is possible. We refer to this type of scenario as the vector-like portal. In some asymmetric dark matter models there are potential naturalness issues for the low energy effective theory. We address that issue in themodels we consider by starting with a Lagrangian that is the most general renormalizable one consistent with the gauge (and discrete) symmetries and showing the low energy effective theory automatically has the required form as a consequence of the symmetries of the full theory. We show that the mass of the dark matter candidate is predicted in these scenarios.

Publication: Journal of High Energy Physics Vol.: 2013 No.: 5 ISSN: 1126-6708

ID: CaltechAUTHORS:20130904-132659017

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Abstract: We enumerate the simplest models that have baryon number violation at the classical level but do not give rise to proton decay. These models have scalar fields in two representations of SU(3)×SU(2)×U(1) and violate baryon number by two units. Some of the models give rise to nn̅ (neutron-antineutron) oscillations, while some also violate lepton number by two units. We discuss the range of scalar masses for which nn̅ oscillations are measurable in the next generation of experiments. We give a brief overview of the phenomenology of these models and then focus on one of them for a more quantitative discussion of nn̅ oscillations, the generation of the cosmological baryon number, the electric dipole moment of the neutron, and K^0-K̅^0 mixing.

Publication: Physical Review D Vol.: 87 No.: 7 ISSN: 2470-0010

ID: CaltechAUTHORS:20130503-094324688

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Abstract: We study direct detection of dark matter in a supersymmetric model where most supersymmetric particles have very high-scale masses beyond the weak scale. In the scenario, a Wino-like or a Higgsino-like neutralino is a good candidate for the dark matter in the Universe. The neutralino scatters off nuclei by a Higgs boson exchange diagram and also electroweak loop diagrams. It is found that the elastic-scattering cross section with nuclei is enhanced or suppressed due to constructive or deconstructive interference among the diagrams. Such a cross section is within the reach of future experiment in some parameter region.

Publication: Physical Review D Vol.: 87 No.: 3 ISSN: 2470-0010

ID: CaltechAUTHORS:20130314-145114630

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Abstract: We present a higher dimensional model where gravity is bound to a brane due to Anderson localization. The extra dimensions are taken to be a disordered crystal of branes, with randomly distributed tensions of order the fundamental scale. Such geometries bind the graviton and thus allow for arbitrarily large extra dimensions even when the curvature is small. Thus this model is quite distinct from that of Randall and Sundrum where localization is a consequence of curvature effects in the bulk. The hierarchy problem can be solved by having the standard model brane live a distance away from the brane on which the graviton is localized. The statistical properties of the system are worked out and it is shown that the scenario leads to a continuum of four dimensional theories with differing strengths of gravitational interactions. We live on one particular brane whose gravitational constant is G_N.

Publication: Physical Review Letters Vol.: 110 No.: 1 ISSN: 0031-9007

ID: CaltechAUTHORS:20130205-161136329

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Abstract: Non-Gaussianity in the inflationary perturbations can couple observable scales to modes of much longer wavelength (even superhorizon), leaving as a signature a large-angle modulation of the observed cosmic microwave background power spectrum. This provides an alternative origin for a power asymmetry that is otherwise often ascribed to a breaking of statistical isotropy. The non-Gaussian modulation effect can be significant even for typical ∼10^(-5) perturbations while respecting current constraints on non-Gaussianity if the squeezed limit of the bispectrum is sufficiently infrared divergent. Just such a strongly infrared-divergent bispectrum has been claimed for inflation models with a non-Bunch-Davies initial state, for instance. Upper limits on the observed cosmic microwave background power asymmetry place stringent constraints on the duration of inflation in such models.

Publication: Physical Review Letters Vol.: 110 No.: 1 ISSN: 0031-9007

ID: CaltechAUTHORS:20130204-135345203

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Abstract: The decay and annihilation cross sections of dark matter particles may depend on the value of a chameleonic scalar field that both evolves cosmologically and takes different values depending on the local matter density. This possibility introduces a separation between the physics relevant for freeze-out and that responsible for dynamics and detection in the late universe. We investigate how such dark sector interactions might be implemented in a particle physics Lagrangian and consider how current and upcoming observations and experiments bound such dark matter candidates. A specific simple model allows for an increase in the annihilation cross section by a factor of 10^6 between freeze-out and today, while more complicated models should also allow for scattering cross sections near the astrophysical bounds.

Publication: Physical Review D Vol.: 86 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:20130131-161047251

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Abstract: In this paper, we discuss the systematics of quarkonium production at the LHC. In particular, we focus on the necessity to sum logs of the form log(Q/p⊥) and log(p⊥/m_Q). We show that the former contributions are power suppressed, while the latter, whose contribution in fragmentation is well known, also arise in the short distance (i.e., nonfragmentation) production mechanisms. Though these contributions are suppressed by powers of m_Q/p⊥, they can be enhanced by inverse powers of v, the relative velocity between heavy quarks in the quarkonium. In the limit p⊥≫m_Q, short-distance production can be thought of as the fragmentation of a pair of partons (i.e., the heavy quark and antiquark) into the final state quarkonium. We derive an all-order factorization theorem for this process in terms of double parton fragmentation functions and calculate the one-loop anomalous dimension matrix for the double parton fragmentation functions.

Publication: Physical Review D Vol.: 86 No.: 9 ISSN: 1550-7998

ID: CaltechAUTHORS:20130114-102047070

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Abstract: We derive general covariant expressions for the six independent observable modes of distortion of ideal standard rulers in a perturbed Friedmann-Robertson-Walker spacetime. Our expressions are gauge invariant and valid on the full sky. These six modes are most naturally classified in terms of their rotational properties on the sphere, yielding two scalars, two vector (spin-1), and two tensor (spin-2) components. One scalar corresponds to the magnification, while the spin-2 components correspond to the shear. The vector components allow for a polar/axial decomposition analogous to the E/B decomposition for the shear. Scalar modes do not contribute to the axial (B-)vector, opening a new avenue to probing tensor modes. Our results apply, but are not limited to, the distortion of correlation functions (of the cosmic microwave background, 21-cm emission, or galaxies) as well as to weak lensing shear and magnification, all of which can be seen as methods relying on "standard rulers."

Publication: Physical Review D Vol.: 86 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:20121116-093009806

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Abstract: Observed angular positions and redshifts of large-scale structure tracers such as galaxies are affected by gravitational waves through volume distortion and magnification effects. Thus, a gravitational wave background can in principle be probed through clustering statistics of large-scale structures. We calculate the observed angular clustering of galaxies in the presence of a gravitational wave background at linear order including all relativistic effects. For a scale-invariant spectrum of gravitational waves, the effects are most significant at the smallest multipoles (2≤ℓ≤5), but typically suppressed by six or more orders of magnitude with respect to scalar contributions for currently allowed amplitudes of the inflationary gravitational wave background. We also discuss the most relevant second-order terms, corresponding to the distortion of tracer correlation functions by gravitational waves. These provide a natural application of the approach recently developed in Schmidt and Jeong {arXiv:1204.3625 [Phys. Rev. D (to be published)]}.

Publication: Physical Review D Vol.: 86 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:20121116-084948137

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Abstract: The B (curl) mode of the correlation of galaxy ellipticities (shear) can be used to detect a stochastic gravitational wave background, such as that predicted by inflation. In this paper, we derive the tensor mode contributions to shear from both gravitational lensing and intrinsic alignments, using the gauge-invariant, full-sky results of Schmidt and Jeong {arXiv:1204.3625 [Phys. Rev. D (to be published)]}. We find that the intrinsic alignment contribution, calculated using the linear alignment model, is larger than the lensing contribution by an order of magnitude or more, if the alignment strength for tensor modes is of the same order as for scalar modes. This contribution also extends to higher multipoles. These results make the prospects for probing tensor modes using galaxy surveys less pessimistic than previously thought, though still very challenging.

Publication: Physical Review D Vol.: 86 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:20121115-133417401

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Abstract: Using the NRGR effective field theory formalism we calculate the remaining source multipole moments necessary to obtain the spin contributions to the gravitational wave amplitude to 2.5 Post-Newtonian (PN) order. We also reproduce the tail contribution to the waveform linear in spin at 2.5PN arising from the nonlinear interaction between the current quadrupole and the mass monopole.

Publication: Journal of Cosmology and Astroparticle Physics Vol.: 2012 No.: 9 ISSN: 1475-7516

ID: CaltechAUTHORS:20121126-113258362

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Abstract: In general relativity, the average velocity field of dark matter around galaxy clusters is uniquely determined by the mass profile. The latter can be measured through weak lensing. We propose a new method of measuring the velocity field (phase space density) by stacking redshifts of surrounding galaxies from a spectroscopic sample. In combination with lensing, this yields a direct test of gravity on scales of 1–30 Mpc. Using N-body simulations, we show that this method can improve upon current constraints on f(R) and Dvali-Gabadadze-Porrati model parameters by several orders of magnitude when applied to upcoming imaging and redshift surveys.

Publication: Physical Review Letters Vol.: 109 No.: 5 ISSN: 0031-9007

ID: CaltechAUTHORS:20120831-134910139

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Abstract: We predict the formation histories, properties and descendants of Lyman-break galaxies (LBGs) in the Λ cold dark matter cosmology. In our model, which incorporates a top-heavy initial mass function in starbursts, we find that most LBGs are starbursts triggered by minor mergers of galaxies. The duration of the LBG phase is predicted to be quite short, ∼20–60 Myr. We investigate the distributions of stellar and halo masses and morphologies for bright (L_(UV) > L^*_(UV)) and faint (L_UV > 0.1L^*_(UV)) LBGs at z= 3, 6 and 10 [where we classify LBGs according to their rest-frame ultraviolet (UV) luminosities relative the observed characteristic luminosity L^*_(UV) at z≈3]. Bright LBGs at z=3 are predicted to have median stellar masses ∼ 1 × 10^9 h^(−1) M_⊙ and host halo masses ∼ 3 × 10^(11) h^(−1) M_⊙, and be typically mildly disc dominated in stellar mass. On the other hand, faint LBGs at z=10 are predicted to have median stellar masses of only ∼ 1 × 10^7 h^(−1) M_⊙ and host halo masses ∼ 2 × 10^(10) h^(−1) M_⊙, and be generally bulge dominated. Bright LBGs at z=3 evolve to present-day galaxies with median stellar mass ∼ 5 × 10^(10) h^(−1) M_⊙ (comparable to the Milky Way), consisting of roughly equal numbers of disc- and bulge-dominated systems, and hosted by haloes with median mass ∼ 2 × 10^(13) h^(−1) M_⊙ (corresponding to medium-size galaxy groups). The model predicts that 40 per cent of Milky Way mass galaxies at the present day have a bright LBG progenitor in the redshift range 3 < z < 4, while 95 per cent have a faint LBG progenitor in the same redshift range and 7 per cent have a faint LBG progenitor at 10 < z < 11. With our multiwavelength model, we also investigate the overlap between the LBG population and that of selected submillimetre galaxies (SMGs); at z=3, only ∼ 1 per cent of bright LBGs are also predicted to be bright SMGs (with an inline imagem flux in excess of 5 mJy).

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 423 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20120807-100755260

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Abstract: We investigate the constraining power of current and future Sunyaev-Zeldovich cluster surveys on the f(R) gravity model. We use a Fisher matrix approach, adopt self-calibration for the mass-observable scaling relation, and evaluate constraints for the South Pole Telescope (SPT), Planck, SPT polarimeter (SPTpol), and Atacama Cosmology Telescope polarimeter (ACTpol) surveys. The modified gravity effects on the mass function, halo bias, matter power spectrum, and mass-observable relation are taken into account. We show that, relying on number counts only, the Planck cluster catalog is expected to reduce current upper limits by about a factor of 4, to σ_f_(R0)=2×10^(-5) (68% confidence level) while SPT, SPTpol, and ACTpol yield about 3×10^(-5). Adding the cluster power spectrum further improves the constraints to σ_f_(R0)=5×10^(-6) for Planck and σ_f_(R0)=2×10^(-5) for SPTpol, pushing cluster constraints significantly beyond the limit where number counts have no constraining power due to the chameleon screening mechanism. Further, the combination of both observables breaks degeneracies, especially with the expansion history (effective dark energy density and equation of state). The constraints are only mildly worsened by the use of self-calibration but depend on the mass threshold and redshift coverage of the cluster samples.

Publication: Physical Review D Vol.: 85 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:20120705-092536225

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Abstract: The observed stellar mass function (SMF) is very different to the halo mass function predicted by Λ cold dark matter (ΛCDM), and it is widely accepted that this is due to energy feedback from supernovae and black holes. However, the strength and form of this feedback is not understood. In this paper, we use the phenomenological model galform to explore how galaxy formation depends on the strength and halo mass dependence of feedback. We focus on 'expulsion' models in which the wind mass loading, β, is proportional to 1/v^n_(disc), with n= 0, 1, 2 and contrast these models with the successful Bower et al. model (B8W7), for which β α 1/v^(3.2)_(disc). A crucial development is that our code explicitly accounts for the recapture of expelled gas as the system’s halo mass (and thus gravitational potential) increases. While models with high wind speed and mass loading result in a poor match to the observed SMF, a model with slower wind speed matches the flat portion of the SMF at M_★∼ 10^9–10^(11) h^(−1) M_⊙. When combined with active galactic nucleus feedback, the model provides a good description of the observed SMF above 10^9 h^(−1) M_⊙. In order to explore the impact of different feedback schemes further, we examine how the expulsion models compare with a further range of observational data, contrasting the results with the B8W7 model. In the expulsion models, the brightest galaxies are assembled more recently, and the specific star formation rates of galaxies decrease strongly with decreasing stellar mass. The expulsion models tend to have a cosmic star formation density that is dominated by lower mass galaxies at z= 1–3, and dominated by high-mass galaxies at low redshift. These trends are in conflict with observational data, but the comparison highlights some deficiencies of the B8W7 model also. The experiments in this paper not only give us important physical insight into the impact of the feedback process on the formation histories of galaxies, but the strong mass dependence of feedback adopted in B8W7 still appears to provide the most promising description of the observed Universe.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 422 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20120620-075426733

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Abstract: Weak-gravitational-lensing distortions to the intensity pattern of 21-cm radiation from the dark ages can be decomposed geometrically into curl and curl-free components. Lensing by primordial gravitational waves induces a curl component, while the contribution from lensing by density fluctuations is strongly suppressed. Angular fluctuations in the 21-cm background extend to very small angular scales, and measurements at different frequencies probe different shells in redshift space. There is thus a huge trove of information with which to reconstruct the curl component of the lensing field, allowing tensor-to-scalar ratios conceivably as small as r∼10^(-9)—far smaller than those currently accessible—to be probed.

Publication: Physical Review Letters Vol.: 108 No.: 21 ISSN: 0031-9007

ID: CaltechAUTHORS:20120619-131243430

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Abstract: We present a new test of gravitational interactions at the r≃(0.2–20) Mpc scale, around the virial radius of dark matter halos measured through cluster-galaxy lensing of maxBCG clusters from the Sloan Digital Sky Survey (SDSS). We employ predictions from self-consistent simulations of f(R) gravity to find an upper bound on the background field amplitude of |f_R0|<3.5×10^(-3) at the 1D-marginalized 95% confidence level. As a model-independent assessment of the constraining power of cluster profiles measured through weak gravitational lensing, we also constrain the amplitude F_0 of a phenomenological modification based on the profile enhancement induced by f(R) gravity when not including effects from the increased cluster abundance in f(R). In both scenarios, dark-matter-only simulations of the concordance model corresponding to |fR0|=0 and F0=0 are consistent with the lensing measurements, i.e., at the 68% and 95% confidence level, respectively.

Publication: Physical Review D Vol.: 85 No.: 10 ISSN: 2470-0010

ID: CaltechAUTHORS:20120604-143012166

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Abstract: We consider a model where two new scalars are introduced in the standard model, assuming classical scale invariance. In this model the scale invariance is broken by quantum corrections and one of the new scalars acquires non-zero vacuum expectation value (VEV), which induces the electroweak symmetry breaking in the standard model, and the other scalar becomes dark matter. It is shown that TeV scale dark matter is realized, independent of the value of the other scalarʼs VEV. The impact of the new scalars on the Higgs potential is also discussed. The Higgs potential is stabilized when the Higgs mass is over ∼120 GeV.

Publication: Physics Letters B Vol.: 710 No.: 1 ISSN: 0370-2693

ID: CaltechAUTHORS:20120608-112642746

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Abstract: We investigate constraints from the observed branching ratio for b → sγ and fine-tuning in the framework of natural supersymmetry. The natural supersymmetry requires the large trilinear coupling of the stop sector, light higgsinos (a small μ parameter) and light stops, in order to reduce the finetuning in the Higgs sector while avoiding the LEP constraint. It is found that in such a scenario 5% (10%) level of fine-tuning is inevitable due to the b→sγ constraint even if the messenger scale is as low as 10^5 GeV (10^4 GeV), provided that the gaugino masses satisfy the GUT relation.

Publication: Physics Letters B Vol.: 710 No.: 1 ISSN: 0370-2693

ID: CaltechAUTHORS:20120608-113439037

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Abstract: Dark matter halo merger trees are now routinely extracted from cosmological simulations of structure formation. These trees are frequently used as inputs to semi-analytic models of galaxy formation to provide the backbone within which galaxy formation takes place. By necessity, these merger trees are constructed from a finite set of discrete ‘snapshots’ of the N-body simulation and so have a limited temporal resolution. To date, there has been little consideration of how this temporal resolution affects the properties of galaxies formed within these trees. In particular, the question of how many snapshots are needed to achieve convergence in galaxy properties has not be answered. Therefore, we study the convergence in the stellar and total baryonic masses of galaxies, distribution of merger times, stellar mass functions and star formation rates in the Galacticus model of galaxy formation as a function of the number of ‘snapshot’ times used to represent dark matter halo merger trees. When utilizing snapshots between z= 20 and 0, we find that at least 128 snapshots are required to achieve convergence to within 5 per cent for galaxy masses, while 64 snapshots give convergence only to within 10 per cent for high-mass haloes. This convergence is obtained for mean quantities averaged over large samples of galaxies – significant variance for individual galaxies remains even when using very large numbers of snapshots. We find only weak dependence of the rate of convergence on the distribution of snapshots in time – snapshots spaced uniformly in the expansion factor, uniformly in the logarithm of expansion factor or uniformly in the logarithm of critical overdensity for collapse work equally well in almost all cases. We provide input parameters to Galacticus which allow this type of convergence study to be tuned to other simulations and to be carried out for other galaxy properties.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 419 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20120316-152525983

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Abstract: We describe a new, free and open source semi-analytic model of galaxy formation, GALACTICUS. The GALACTICUS model was designed to be highly modular to facilitate expansion and the exploration of alternative descriptions of key physical ingredients. We detail the GALACTICUS engine for evolving galaxies through a merging hierarchy of dark matter halos and give details of the specific implementations of physics currently available in GALACTICUS. Finally, we show results from an example model that is in reasonably good agreement with several observational datasets. We use this model to explore numerical convergence and to demonstrate the types of information which can be extracted from GALACTICUS.

Publication: New Astronomy Vol.: 17 No.: 2 ISSN: 1384-1076

ID: CaltechAUTHORS:20120120-151217510

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Abstract: Despite the importance of the Second Law of Thermodynamics, it is not absolute. Statistical mechanics implies that, given sufficient time, systems near equilibrium will spontaneously fluctuate into lower-entropy states, locally reversing the thermodynamic arrow of time. We study the time development of such fluctuations, especially the very large fluctuations relevant to cosmology. Under fairly general assumptions, the most likely history of a fluctuation out of equilibrium is simply the CPT conjugate of the most likely way a system relaxes back to equilibrium. We use this idea to elucidate the spacetime structure of various fluctuations in (stable and metastable) de Sitter space and thermal anti-de Sitter space.

Publication: Journal of Cosmology and Astroparticle Physics Vol.: 2012 No.: 2 ISSN: 1475-7516

ID: CaltechAUTHORS:20120516-092014134

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Abstract: We use a coupled model of the formation and evolution of galaxies and black holes (BHs) to study the evolution of active galactic nuclei (AGNs) in a cold dark matter universe. The model is embedded in the galaxy formation code galform and predicts the masses, spins and mass accretion histories of BHs in tandem with the formation of their host galaxies. BHs grow by accretion during starbursts, triggered by discs becoming dynamically unstable or by galaxy mergers, and accretion from quasi-hydrostatic hot gas haloes. Using an empirical law for AGN obscuration, our model matches the observed luminosity functions (LFs) of AGNs over a wide range of redshifts. Due to the suppression of cooling in massive haloes by AGN feedback, at low redshift (z≲2), the brightest quasars (L_(bol)≳ 10^(46) erg s^(−1)) are predicted preferentially to inhabit haloes with masses ≃ 10^(12)–10^(13) M_⊙. The model predicts a hierarchical buildup of BH mass, with the typical mass of actively growing BHs increasing with decreasing redshift. Nevertheless, the model displays clear ‘downsizing’ as reflected in the differential evolution of the space density of faint and bright AGNs. This arises naturally from the interplay between the starburst and hot gas halo accretion modes. The faint end of the LF is dominated by massive BHs accreting at low rates via a thick disc, primarily during the hot-halo mode. The bright end is populated by BHs accreting close to or above the Eddington limit during the starburst mode. Obscuration plays a central role in determining the observed abundance of AGNs and, hence, in their implied cosmic evolution.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 419 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20120319-104412300

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Abstract: Radiative corrections to the decay rate of charged fermions caused by the presence of a thermal bath of photons are calculated in the limit when temperatures are below the masses of all charged particles involved. The cancellation of finite-temperature infrared divergences in the decay rate is described in detail. Temperature-dependent radiative corrections to a two-body decay of a hypothetical charged fermion and to electroweak decays of a muon μ→eν_μν̅_e are given. We touch upon possible implications of these results for charged particles in the early Universe.

Publication: Physical Review D Vol.: 85 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:20120306-065630130

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Abstract: Bipolar spherical harmonics (BiPoSHs) provide a general formalism for quantifying departures in the cosmic microwave background (CMB) from statistical isotropy and from Gaussianity. However, prior work has focused only on BiPoSHs with even parity. Here we show that there is another set of BiPoSHs with odd parity, and we explore their cosmological applications. We describe systematic artifacts in a CMB map that could be sought by measurement of these odd-parity BiPoSH modes. These BiPoSH modes may also be produced cosmologically through lensing by gravitational waves (GWs), among other sources. We derive expressions for the BiPoSH modes induced by the weak lensing of both scalar and tensor perturbations. We then investigate the possibility of detecting parity-breaking physics, such as chiral GWs, by cross-correlating opposite-parity BiPoSH modes with multipole moments of the CMB polarization. We find that the expected signal-to-noise of such a detection is modest.

Publication: Physical Review D Vol.: 85 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:20121011-091507881

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Abstract: Weak lensing is commonly measured using shear through galaxy ellipticities or using the effect of magnification bias on galaxy number densities. Here, we report on the first detection of weak-lensing magnification with a new, independent technique using the distribution of galaxy sizes and magnitudes. These data come for free in galaxy surveys designed for measuring shear. We present the magnification estimator and apply it to an X-ray-selected sample of galaxy groups in the COSMOS Hubble Space Telescope survey. The measurement of the projected surface density Σ(r) is consistent with the shear measurements within the uncertainties and has roughly 40% of the signal to noise of the latter. We discuss systematic issues and challenges to realizing the potential of this new probe of weak lensing.

Publication: Astrophysical Journal Letters Vol.: 744 No.: 2 ISSN: 2041-8205

ID: CaltechAUTHORS:20120312-110204684

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Abstract: Several recent studies have shown how to properly calculate the observed clustering of galaxies in a relativistic context, and uncovered corrections to the Newtonian calculation that become significant on scales near the horizon. Here, we retrace these calculations and show that, on scales approaching the horizon, the observed galaxy power spectrum depends strongly on which gauge is assumed to relate the intrinsic fluctuations in galaxy density to matter perturbations through a linear bias relation. Starting from simple physical assumptions, we derive a gauge-invariant expression relating galaxy density perturbations to matter density perturbations on large scales, and show that it reduces to a linear bias relation in a synchronous-comoving gauge, corroborating an assumption made in several recent papers. We evaluate the resulting observed galaxy power spectrum, and show that it leads to corrections similar to an effective non-Gaussian bias corresponding to a local f_(NL,eff)≲0.5. This number can serve as a guideline as to which surveys need to take into account relativistic effects. We also discuss the scale-dependent bias induced by primordial non-Gaussianity in the relativistic context, which again is simplest in a synchronous-comoving gauge.

Publication: Physical Review D Vol.: 85 No.: 2 ISSN: 1550-7998

ID: CaltechAUTHORS:20120206-151838806

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Abstract: If the conformal invariance of electromagnetism is broken during inflation, then primordial magnetic fields may be produced. If this symmetry breaking is generated by the coupling between electromagnetism and a scalar field—e.g. the inflaton, curvaton, or Ricci scalar—then these magnetic fields may be correlated with primordial density perturbations, opening a new window to the study of non-Gaussianity in cosmology. In order to illustrate, we couple electromagnetism to an auxiliary scalar field in a de Sitter background. We calculate the power spectra for scalar-field perturbations and magnetic fields, showing how a scale-free magnetic-field spectrum with rms amplitude of ∼nG at Mpc scales may be achieved. We explore the Fourier-space dependence of the cross correlation between the scalar field and magnetic fields, showing that the dimensionless amplitude, measured in units of the power spectra, can grow as large as ∼500H_I/M, where H_I is the inflationary Hubble constant and M is the effective mass scale of the coupling.

Publication: Physical Review D Vol.: 84 No.: 12 ISSN: 1550-7998

ID: CaltechAUTHORS:20120210-112540018

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Abstract: We review the effects of modified gravity on large-scale structure in the nonlinear regime, focusing on f(R) gravity and the Dvali–Gabadadze–Porrati model, for which full N-body simulations have been performed. In particular, we discuss the abundance of massive halos, the nonlinear matter power spectrum and the dynamics within clusters and galaxies, with particular emphasis on the screening mechanisms present in these models.

Publication: Philosophical Transactions A: Mathematical, Physical and Engineering Sciences Vol.: 369 No.: 1957 ISSN: 1364-503X

ID: CaltechAUTHORS:20111212-092417964

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Abstract: Measurements of cosmic microwave background (CMB) anisotropies constrain isocurvature fluctuations between photons and nonrelativistic particles to be subdominant to adiabatic fluctuations. Perturbations in the relative number densities of baryons and dark matter, however, are surprisingly poorly constrained. In fact, baryon-density perturbations of fairly large amplitude may exist if they are compensated by dark-matter perturbations, so that the total density remains unchanged. These compensated isocurvature perturbations (CIPs) leave no imprint on the CMB at observable scales, at linear order. B modes in the CMB polarization are generated at reionization through the modulation of the optical depth by CIPs, but this induced polarization is small. The strongest known constraint ≲10% to the CIP amplitude comes from galaxy-cluster baryon fractions. Here, it is shown that modulation of the baryon density by CIPs at and before the decoupling of Thomson scattering at z∼1100 gives rise to CMB effects several orders of magnitude larger than those considered before. Polarization B modes are induced, as are correlations between temperature/polarization spherical-harmonic coefficients of different lm. It is shown that the CIP field at the surface of last scatter can be measured with these off-diagonal correlations. The sensitivity of ongoing and future experiments to these fluctuations is estimated. Data from the WMAP, ACT, SPT, and Spider experiments will be sensitive to fluctuations with amplitude ∼5–10%. The Planck satellite and Polarbear experiment will be sensitive to fluctuations with amplitude ∼3%. SPTPol, ACTPol, and future space-based polarization methods will probe amplitudes as low as ∼0.4%–0.6%. In the cosmic-variance limit, the smallest CIPs that could be detected with the CMB are of amplitude ∼0.05%.

Publication: Physical Review D Vol.: 84 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:20120202-112949558

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Abstract: In this Letter we explore the direct detection of the dark matter in the universe, assuming the dark matter particles are degenerate in mass with new colored particles below TeV scale. The scenario with such a mass spectrum is difficult to be confirmed or excluded by the present analysis at the LHC experiments because the QCD jets in the cascade decay of the new particles produced in the proton-proton collision are too soft to be triggered in the event selection. It is shown that both of the spin-independent and spin-dependent couplings of the dark matter with a nucleon are enhanced and the scattering cross section may reach even the current bound of the direct detection experiments. Then such a degenerate scenario may be tested in the direct detection experiments.

Publication: Physics Letters B Vol.: 706 No.: 2-3 ISSN: 0370-2693

ID: CaltechAUTHORS:20120120-101418660

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Abstract: We study the evolution of the cold gas content of galaxies by splitting the interstellar medium into its atomic and molecular hydrogen components, using the galaxy formation model galform in the Λ cold dark matter framework. We calculate the molecular-to-atomic hydrogen mass ratio, H₂/H i, in each galaxy using two different approaches, the pressure-based empirical relation of Blitz & Rosolowsky and the theoretical model of Krumholz, McKeee & Tumlinson, and apply them to consistently calculate the star formation rates of galaxies. We find that the model based on the Blitz & Rosolowsky law predicts an H i mass function, ¹²CO (1–0) luminosity function, correlations between H₂/H i and stellar and cold gas mass, and infrared–¹²CO molecule luminosity relation in good agreement with local and high-redshift observations. The H i mass function evolves weakly with redshift, with the number density of high-mass galaxies decreasing with increasing redshift. In the case of the H₂ mass function, the number density of massive galaxies increases strongly from z = 0 to 2, followed by weak evolution up to z = 4. We also find that H₂/H i of galaxies is strongly dependent on stellar and cold gas mass, and also on redshift. The slopes of the correlations between H₂/H i and stellar and cold gas mass hardly evolve, but the normalization increases by up to two orders of magnitude from z = 0 to 8. The strong evolution in the H₂ mass function and H₂/H i is primarily due to the evolution in the sizes of galaxies and, secondarily, in the gas fractions. The predicted cosmic density evolution of H i agrees with the observed evolution inferred from damped Lyα systems, and is always dominated by the H i content of low- and intermediate-mass haloes. We find that previous theoretical studies have largely overestimated the redshift evolution of the global H₂/H i due to limited resolution. We predict a maximum of ρH₂/ρH i ≈ 1.2 at z ≈ 3.5.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 418 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20120217-122730662

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Abstract: In this paper we analyze a suite of cosmological simulations of modied gravitational action f(R) models, where cosmic acceleration is induced by a scalar eld that acts as a fifth force on all forms of matter. In particular, we focus on the bispectrum of the dark matter density eld on mildly non-linear scales. For models with the same initial power spectrum, the dark matter bispectrum shows signicant differences for cases where the final dark matter power spectrum also diers. Given the different dependence on bias of the galaxy power spectrum and bispectrum, bispectrum measurements can close the loophole of galaxy bias hiding differences in the power spectrum. Alternatively, changes in the initial power spectrum can also hide differences. By constructing CDM models with very similar final non-linear power spectra, we show that the differences in the bispectrum are reduced (≾ 4%) and are comparable with differences in the imperfectly matched power spectra. These results indicate that the bispectrum depends mainly on the power spectrum and less sensitively on the gravitational signatures of the f(R) model. This weak dependence of the matter bispectrum on gravity makes it useful for breaking degeneracies associated with galaxy bias, even for models beyond general relativity.

Publication: Journal of Cosmology and Astroparticle Physics Vol.: 2011 No.: 11 ISSN: 1475-7516

ID: CaltechAUTHORS:20120203-133112782

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Abstract: The first stars and quasars are known sources of hard ionizing radiation in the first billion years of the Universe. We examine the joint effects of X-rays and hard ultraviolet (UV) radiation from such first-light sources on the hydrogen and helium reionization of the intergalactic medium (IGM) at early times, and the associated heating. We study the growth and evolution of individual H ii, He ii and He iii regions around early galaxies with first stars and/or quasi-stellar object populations. We find that in the presence of helium-ionizing radiation, X-rays may not dominate the ionization and thermal history of the IGM at z ~ 10–20, contributing relatively modest increases to IGM ionization and heating up to ~10^3–10^5 K in IGM temperatures. We also calculate the 21-cm signal expected from a number of scenarios with metal-free starbursts and quasars in varying combinations and masses at these redshifts. The peak values for the spin temperature reach ~10^4–10^5 K in such cases. The maximum values for the 21-cm brightness temperature are around 30–40 mK in emission, while the net values of the 21-cm absorption signal range from ~a few to 60 mK on scales of 0.01–1 Mpc. We find that the 21-cm signature of X-ray versus UV ionization could be distinct, with the emission signal expected from X-rays alone occurring at smaller scales than that from UV radiation, resulting from the inherently different spatial scales at which X-ray and UV ionization/heating manifests. This difference is time-dependent and becomes harder to distinguish with an increasing X-ray contribution to the total ionizing photon production. Such differing scale-dependent contributions from X-ray and UV photons may therefore ‘blur’ the 21-cm signature of the percolation of ionized bubbles around early haloes (depending on whether a cosmic X-ray or UV background is built up first) and affect the interpretation of 21-cm data constraints on reionization.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 417 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20111213-103109741

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Abstract: Many models of inflation predict oscillatory features in the bispectrum of primordial fluctuations. Since it has been shown that primordial non-Gaussianity can lead to a scale-dependent halo bias, we investigate the effect of oscillations in the three-point function on the clustering of dark-matter halos. Interestingly, we find that features in the inflaton potential such as oscillations or sharp steps get imprinted in the mass dependence of the non-Gaussian halo bias. In this paper, we focus on models displaying a sharp feature in the inflaton potential as well as resonant non-Gaussianity. In both cases, we find a strong scale dependence for the non-Gaussian halo bias with a slope similar to that of the local model. In the resonant case, we find that the non-Gaussian bias oscillates with halo mass, a novel feature that is unique to this type of models. In the case of a sharp feature in the inflaton potential, we find that the clustering of halos is enhanced at the mass scale corresponding to the Fourier mode that exited the horizon when the inflaton was crossing the feature in the potential. Both of these are new effects that open the possibility of characterizing the inflationary potential with large-scale-structure surveys. We briefly discuss the prospects for detecting these non-Gaussian effects.

Publication: Physical Review D Vol.: 84 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:20111122-103056677

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Abstract: We present a model for the satellites of the Milky Way in which galaxy formation is followed using semi-analytic techniques applied to the six high-resolution N-body simulations of galactic haloes of the Aquarius project. The model, calculated using the galform code, incorporates improved treatments of the relevant physics in the Λ cold dark matter cosmogony, particularly a self-consistent calculation of reionization by ultraviolet (UV) photons emitted by the forming galaxy population, including the progenitors of the central galaxy. Along the merger tree of each halo, the model calculates gas cooling (by Compton scattering off cosmic microwave background photons, molecular hydrogen and atomic processes), gas heating (from hydrogen photoionization and supernova energy), star formation and evolution. The evolution of the intergalactic medium is followed simultaneously with that of the galaxies. Star formation in the more massive progenitor subhaloes is suppressed primarily by supernova feedback, while for smaller subhaloes, it is suppressed primarily by photoionization due to external and internal sources. The model is constrained to match a wide range of properties of the present-day galaxy population as a whole, but at high redshift it requires an escape fraction of UV photons near unity in order to completely reionize the universe by redshift z ≳ 8. In the most successful model, the local sources photoionize the pre-galactic region completely by z ≃ 10. In addition to the luminosity function of Milky Way satellites, the model matches their observed luminosity–metallicity relation, their radial distribution and the inferred values of the mass within 300 pc, which in the models increase slowly but significantly with luminosity. There is a large variation in satellite properties from halo to halo, with the luminosity function, for example, varying by a factor of ∼2 among the six simulations.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 417 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:20120104-075005808

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Abstract: It is possible that there are additional vectorlike generations where the quarks have mass terms that do not originate from weak symmetry breaking, but the leptons only get mass through weak symmetry breaking. We discuss the impact that the new leptons have on Higgs boson decay branching ratios and on the range of allowed Higgs masses in such a model (with a single new vectorlike generation). We find that if the fourth generation leptons are too heavy to be produced in Higgs decay, then the new leptons reduce the branching ratio for h→γγ to about 30% of its standard-model value. The dependence of this branching ratio on the new charged lepton masses is weak. Furthermore the expected Higgs production rate at the LHC is very near its standard-model value if the new quarks are much heavier than the weak scale. If the new quarks have masses near the cutoff for the theory, then for cutoffs greater than 10^(15) GeV, the new lepton masses cannot be much heavier than about 100 GeV and the Higgs mass must have a value around 175 GeV.

Publication: Physical Review D Vol.: 84 No.: 5 ISSN: 2470-0010

ID: CaltechAUTHORS:20111021-142612846

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Abstract: One of the principle efforts in cosmic microwave background (CMB) research is measurement of the parameter fnl that quantifies the departure from Gaussianity in a large class of nonminimal inflationary (and other) models. Estimators for f_(nl) are composed of a sum of products of the temperatures in three different pixels in the CMB map. Since the number ~N_(pix)^2 of terms in this sum exceeds the number N_(pix) of measurements, these ~N_(pix)^2 terms cannot be statistically independent. Therefore, the central-limit theorem does not necessarily apply, and the probability distribution function (PDF) for the f_(nl) estimator does not necessarily approach a Gaussian distribution for N_(pix)≫1. Although the variance of the estimators is known, the significance of a measurement of fnl depends on knowledge of the full shape of its PDF. Here we use Monte Carlo realizations of CMB maps to determine the PDF for two minimum-variance estimators: the standard estimator, constructed under the null hypothesis (f_(nl)=0), and an improved estimator with a smaller variance for f_(nl) ≠ 0. While the PDF for the null-hypothesis estimator is very nearly Gaussian when the true value of f_(nl) is zero, the PDF becomes significantly non-Gaussian when f_(nl) ≠ 0. In this case we find that the PDF for the null-hypothesis estimator f_(nl) is skewed, with a long non-Gaussian tail at f_(nl)>|f_(nl)| and less probability at f_(nl)<|f_(nl)| than in the Gaussian case. We provide an analytic fit to these PDFs. On the other hand, we find that the PDF for the improved estimator is nearly Gaussian for observationally allowed values of f_(nl). We discuss briefly the implications for trispectrum (and other higher-order correlation) estimators.

Publication: Physical Review D Vol.: 84 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20111018-073820503

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Abstract: Recent results of N-body simulations have shown that current theoretical models are not able to correctly predict the amplitude of the scale-dependent halo bias induced by primordial non-Gaussianity, for models going beyond the simplest, local quadratic case. Motivated by these discrepancies, we carefully examine three theoretical approaches based on (1) the statistics of thresholded regions, (2) a peak-background split method based on separation of scales, and (3) a peak-background split method using the conditional mass function. We first demonstrate that the statistics of thresholded regions, which is shown to be equivalent at leading order to a local bias expansion, cannot explain the mass-dependent deviation between theory and N-body simulations. In the two formulations of the peak-background split on the other hand, we identify an important, but previously overlooked, correction to the non-Gaussian bias that strongly depends on halo mass. This new term is in general significant for any primordial non-Gaussianity going beyond the simplest local fNL model. In a separate paper (to be published in PRD rapid communication), the authors compare these new theoretical predictions with N-body simulations, showing good agreement for all simulated types of non-Gaussianity.

Publication: Physical Review D Vol.: 84 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20110930-130454310

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Abstract: The large-scale clustering of galaxies can serve as a probe of primordial non-Gaussianity in the Universe competitive with the anisotropies of the cosmic microwave background. Here, we present improved theoretical predictions which include an important, previously overlooked correction to the bias. We demonstrate that the new predictions are able to reproduce the results of N-body simulations, explaining the significant departures seen from previous theoretical results. These refined predictions open the way to accurate constraints on primordial physics with large-scale structure surveys.

Publication: Physical Review D Vol.: 84 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20110927-112035978

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Abstract: In this paper, we complete formulae for the elastic scattering cross section of general vector dark matter with nucleons in the direct detection at the leading order of the strong coupling constant α_s, assuming that the dark matter is composed of vector particles and interacts with heavy fermions with color charge as well as standard-model quark. As an application of our formulae, the direct detection of the first Kaluza-Klein photon in the minimal universal extra dimension model is discussed. It is found that the scattering cross section is larger than those in the previous works by up to a factor of ten.

Publication: Progress of Theoretical Physics Vol.: 126 No.: 3 ISSN: 0033-068X

ID: CaltechAUTHORS:20111107-115404879

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Abstract: We investigate the consequences of applying different star formation laws in the galaxy formation model galform. Three broad star formation laws are implemented: the empirical relations of Kennicutt and Schmidt and Blitz & Rosolowsky and the theoretical model of Krumholz, McKee & Tumlinson. These laws have no free parameters once calibrated against observations of the star formation rate (SFR) and gas surface density in nearby galaxies. We start from published models, and investigate which observables are sensitive to a change in the star formation law, without altering any other model parameters. We show that changing the star formation law (i) does not significantly affect either the star formation history of the universe or the galaxy luminosity functions in the optical and near-infrared, due to an effective balance between the quiescent and burst star formation modes, (ii) greatly affects the cold gas contents of galaxies and (iii) changes the location of galaxies in the SFR versus stellar mass plane, so that a second sequence of ‘passive’ galaxies arises, in addition to the known ‘active’ sequence. We show that this plane can be used to discriminate between the star formation laws.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 416 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:20111017-111848030

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Abstract: Theories for new particle and early-Universe physics abound with pseudo-Nambu-Goldstone fields that arise when global symmetries are spontaneously broken. The coupling of these fields to the Chern-Simons term of electromagnetism may give rise to cosmological birefringence (CB), a frequency-independent rotation of the linear polarization of photons as they propagate over cosmological distances. Inhomogeneities in the CB-inducing field may yield a rotation angle that varies across the sky. Here we note that such a spatially-varying birefringence may be correlated with the cosmic microwave background (CMB) temperature. We describe quintessence scenarios where this cross-correlation exists and other scenarios where the scalar field is simply a massless spectator field, in which case the cross-correlation does not exist. We discuss how the cross-correlation between CB-rotation angle and CMB temperature may be measured with CMB polarization. This measurement may improve the sensitivity to the CB signal, and it can help discriminate between different models of CB.

Publication: Physical Review D Vol.: 84 No.: 4 ISSN: 2470-0010

ID: CaltechAUTHORS:20110822-092055341

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Abstract: We present a global analysis of kinematics and metallicity in the nearest S0 galaxy, NGC 3115, along with implications for its assembly history. The data include high-quality wide-field imaging from Suprime-Cam on the Subaru telescope, and multi-slit spectra of the field stars and globular clusters (GCs) obtained using Keck-DEIMOS/LRIS and Magellan-IMACS. Within two effective radii, the bulge (as traced by the stars and metal-rich GCs) is flattened and rotates rapidly (v/σ ≳ 1.5). At larger radii, the rotation declines dramatically to v/σ ~ 0.7, but remains well aligned with the inner regions. The radial decrease in characteristic metallicity of both the metal-rich and metal-poor GC subpopulations produces strong gradients with power-law slopes of –0.17 ± 0.04 and –0.38 ± 0.06 dex dex^(–1), respectively. We argue that this pattern is not naturally explained by a binary major merger, but instead by a two-phase assembly process where the inner regions have formed in an early violent, dissipative phase, followed by the protracted growth of the outer parts via minor mergers with typical mass ratios of ~15-20:1.

Publication: Astrophysical Journal Letters Vol.: 736 No.: 2 ISSN: 2041-8205

ID: CaltechAUTHORS:20110830-081227632

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Abstract: We analyze the finite temperature structure of the standard model coupled to gravity with one and two dimensions compactified (on a circle and a torus). We find that finite temperature effects wash out any vacua which exist at zero temperature. We also discuss the possibility of transitions between the four-dimensional universe and the lower-dimensional spacetimes.

Publication: Journal of High Energy Physics Vol.: 2011 No.: 8 ISSN: 1126-6708

ID: CaltechAUTHORS:20111018-111238782

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Abstract: Large catalogs of shear-selected peaks have recently become a reality. In order to properly interpret the abundance and properties of these peaks, it is necessary to take into account the effects of the clustering of source galaxies, among themselves and with the lens. In addition, the preferred selection of magnified galaxies in a flux- and size-limited sample leads to fluctuations in the apparent source density that correlate with the lensing field. In this paper, we investigate these issues for two different choices of shear estimators that are commonly in use today: globally normalized and locally normalized estimators. While in principle equivalent, in practice these estimators respond differently to systematic effects such as magnification and cluster member dilution. Furthermore, we find that the answer to the question of which estimator is statistically superior depends on the specific shape of the filter employed for peak finding; suboptimal choices of the estimator+filter combination can result in a suppression of the number of high peaks by orders of magnitude. Magnification and size bias generally act to increase the signal-to-noise ν of shear peaks; for high peaks the boost can be as large as Δν ≈ 1-2. Due to the steepness of the peak abundance function, these boosts can result in a significant increase in the observed abundance of shear peaks. A companion paper investigates these same issues within the context of stacked weak-lensing mass estimates.

Publication: Astrophysical Journal Vol.: 735 No.: 2 ISSN: 0004-637X

ID: CaltechAUTHORS:20110712-113548421

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Abstract: Assuming that the lightest neutral component in an SU(2)_L gauge multiplet is the main ingredient of dark matter in the universe, we calculate the elastic scattering cross section of the dark matter with nucleon, which is an important quantity for the direct detection experiments. When the dark matter is a real scalar or a Majorana fermion which has only electroweak gauge interactions, the scattering with quarks and gluon are induced through one- and two-loop quantum processes, respectively, and both of them give rise to comparable contributions to the elastic scattering cross section. We evaluate all of the contributions at the leading order and find that there is an accidental cancellation among them. As a result, the spin-independent cross section is found to be O(10^(−(46−48))) cm^2, which is far below the current experimental bounds.

Publication: Journal of High Energy Physics Vol.: 2011 No.: 7 ISSN: 1126-6708

ID: CaltechAUTHORS:20111007-100722920

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Abstract: In the picture of eternal inflation as driven by a scalar potential with multiple minima, our observable universe resides inside one of many bubbles formed from transitions out of a false vacuum. These bubbles necessarily collide, upsetting the homogeneity and isotropy of our bubble interior, and possibly leading to detectable signatures in the observable portion of our bubble, potentially in the cosmic microwave background or other precision cosmological probes. This constitutes a direct experimental test of eternal inflation and the landscape of string theory vacua. Assessing this possibility roughly splits into answering three questions: What happens in a generic bubble collision? What observational effects might be expected? How likely are we to observe a collision? In this review we report the current progress on each of these questions, improve upon a few of the existing results and attempt to lay out directions for future work.

Publication: Reports on Progress in Physics Vol.: 74 No.: 7 ISSN: 0034-4885

ID: CaltechAUTHORS:20110713-155535882

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Abstract: Standard big-bang nucleosynthesis (BBN) predicts only a trace abundance of lithium and no heavier elements, but some alternatives predict a nonzero primordial metallicity. Here we explore whether CMB measurements may set useful constraints to the primordial metallicity and/or whether the standard CMB calculations are robust, within the tolerance of forthcoming CMB maps, to the possibility of primordial metals. Metals would affect the recombination history (and thus CMB power spectra) in three ways: (1) Lyα photons can be removed (and recombination thus accelerated) by photoionizing metals; (2) The Bowen resonance-fluorescence mechanism may degrade Lyβ photons and thus enhance the Lyβ escape probability and speed up recombination; (3) Metals could affect the low-redshift tail of the CMB visibility function by providing additional free electrons. The last two of these provide the strongest CMB signal. However, the effects are detectable in the Planck satellite only if the primordial metal abundance is at least a few hundredths of solar for (2) and a few tenths of solar for (3). We thus conclude that Planck will not be able to improve upon current constraints to primordial metallicity, at the level of a thousandth of solar, from the Lyman-α forest and ultra-metal-poor halo stars, and that the CMB power-spectrum predictions for Planck suffer no uncertainty arising from the possibility that there may be primordial metals.

Publication: Physical Review D Vol.: 83 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:20110429-142624622

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Abstract: We investigate the spectrum and wave functions of q̅ ′q′ bound states for heavy fourth generation quarks (q′) that have a very small mixing with the three observed generations of standard model quarks. Such bound states come with different color, spin and flavor quantum numbers. Since the fourth generation Yukawa coupling, λ_q′, is large we include all perturbative corrections to the potential between the heavy quark and antiquark of order λ_(q′)^(2)N_c/16π^2 where N_c is the number of colors, as well as relativistic corrections suppressed by (v/c)^2. We find that the lightest fourth generation quark masses for which a bound state exists for color octet states. For the color singlet states, which always have a bound state, we analyze the influence that the Higgs couplings have on the size and binding energy of the bound states.

Publication: Physical Review D Vol.: 83 No.: 7 ISSN: 2470-0010

ID: CaltechAUTHORS:20110502-112029805

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Abstract: As one of the most powerful probes of cosmological structure formation, the abundance of massive galaxy clusters is a sensitive probe of modifications to gravity on cosmological scales. In this paper, we present results from N-body simulations of a general class of f(R) models, which self-consistently solve the nonlinear field equation for the enhanced forces. Within this class we vary the amplitude of the field, which controls the range of the enhanced gravitational forces, both at the present epoch and as a function of redshift. Most models in the literature can be mapped onto the parameter space of this class. Focusing on the abundance of massive dark matter halos, we compare the simulation results to a simple spherical collapse model. Current constraints lie in the large-field regime, where the chameleon mechanism is not important. In this regime, the spherical collapse model works equally well for a wide range of models and can serve as a model-independent tool for placing constraints on f(R) gravity from cluster abundance. Using these results, we show how constraints from the observed local abundance of X-ray clusters on a specific f(R) model can be mapped onto other members of this general class of models.

Publication: Physical Review D Vol.: 83 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20110322-091231806

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Abstract: We have analysed high-resolution N-body simulations of dark-matter (DM) haloes, focusing specifically on the evolution of angular momentum. We find that not only is individual particle angular momentum not conserved, but the angular momentum of radial shells also varies over the age of the Universe by up to factors of a few. We find that torques from external structure are the most likely cause for this distribution shift. Since the model of adiabatic contraction that is often applied to model the effects of galaxy evolution on the DM density profile in a halo assumes angular momentum conservation, this variation implies that there is a fundamental limit on the possible accuracy of the adiabatic contraction model in modelling the response of DM haloes to the growth of galaxies.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 411 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20110322-091233054

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Abstract: A generic expectation for gas accreted by high-mass haloes is that it is shock-heated to the virial temperature of the halo. In low-mass haloes, or at high redshift, however, the gas cooling rate is sufficiently rapid that an accretion shock is unlikely to form close to the virial radius. Instead, the accretion shock will form at smaller radii, perhaps close to the central galaxy. Semi-analytic models have always made a clear distinction between the regimes in which accretion is limited by the cooling time of hot gas and by the dynamical time-scale of the halo, using simple estimates of the mass-scale at which the transition from rapid to slow cooling occurs. In this work, we revisit this issue using the latest understanding and calibration of accretion shock formation. Starting from the well-established Galform code, we investigate the effect of accounting for the presence or otherwise of an accretion shock close to the virial radius using the shock stability model of Birnboim & Dekel. As expected, when we modify the code so that there is no effective feedback from galaxy formation, we find that so-called ‘cold-mode’ accretion is the dominant channel for feeding gas into the galaxies at high redshifts, such that 90 per cent of baryons in galaxies (averaged over all galaxies) arrive via this channel. The mass-scale at which the rapid to slow cooling transition occurs is significantly affected at high redshifts and accretion rates become dominated by cold-mode accretion. However, the impact of this change in the cooling channel on galaxies properties is mitigated by compensating effects in the star formation and feedback cycle. When effective feedback, which reheats gas from galaxies to the virial temperature but which allows no gas to escape from a halo, is included in the model, we find that the ‘cold mode’ is even less apparent because of the presence of gas ejected from the galaxy's disc, although it can still contribute almost 50 per cent of the net inflow rate when averaged over all galaxies. Thus, the inclusion of the latest calibration of accretion shock physics makes little difference to basic results from earlier semi-analytic models, which used a simpler treatment. We conclude that this ‘cold-mode’ physics is already adequately accounted for in semi-analytic models and that feedback represents a much larger uncertainty than any of these effects.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 410 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20110301-084205905

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Abstract: Minimum-variance estimators for the parameter f_(nl) that quantifies local-model non-Gaussianity can be constructed from the cosmic microwave background (CMB) bispectrum (three-point function) and also from the trispectrum (four-point function). Some have suggested that a comparison between the estimates for the values of f_(nl) from the bispectrum and trispectrum allow a consistency test for the model. But others argue that the saturation of the Cramer-Rao bound—which gives a lower limit to the variance of an estimator—by the bispectrum estimator implies that no further information on f_(nl) can be obtained from the trispectrum. Here, we elaborate the nature of the correlation between the bispectrum and trispectrum estimators for f_(nl). We show that the two estimators become statistically independent in the limit of large number of CMB pixels, and thus that the trispectrum estimator does indeed provide additional information on f_(nl) beyond that obtained from the bispectrum. We explain how this conclusion is consistent with the Cramer-Rao bound. Our discussion of the Cramer-Rao bound may be of interest to those doing Fisher-matrix parameter-estimation forecasts or data analysis in other areas of physics as well.

Publication: Physical Review D Vol.: 83 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:20110302-093713796

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Abstract: Measurement of the CMB bispectrum, or three-point correlation function, has now become one of the principle efforts in early-Universe cosmology. Here we show that there is an odd-parity component of the CMB bispectrum that has been hitherto unexplored. We argue that odd-parity temperature-polarization bispectra can arise, in principle, through weak lensing of the CMB by chiral gravitational waves or through cosmological birefringence, although the signals will be small even in the best-case scenarios. Measurement of these bispectra requires only modest modifications to the usual data-analysis algorithms. They may be useful as a consistency test in searches for the usual bispectrum and to search for surprises in the data.

Publication: Physical Review D Vol.: 83 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:20110302-101406467

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Abstract: We consider constraints on a phenomenological dark-matter model consisting of two nearly degenerate particle species using observed properties of the Milky Way satellite galaxy population. The two parameters of this model, assuming the particle masses are ≳ GeV, are v_k, the recoil speed of the daughter particle, and τ, the lifetime of the parent particle. The satellite constraint that spans the widest range of v_k is the number of satellites that have a mass within 300 pc M_(300)>5×10^6M_⊙, although constraints based on M_300 in the classical dwarfs and the overall velocity function are competitive for vk≳50 km s^(-1). In general, we find that τ≲30 Gyr is ruled out for 20 km s^(-1)≲v_k≲200 km s^(-1), although we find that the limits on τ for fixed vk can change by a factor of ~3 depending on the star-formation histories of the satellites. We advocate using the distribution of M300 in Milky Way satellites, determined by next-generation all-sky surveys and follow-up spectroscopy, as a probe of dark-matter physics.

Publication: Physical Review D Vol.: 82 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:20110307-112621455

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Abstract: We show how the peak-background split (PBS) can be generalized to predict the effect of nonlocal primordial non-Gaussianity on the clustering of halos. Our approach is applicable to arbitrary primordial bispectra. We show that the scale dependence of halo clustering predicted in the peak-background split agrees with that of the local-biasing model on large scales. On smaller scales, k ≳ 0:01h Mpc^(-1), the predictions diverge, a consequence of the assumption of separation of scales in the peak-background split. Even on large scales, PBS and local biasing do not generally agree on the amplitude of the effect outside of the high-peak limit. The scale dependence of the biasing—the effect that provides strong constraints to the local-model bispectrum—is far weaker for the equilateral and self-ordering-scalar-field models of non-Gaussianity. The bias scale dependence for the orthogonal and folded models is weaker than in the local model (~k^(-1)), but likely still strong enough to be constraining.We show that departures from scale-invariance of the primordial power spectrum may lead to order-unity corrections, relative to predictions made assuming scale-invariance—to the non-Gaussian bias in some of these nonlocal models for non-Gaussianity. An Appendix shows that a nonlocal model can produce the local-model bispectrum, a mathematical curiosity we uncovered in the course of this investigation.

Publication: Physical Review D Vol.: 82 No.: 10 ISSN: 1550-7998

ID: CaltechAUTHORS:20101207-091750993

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Abstract: Semi-analytic models are a powerful tool for studying the formation of galaxies. However, these models inevitably involve a significant number of poorly constrained parameters that must be adjusted to provide an acceptable match to the observed Universe. In this paper, we set out to quantify the degree to which observational data sets can constrain the model parameters. By revealing degeneracies in the parameter space we can hope to better understand the key physical processes probed by the data. We use novel mathematical techniques to explore the parameter space of the galform semi-analytic model. We base our investigation on the Bower et al. version of galform, adopting the same methodology of selecting model parameters based on an acceptable match to the local bJ and K luminosity functions. Since the galform model is inherently approximate, we explicitly include a model discrepancy term when deciding if a match is acceptable or not. The model contains 16 parameters that are poorly constrained by our prior understanding of the galaxy formation processes and that can plausibly be adjusted between reasonable limits. We investigate this parameter space using the Model Emulator technique, constructing a Bayesian approximation to the galform model that can be rapidly evaluated at any point in parameter space. The emulator returns both an expectation for the galform model and an uncertainty which allows us to eliminate regions of parameter space in which it is implausible that a galform run would match the luminosity function data. By combining successive waves of emulation, we show that only 0.26 per cent of the initial volume is of interest for further exploration. However, within this region we show that the Bower et al. model is only one choice from an extended subspace of model parameters that can provide equally acceptable fits to the luminosity function data. We explore the geometry of this region and begin to explore the physical connections between parameters that are exposed by this analysis. We also consider the impact of adding additional observational data to further constrain the parameter space. We see that the known tensions existing in the Bower et al. model lead to a further reduction in the successful parameter space.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 407 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20101022-082310974

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Abstract: We review the current theory of how galaxies form within the cosmological framework provided by the cold dark matter paradigm for structure formation. Beginning with the pre-galactic evolution of baryonic material we describe the analytical and numerical understanding of how baryons condense into galaxies, what determines the structure of those galaxies and how internal and external processes (including star formation, merging, active galactic nuclei, etc.) determine their gross properties and evolution. Throughout, we highlight successes and failures of current galaxy formation theory. We include a review of computational implementations of galaxy formation theory and assess their ability to provide reliable modeling of this complex phenomenon. We finish with a discussion of several “hot topics” in contemporary galaxy formation theory and assess future directions for this field.

Publication: Physics Reports Vol.: 495 No.: 2-3 ISSN: 0370-1573

ID: CaltechAUTHORS:20101111-090323470

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Abstract: During eternal inflation, a landscape of vacua can be populated by the nucleation of bubbles. These bubbles inevitably collide, and collisions sometimes displace the field into a new minimum in a process known as a classical transition. In this paper, we examine some new features of classical transitions that arise when gravitational effects are included. Using the junction condition formalism, we study the conditions for energy conservation in detail, and solve explicitly for the types of allowed classical transition geometries. We show that the repulsive nature of domain walls, and the de Sitter expansion associated with a positive energy minimum, can allow for classical transitions to vacua of higher energy than that of the colliding bubbles. Transitions can be made out of negative or zero energy (terminal) vacua to a de Sitter phase, restarting eternal inflation, and populating new vacua. However, the classical transition cannot produce vacua with energy higher than the original parent vacuum, which agrees with previous results on the construction of pockets of false vacuum. We briefly comment on the possible implications of these results for various measure proposals in eternal inflation.

Publication: Physical Review D Vol.: 82 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20101022-101219288

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Abstract: Recent focus on the importance of cold, unshocked gas accretion in galaxy formation – not explicitly included in semi-analytic studies – motivates the following detailed comparison between two inherently different modelling techniques: direct hydrodynamical simulation and semi-analytic modelling. By analysing the physical assumptions built into the gasoline simulation, formulae for the emergent behaviour are derived which allow immediate and accurate translation of these assumptions to the galform semi-analytic model. The simulated halo merger history is then extracted and evolved using these equivalent equations, predicting a strikingly similar galactic system. This exercise demonstrates that it is the initial conditions and physical assumptions which are responsible for the predicted evolution, not the choice of modelling technique. On this level playing field, a previously published galform model is applied (including additional physics such as chemical enrichment and feedback from active galactic nuclei) which leads to starkly different predictions.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 407 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20100915-094131397

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Abstract: We study the peculiar velocities of density peaks in the presence of primordial non-Gaussianity. Rare, high-density peaks in the initial density field can be identified with tracers such as galaxies and clusters in the evolved matter distribution. The distribution of relative velocities of peaks is derived in the large-scale limit using two different approaches based on a local biasing scheme. Both approaches agree, and show that halos still stream with the dark matter locally as well as statistically, i.e. they do not acquire a velocity bias. Nonetheless, even a moderate degree of (not necessarily local) non-Gaussianity induces a significant skewness (~0.1–0.2) in the relative velocity distribution, making it a potentially interesting probe of non-Gaussianity on intermediate to large scales. We also study two-point correlations in redshift space. The well-known Kaiser formula is still a good approximation on large scales, if the Gaussian halo bias is replaced with its (scale-dependent) non-Gaussian generalization. However, there are additional terms not encompassed by this simple formula which become relevant on smaller scales (k ≳ 0.01h/Mpc). Depending on the allowed level of non-Gaussianity, these could be of relevance for future large spectroscopic surveys.

Publication: Physical Review D Vol.: 82 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20100920-090928112

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Abstract: There is no direct evidence for radiation domination prior to big-bang nucleosynthesis, and so it is useful to consider how constraints to thermally-produced axions change in non-standard thermal histories. In the low-temperature-reheating scenario, radiation domination begins as late as ~1 MeV, and is preceded by significant entropy generation. Axion abundances are then suppressed, and cosmological limits to axions are significantly loosened. In a kination scenario, a more modest change to axion constraints occurs. Future possible constraints to axions and low-temperature reheating are discussed.

No.: 1274
ID: CaltechAUTHORS:20110318-145420435

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Abstract: We use stripped-down versions of three semi-analytic galaxy formation models to study the influence of different assumptions about gas cooling and galaxy mergers. By running the three models on identical sets of merger trees extracted from high-resolution cosmological N-body simulations, we are able to perform both statistical analyses and halo-by-halo comparisons. This paper demonstrates that there is a good statistical agreement between the three models used here, when operating on the same merger trees, reflecting a general agreement in the underlying framework for semi-analytic models. We also show, however, that various assumptions that are commonly adopted to treat gas cooling and galaxy mergers can lead to significantly different results, at least in some regimes. In particular, we find that the different models adopted for gas cooling lead to similar results for mass scales comparable to that of our own Galaxy. Significant differences, however, arise at larger mass scales. These are largely (but not entirely) due to different treatments of the ‘rapid cooling’ regime, and different assumptions about the hot gas distribution. At this mass regime, the predicted cooling rates can differ up to about one order of magnitude, with important implications on the relative weight that these models give to feedback from active galactic nuclei in order to counteract excessive gas condensation in relatively massive haloes at low redshift. Different assumptions in the modelling of galaxy mergers can also result in significant differences in the timings of mergers, with important consequences for the formation and evolution of massive galaxies.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 406 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20100903-105425904

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Abstract: We present six simulations of galactic stellar haloes formed by the tidal disruption of accreted dwarf galaxies in a fully cosmological setting. Our model is based on the Aquarius project, a suite of high-resolution N-body simulations of individual dark matter haloes. We tag subsets of particles in these simulations with stellar populations predicted by the galform semi-analytic model. Our method self-consistently tracks the dynamical evolution and disruption of satellites from high redshift. The luminosity function (LF) and structural properties of surviving satellites, which agree well with observations, suggest that this technique is appropriate. We find that accreted stellar haloes are assembled between 1 < z < 7 from less than five significant progenitors. These progenitors are old, metal-rich satellites with stellar masses similar to the brightest Milky Way dwarf spheroidals (10^(7)–10^(8) M_⊙). In contrast to previous stellar halo simulations, we find that several of these major contributors survive as self-bound systems to the present day. Both the number of these significant progenitors and their infall times are inherently stochastic. This results in great diversity among our stellar haloes, which amplifies small differences between the formation histories of their dark halo hosts. The masses (~10^(8)–10^(9) M_⊙) and density/surface-brightness profiles of the stellar haloes (from 10 to 100 kpc) are consistent with expectations from the Milky Way and M31. Each halo has a complex structure, consisting of well-mixed components, tidal streams, shells and other subcomponents. This structure is not adequately described by smooth models. The central regions (<10 kpc) of our haloes are highly prolate (c/a ~ 0.3), although we find one example of a massive accreted thick disc. Metallicity gradients in our haloes are typically significant only where the halo is built from a small number of satellites. We contrast the ages and metallicities of halo stars with surviving satellites, finding broad agreement with recent observations.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 406 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:20100903-105537495

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Abstract: We use N-body simulations to study the effects that a divergent (i.e. ‘cuspy’) dark matter profile introduces on the tidal evolution of dwarf spheroidal galaxies (dSphs). Our models assume cosmologically motivated initial conditions where dSphs are dark-matter-dominated systems on eccentric orbits about a host galaxy composed of a dark halo and a baryonic disc. We find that the resilience of dSphs to tidal stripping is extremely sensitive to the cuspiness of the inner halo profile; whereas dwarfs with a cored profile can be easily destroyed by the disc component, those with cusps always retain a bound remnant, even after losing more than 99.99 per cent of the original mass. For a given halo profile, the evolution of the structural parameters as driven by tides is controlled solely by the total amount of mass lost. This information is used to construct a semi-analytic code that follows the tidal evolution of individual satellites as they fall into a more massive host, which allows us to simulate the hierarchical build-up of spiral galaxies assuming different halo profiles and disc masses. We find that tidal encounters with discs tend to decrease the average mass of satellite galaxies at all galactocentric radii. Of all satellites, those accreted before re-ionization (z ≳ 6), which may be singled out by anomalous metallicity patterns, provide the strongest constraints on the inner profile of dark haloes. These galaxies move on orbits that penetrate the disc repeatedly and survive to the present day only if haloes have an inner density cusp. We show that the size–mass relationship established from Milky Way (MW) dwarfs strongly supports the presence of cusps in the majority of these systems, as cored models systematically underestimate the masses of the known ultra-faint dSphs. Our models also indicate that a massive M31 disc may explain why many of its dSphs with suitable kinematic data fall below the size–mass relationship derived from MW dSphs. We also examine whether our modelling can constrain the mass threshold below which star formation is suppressed in dark matter haloes. We find that luminous satellites must be accreted with masses above 10^(8)–10^(9) M_⊙ in order to explain the size–mass relation observed in MW dwarfs.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 406 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:20100831-152446337

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Abstract: Light gravitinos, with mass in the eV to MeV range, are well motivated in particle physics, but their status as dark-matter candidates is muddled by early-Universe uncertainties. We investigate how upcoming data from colliders may clarify this picture. Light gravitinos are produced primarily in the decays of the next-to-lightest supersymmetric particle, resulting in spectacular signals, including di-photons, delayed and nonpointing photons, kinked charged tracks, and heavy metastable charged particles. We find that the Tevatron with 20 fb^(-1) and the 7 TeV LHC with 1 fb^(-1) may both see evidence for hundreds of light-gravitino events. Remarkably, this collider data is also well suited to distinguish between currently viable light-gravitino scenarios, with striking implications for structure formation, inflation, and other early-Universe cosmology.

Publication: Physical Review D Vol.: 82 No.: 1 ISSN: 1550-7998

ID: CaltechAUTHORS:20100816-154924446

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Abstract: Over the past several decades, galaxy formation theory has met with significant successes. In order to test current theories thoroughly we require predictions for as yet unprobed regimes. To this end, we describe a new implementation of the Galform semi-analytic model of galaxy formation. Our motivation is the success of the model described by Bower et al. in explaining many aspects of galaxy formation. Despite this success, the Bower et al. model fails to match some observational constraints, and certain aspects of its physical implementation are not as realistic as we would like. The model described in this work includes substantially updated physics, taking into account developments in our understanding over the past decade, and removes certain limiting assumptions made by these (and most other) semi-analytic models. This allows it to be exploited reliably in high-redshift and low-mass regimes. Furthermore, we have performed an exhaustive search of model parameter space to find a particular set of model parameters which produce results in good agreement with a wide range of observational data (luminosity functions, galaxy sizes and dynamics, clustering, colours, metal content) over a wide range of redshifts. This model represents a solid basis on which to perform calculations of galaxy formation in as yet unprobed regimes.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 405 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20100714-143601301

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Abstract: Chiral gravity and cosmological birefringence both provide physical mechanisms to produce parity-violating TB and EB correlations in the cosmic microwave background (CMB) temperature/polarization. Here, we study how well these two mechanisms can be distinguished if nonzero TB/EB correlations are found. To do so, we evaluate the correlation matrix, including new TB-EB covariances. We find that the effects of these two mechanisms on the CMB are highly orthogonal, and can thus be distinguished fairly well in the case of a high-signal-to-noise detection of TB/EB correlations. Appendix evaluates the relative sensitivities of the BB, TB, and EB signals for detecting a chiral gravitational-wave background.

Publication: Physical Review D Vol.: 81 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:20100713-100910867

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Abstract: We use a model for the evolution of galaxies in the far-infrared (far-IR) based on the Λ-cold dark matter cosmology to make detailed predictions for the upcoming cosmological surveys with the Herschel Space Observatory. We use the combined GALFORM semi-analytical galaxy formation model and GRASIL spectrophotometric code to compute galaxy spectral energy distribution (SEDs) including the reprocessing of radiation by dust. The model, which is the same as that in Baugh et al., assumes two different initial mass functions (IMFs): a normal solar neighbourhood IMF for quiescent star formation in discs, and a very top-heavy IMF in starbursts triggered by galaxy mergers. We have shown previously that the top-heavy IMF appears necessary to explain the number counts and redshifts of faint submillimetre galaxies. In this paper, we present predictions for galaxy luminosity functions, number counts and redshift distributions in the Herschel imaging bands. We find that source confusion will be a serious problem in the deepest planned surveys. We also show predictions for physical properties such as star formation rates and stellar, gas and halo masses, together with fluxes at other wavelengths (from the far-ultraviolet to the radio) relevant for multi-wavelength follow-up observations. We investigate what fraction of the total IR emission from dust and of the high-mass star formation over the history of the Universe should be resolved by planned surveys with Herschel, and find a fraction ~30–50 per cent, depending on confusion. Finally, we show that galaxies in Herschel surveys should be significantly clustered.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 405 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:20100629-091542525

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Abstract: Recent observations of galaxy clusters have shown that environmental effects apparently associated with the cluster begin to lower the star formation rates of galaxies at distances as great as three times the cluster virial radius. These observations may indicate preprocessing of cluster galaxies in groups or in the cluster core for galaxies on highly elliptical orbits, but may also imply that the environmental effects due to the cluster are directly affecting galaxies on their first infall. To explore these issues, we investigate different models of ram pressure stripping (RPS) as it acts on satellite galaxies in clusters and compare to observations of the radial star formation gradient in clusters. We calculate the location of the accretion shock around model clusters and use this as the radius of onset of RPS in the GALFORM semi-analytic model of galaxy formation. Comparison of the results of our model and previously considered, simpler ram pressure models with recent observations indicates that current data are unable to strongly discriminate between models of RPS due to the complex interplay of preprocessing effects at work. However, future observations of a larger sample of clusters will likely be able to place stronger constraints on the process of RPS and its role in shaping radial trends in and around clusters.

Publication: Astrophysical Journal Vol.: 716 No.: 1 ISSN: 0004-637X

ID: CaltechAUTHORS:20100617-130622638

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Abstract: The Universe may harbor relics of the post-inflationary epoch in the form of a network of self-ordered scalar fields. Such fossils, while consistent with current cosmological data at trace levels, may leave too weak an imprint on the cosmic microwave background and the large-scale distribution of matter to allow for direct detection. The non-Gaussian statistics of the density perturbations induced by these fields, however, permit a direct means to probe for these relics. Here we calculate the bispectrum that arises in models of self-ordered scalar fields. We find a compact analytic expression for the bispectrum, evaluate it numerically, and provide a simple approximation that may be useful for data analysis. The bispectrum is largest for triangles that are aligned (have edges k_1≃2k_2≃2k_3) as opposed to the local-model bispectrum, which peaks for squeezed triangles (k_1≃k_2≫k_3), and the equilateral bispectrum, which peaks at k_1≃k_2≃k_3. We estimate that this non-Gaussianity should be detectable by the Planck satellite if the contribution from self-ordering scalar fields to primordial perturbations is near the current upper limit.

Publication: Physical Review D Vol.: 81 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:20100625-080640927

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Abstract: We consider models in which a dark-matter particle decays to a slightly less massive daughter particle and a noninteracting massless particle. The decay gives the daughter particle a small velocity kick. Self-gravitating dark-matter halos that have a virial velocity smaller than this velocity kick may be disrupted by these particle decays, while those with larger virial velocities will be heated. We use numerical simulations to follow the detailed evolution of the total mass and density profile of self-gravitating systems composed of particles that undergo such velocity kicks as a function of the kick speed (relative to the virial velocity) and the decay time (relative to the dynamical time). We show how these decays will affect the halo mass-concentration relation and mass function. Using measurements of the halo mass-concentration relation and galaxy-cluster mass function to constrain the lifetime–kick-velocity parameter space for decaying dark matter, we find roughly that the observations rule out the combination of kick velocities greater than 100 km s^(-1) and decay times less than a few times the age of the Universe.

Publication: Physical Review D Vol.: 81 No.: 10 ISSN: 2470-0010

ID: CaltechAUTHORS:20100624-142330707

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Abstract: Differences in masses inferred from dynamics, such as velocity dispersions or x rays, and those inferred from lensing are a generic prediction of modified gravity theories. Viable models, however, must include some nonlinear mechanism to restore general relativity (GR) in dense environments, which is necessary to pass Solar System constraints on precisely these deviations. In this paper, we study the dynamics within virialized structures in the context of two modified gravity models, f(R) gravity and Dvali-Gabadadze-Porrati (DGP). The nonlinear mechanisms to restore GR, which f(R) and DGP implement in very different ways, have a strong impact on the dynamics in bound objects; they leave distinctive signatures in the dynamical mass-lensing mass relation as a function of mass and radius. We present measurements from N-body simulations of f(R) and DGP, as well as semianalytical models that match the simulation results to surprising accuracy in both cases. The semianalytical models are useful for making the connection to observations. Our results confirm that the environment and scale dependence of the modified gravity effects have to be taken into account when confronting gravity theories with observations of dynamics in galaxies and clusters.

Publication: Physical Review D Vol.: 81 No.: 10 ISSN: 2470-0010

ID: CaltechAUTHORS:20100624-143823509

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Abstract: Primordial quantum fluctuations produced by inflation are conventionally assumed to be statistically homogeneous, a consequence of translational invariance. In this paper we quantify the potentially observable effects of a small violation of translational invariance during inflation, as characterized by the presence of a preferred point, line, or plane.We explore the imprint such a violation would leave on the cosmic microwave background anisotropy, and provide explicit formulas for the expected amplitudes <а_(lm)а^*_(l'm')> of the spherical-harmonic coefficients.

Publication: Physical Review D Vol.: 81 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:20100524-141735641

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Abstract: A long-range intergalactic force between dark matter (DM) particles, mediated by an ultralight scalar, is tightly constrained by galactic dynamics and large scale structure formation. We examine the implications of such a ‘‘dark force” for several terrestrial experiments, including Eötvös tests of the Weak Equivalence Principle (WEP), direct-detection DM searches, and collider studies. The presence of a dark force implies a nonvanishing effect in Eötvös tests that could be probed by current and future experiments depending on the DM model. For scalar DM that is a singlet under the standard model gauge groups, a dark force of astrophysically relevant magnitude is ruled out in large regions of parameter space by the DM relic density and WEP constraints. WEP tests also imply constraints on the Higgs-exchange contributions to the spin-independent (SI) DM-nucleus direct-detection cross section. For WIMP scenarios, these considerations constrain Higgs-exchange contributions to the SI cross section to be subleading compared to gauge-boson mediated contributions. In multicomponent DM scenarios, a dark force would preclude large shifts in the rate for Higgs decay to two photons associated with DM-multiplet loops that might otherwise lead to measurable deviations at the LHC or a future linear collider. The combination of observations from galactic dynamics, large scale structure formation, Eötvös experiments, DM-direct-detection experiments, and colliders can further constrain the size of new long-range forces in the dark sector.

Publication: Physical Review D Vol.: 81 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20100520-140754035

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Abstract: We present projected constraints on modified gravity models from the observational technique known as 21 cm intensity mapping, where cosmic structure is detected without resolving individual galaxies. The resulting map is sensitive to both baryon acoustic oscillations and weak lensing, two of the most powerful cosmological probes. It is found that a 200 m × 200 m cylindrical telescope, sensitive out to z=2.5, would be able to distinguish Dvali, Gabadadze, and Porrati from most dark energy models, and constrain the Hu and Sawicki f(R) model to |f_(R0)|<9×10^(-6) at 95% confidence. The latter constraint makes extensive use of the lensing spectrum in the nonlinear regime. These results show that 21 cm intensity mapping is not only sensitive to modifications of the standard model’s expansion history, but also to structure growth. This makes intensity mapping a powerful and economical technique, achievable on much shorter time scales than optical experiments that would probe the same era.

Publication: Physical Review D Vol.: 81 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20100521-091332397

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Abstract: We present a detailed study of the collapse of a spherical perturbation in Dvali-Gabadadze-Porrati (DGP) braneworld gravity for the purpose of modeling simulation results for the halo mass function, bias, and matter power spectrum. The presence of evolving modifications to the gravitational force in the form of the scalar brane-bending mode leads to qualitative differences to the collapse in ordinary gravity. In particular, differences in the energetics of the collapse necessitate a new, generalized method for defining the virial radius which does not rely on strict energy conservation. These differences and techniques apply to smooth dark energy models with w ≠ -1 as well. We also discuss the impact of the exterior of the perturbation on collapse quantities due to the lack of a Birkhoff theorem in DGP. The resulting predictions for the mass function, halo bias, and power spectrum are in good overall agreement with DGP N-body simulations on both the self-accelerating and normal branch. In particular, the impact of the Vainshtein mechanism as measured in the full simulations is matched well. The model and techniques introduced here can serve as practical tools for placing consistent constraints on braneworld models using observations of large-scale structure.

Publication: Physical Review D Vol.: 81 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20100517-101855245

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Abstract: The galform semi-analytic model of galaxy formation is used to explore the mechanisms primarily responsible for the three types of galaxies seen in the local universe: bulge, bulge+disc and disc, identified with the visual morphological types E, S0/a-Sbc and Sc-Scd, respectively. With a suitable choice of parameters the galform model can accurately reproduce the observed local K_s-band luminosity function (LF) for galaxies split by visual morphological type. The successful set of model parameters is used to populate the Millennium Simulation with 9.4 million galaxies and their dark matter haloes. The resulting catalogue is then used to explore the evolution of galaxies through cosmic history. The model predictions concur with recent observational results including the galaxy merger rate, the star formation rate and the seemingly antihierarchical evolution of ellipticals. However, the model also predicts significant evolution of the elliptical galaxy LF that is not observed. The discrepancy raises the possibility that samples of z~1 galaxies which have been selected using colour and morphological criteria may be contaminated with galaxies that are not actually ellipticals.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 402 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20100330-151338852

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Abstract: We study the effects of substructure on the rate of dark-matter annihilation in the Galactic halo. We use an analytic model for substructure that can extend numerical simulation results to scales too small to be resolved by the simulations. We first calibrate the analytic model to numerical simulations, and then determine the annihilation boost factor, for standard weakly interacting massive particle (WIMP) models as well as those with Sommerfeld (or other) enhancements, as a function of galactocentric radius in the Milky Way. We provide an estimate of the dependence of the gamma-ray intensity of WIMP annihilation as a function of angular distance from the Galactic center. This methodology, coupled with future numerical simulation results can be a powerful tool that can be used to constrain WIMP properties using Fermi all-sky data.

Publication: Physical Review D Vol.: 81 No.: 4 ISSN: 2470-0010

ID: CaltechAUTHORS:20100414-091218940

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Abstract: We explore the stability of domain wall and bubble solutions in theories with compact extra dimensions. The energy density stored inside of the wall can destabilize the volume modulus of a compactification, leading to solutions containing either a timelike singularity or a region where space decompactifies, depending on the metric ansatz. We determine the structure of such solutions both analytically and using numerical simulations, and analyze how they arise in compactifications of Einstein-Maxwell theory and type IIB string theory. The existence of instabilities has important implications for the formation of networks of topological defects and the population of vacua during eternal inflation.

Publication: Physical Review D Vol.: 81 No.: 4 ISSN: 2470-0010

ID: CaltechAUTHORS:20100413-111150524

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Abstract: We present new determinations of the cosmic expansion history from red-envelope galaxies. We have obtained for this purpose high-quality spectra with the Keck-LRIS spectrograph of red-envelope galaxies in 24 galaxy clusters in the redshift range 0.2 < z < 1.0. We complement these Keck spectra with high-quality, publicly available archival spectra from the SPICES and VVDS surveys. We improve over our previous expansion history measurements in Simon et al. (2005) by providing two new determinations of the expansion history: H(z) = 97±62 km sec^(−1) Mpc^(−1) at z ≃ 0.5 and H(z) = 90±40 km sec^(−1) Mpc^(−1) at z ≃ 0.9. We discuss the uncertainty in the expansion history determination that arises from uncertainties in the synthetic stellar-population models. We then use these new measurements in concert with cosmic-microwave-background (CMB) measurements to constrain cosmological parameters, with a special emphasis on dark-energy parameters and constraints to the curvature. In particular, we demonstrate the usefulness of direct H(z) measurements by constraining the dark-energy equation of state parameterized by w_0 and w_a and allowing for arbitrary curvature. Further, we also constrain, using only CMB and H(z) data, the number of relativistic degrees of freedom to be 4±0.5 and their total mass to be < 0.2 eV, both at 1σ.

Publication: Journal of Cosmology and Astroparticle Physics Vol.: 2010 No.: 2 ISSN: 1475-7516

ID: CaltechAUTHORS:20100603-113559863

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Abstract: We explore the feasibility and astrophysical consequences of a new long-range U(1) gauge field ("dark electromagnetism") that couples only to dark matter, not to the Standard Model. The dark matter consists of an equal number of positive and negative charges under the new force, but annihilations are suppressed if the dark matter mass is sufficiently high and the dark fine-structure constant α is sufficiently small. The correct relic abundance can be obtained if the dark matter also couples to the conventional weak interactions, and we verify that this is consistent with particle-physics constraints. The primary limit on a comes from the demand that the dark matter be effectively collisionless in galactic dynamics, which implies α ≾ 10^(-3) for TeV-scale dark matter. These values are easily compatible with constraints from structure formation and primordial nucleosynthesis. We raise the prospect of interesting new plasma effects in dark matter dynamics, which remain to be explored. This proceedings is based on the work presented originally in.(1)

ID: CaltechAUTHORS:20100715-111557235

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Abstract: We introduce a cosmological model based on the normal branch of Dvali-Gabadadze-Porrati (DGP) braneworld gravity with a smooth dark energy component on the brane. The expansion history in this model is identical to ΛCDM, thus evading all geometric constraints on the DGP crossover scale r_c. This well-defined model can serve as a first approximation to more general braneworld models whose cosmological solutions have not been obtained yet. We study the formation of large-scale structure in this model in the linear and nonlinear regime using N-body simulations for different values of r_c. The simulations use the code presented in 25 and solve the full nonlinear equation for the brane-bending mode in conjunction with the usual gravitational dynamics. The brane-bending mode is attractive rather than repulsive in the DGP normal branch, hence the sign of the modified gravity effects is reversed compared to those presented in 25. We compare the simulation results with those of ordinary ΛCDM simulations run using the same code and initial conditions. We find that the matter power spectrum in this model shows a characteristic enhancement peaking at k ~ 0.7h/Mpc. We also find that the abundance of massive halos is significantly enhanced. Other results presented here include the density profiles of dark matter halos, and signatures of the brane-bending mode self-interactions (Vainshtein mechanism) in the simulations. Independently of the expansion history, these results can be used to place constraints on the DGP model and future generalizations through their effects on the growth of cosmological structure.

Publication: Physical Review D Vol.: 80 No.: 12 ISSN: 1550-7998

ID: CaltechAUTHORS:20100119-135921946

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Abstract: We show that D-dimensional de Sitter space is unstable to the nucleation of non-singular geometries containing spacetime regions with different numbers of macroscopic dimensions, leading to a dynamical mechanism of compactification. These and other solutions to Einstein gravity with flux and a cosmological constant are constructed by performing a dimensional reduction under the assumption of q-dimensional spherical symmetry in the full D-dimensional geometry. In addition to the familiar black holes, black branes, and compactification solutions we identify a number of new geometries, some of which are completely non-singular. The dynamical compactification mechanism populates lower-dimensional vacua very differently from false vacuum eternal inflation, which occurs entirely within the context of four-dimensions. We outline the phenomenology of the nucleation rates, finding that the dimensionality of the vacuum plays a key role and that among vacua of the same dimensionality, the rate is highest for smaller values of the cosmological constant. We consider the cosmological constant problem and propose a novel model of slow-roll inflation that is triggered by the compactification process.

Publication: Journal of High Energy Physics Vol.: 11ISSN: 1126-6708

ID: CaltechAUTHORS:20100120-093214799

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Abstract: Taking the extremal limit of a non-extremal Reissner-Nordström black hole (by externally varying the mass or charge), the region between the inner and outer event horizons experiences an interesting fate — while this region is absent in the extremal case, it does not disappear in the extremal limit but rather approaches a patch of AdS_2 × S^2. In other words, the approach to extremality is not continuous, as the non-extremal Reissner-Nordström solution splits into two spacetimes at extremality: an extremal black hole and a disconnected AdS space. We suggest that the unusual nature of this limit may help in understanding the entropy of extremal black holes.

Publication: Journal of High Energy Physics Vol.: 2009 No.: 11 ISSN: 1126-6708

ID: CaltechAUTHORS:20100120-093837694

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Abstract: The discovery that the cosmic expansion is accelerating has been followed by an intense theoretical and experimental response in physics and astronomy. The discovery implies that our most basic notions about how gravity works are violated on cosmological distance scales. A simple fix is to introduce a cosmological constant into the field equations for general relativity. However, the extremely small value of the cosmological constant, relative to theoretical expectations, has led theorists to explore numerous alternative explanations that involve the introduction of an exotic negative-pressure fluid or a modification of general relativity. Here we review the evidence for cosmic acceleration. We then survey some of the theoretical attempts to account for it, including the cosmological constant, quintessence and its variants, mass-varying neutrinos, and modifications of general relativity. We discuss experimental and observational tests that may allow us to distinguish among some of the theoretical ideas that have been proposed.

Publication: Annual Review of Nuclear and Particle Science Vol.: 59ISSN: 0163-8998

ID: CaltechAUTHORS:20100111-100032725

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Abstract: If the hemispherical power asymmetry observed in the cosmic microwave background (CMB) on large angular scales is attributable to a superhorizon curvaton fluctuation, then the simplest model predicts that the primordial density fluctuations should be similarly asymmetric on all smaller scales. The distribution of high-redshift quasars was recently used to constrain the power asymmetry on scales k ≃ 1.5h Mpc^(-1), and the upper bound on the amplitude of the asymmetry was found to be a factor of 6 smaller than the amplitude of the asymmetry in the CMB. We show that it is not possible to generate an asymmetry with this scale dependence by changing the relative contributions of the inflaton and curvaton to the adiabatic power spectrum. Instead, we consider curvaton scenarios in which the curvaton decays after dark matter freezes out, thus generating isocurvature perturbations. If there is a superhorizon fluctuation in the curvaton field, then the rms amplitude of these perturbations will be asymmetric, and the asymmetry will be most apparent on large angular scales in the CMB. We find that it is only possible to generate the observed asymmetry in the CMB while satisfying the quasar constraint if the curvaton's contribution to the total dark matter density is small, but nonzero. The model also requires that the majority of the primordial power comes from fluctuations in the inflaton field. Future observations and analyses of the CMB will test this model because the power asymmetry generated by this model has a specific spectrum, and the model requires that the current upper bounds on isocurvature power are nearly saturated.

Publication: Physical Review D Vol.: 80 No.: 8 ISSN: 1550-7998

ID: CaltechAUTHORS:20091124-151956283

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Abstract: We consider the effects of Galactic substructure on energetic neutrinos from annihilation of weakly interacting massive particles that have been captured by the Sun and Earth. Substructure gives rise to a time-varying capture rate and thus to time variation in the annihilation rate and resulting energetic-neutrino flux. However, there may be a time lag between the capture and annihilation rates. The energetic-neutrino flux may then be determined by the density of dark matter in the Solar System's past trajectory, rather than the local density. The signature of such an effect may be sought in the ratio of the direct- to indirect-detection rates.

Publication: Physical Review Letters Vol.: 103 No.: 12 ISSN: 0031-9007

ID: CaltechAUTHORS:20091013-091141467

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Abstract: There is no direct evidence for radiation domination prior to big-bang nucleosynthesis, and so it is useful to consider how constraints to thermally-produced axions change in non-standard thermal histories. In the low-temperature-reheating scenario, radiation domination begins as late as ~1 MeV, and is preceded by significant entropy generation. Axion abundances are then suppressed, and cosmological limits to axions are significantly loosened. In a kination scenario, a more modest change to axion constraints occurs. Future possible constraints to axions and low-temperature reheating are discussed.

No.: 1166
ID: CaltechAUTHORS:20100804-142905281

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Abstract: The huge size and uniformity of the Sloan Digital Sky Survey (SDSS) make possible an exacting test of current models of galaxy formation. We compare the predictions of the galform semi-analytical galaxy formation model for the luminosities, morphologies, colours and scalelengths of local galaxies. galform models the luminosity and size of the disc and bulge components of a galaxy, and so we can compute quantities which can be compared directly with SDSS observations, such as the Petrosian magnitude and the Sérsic index. We test the predictions of two published models set in the cold dark matter cosmology: the Baugh et al. model, which assumes a top-heavy initial mass function (IMF) in starbursts and superwind feedback, and the Bower et al. model, which uses active galactic nucleus feedback and a standard IMF. The Bower et al. model better reproduces the overall shape of the luminosity function, the morphology–luminosity relation and the colour bimodality observed in the SDSS data, but gives a poor match to the size–luminosity relation. The Baugh et al. model successfully predicts the size–luminosity relation for late-type galaxies. Both models fail to reproduce the sizes of bright early-type galaxies. These problems highlight the need to understand better both the role of feedback processes in determining galaxy sizes, in particular the treatment of the angular momentum of gas reheated by supernovae, and the sizes of the stellar spheroids formed by galaxy mergers and disc instabilities.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 397 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20090827-150331186

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Abstract: Mechanisms have been proposed that might rotate the linear polarization of the cosmic microwave background (CMB) as it propagates from the surface of last scatter. In the simplest scenario, the rotation will be uniform across the sky, but the rotation angle may also vary across the sky. We develop in detail the complete set of full-sky quadratic estimators for the rotation of the CMB polarization that can be constructed from the CMB temperature and polarization. We derive the variance with which these estimators can be measured and show that these variances reduce to the simpler flat-sky expressions in the appropriate limit. We evaluate the variances numerically. While the flat-sky formalism may be suitable if the rotation angle arises as a realization of a random field, the full-sky formalism will be required to search for rotations that vary slowly across the sky as well as for models in which the angular power spectrum for the rotation angle peaks at large angles.

Publication: Physical Review D Vol.: 80 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:20090817-144813013

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Abstract: We consider a theory of gravity in which a symmetric two-index tensor in Minkowski spacetime acquires a vacuum expectation value (vev) via a potential, thereby breaking Lorentz invariance spontaneously. When the vev breaks all the generators of the Lorentz group, six Goldstone modes emerge, two linear combinations of which have properties that are identical to those of the graviton in general relativity. Integrating out massive modes yields an infinite number of Lorentz-violating radiative-correction terms in the low-energy effective Lagrangian. We examine a representative subset of these terms and show that they modify the dispersion relation of the two propagating graviton modes such that their phase velocity is direction dependent. If the phase velocity of the Goldstone gravitons is subluminal, cosmic rays can emit gravi-Cherenkov radiation, and the detection of high-energy cosmic rays can be used to constrain these radiative-correction terms. Test particles in the vicinity of the Goldstone gravitons undergo longitudinal oscillations in addition to the usual transverse oscillations as predicted by general relativity. Finally, we discuss the possibility of having vevs that do not break all six generators and examine in detail one such theory.

Publication: Physical Review D Vol.: 80 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:20090817-144813345

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Abstract: Unbound outflows in the form of highly collimated jets and broad winds appear to be a ubiquitous feature of accreting black hole systems. The most powerful jets are thought to derive a significant fraction, if not the majority, of their power from the rotational energy of the black hole. Whatever the precise mechanism that causes them, these jets must, therefore, exert a braking torque on the black hole. Consequently, we expect jet production to play a significant role in limiting the maximum spin attainable by accreting black holes. We calculate the spin-up function – the rate of change of black hole spin normalized to the black hole mass and accretion rate – for an accreting black hole, accounting for this braking torque. We assume that the accretion flow on to a Kerr black hole is advection-dominated (ADAF) and construct easy-to-use analytic fits to describe the global structure of such flows based on the numerical solutions of Popham & Gammie. We find that the predicted black hole spin-up function depends only on the black hole spin and dimensionless parameters describing the accretion flow. Using recent relativistic magnetohydrodynamical (MHD) numerical simulation results to calibrate the efficiency of angular momentum transfer in the flow, we find that an ADAF flow will spin a black hole up (or down) to an equilibrium value of about 96 per cent of the maximal spin value in the absence of jets. Combining our ADAF system with a simple model for jet power, we demonstrate that an equilibrium is reached at approximately 93 per cent of the maximal spin value, as found in the numerical simulation studies of the spin-up of accreting black holes, at which point the spin-up of the hole by accreted material is balanced by the braking torque arising from jet production. The existence of equilibrium spin means that optically dim active galactic nuclei (AGNs) that have grown via accretion from an advection-dominated flow will not be maximally rotating. It also offers a possible explanation for the tight correlation observed by Allen et al. between the Bondi accretion rate and jet power in nine, nearby, X-ray luminous giant elliptical galaxies. We suggest that the black holes in these galaxies must all be rotating close to their equilibrium value. Our model also yields a relationship between jet efficiency and black hole spin that is in surprisingly good agreement with that seen in the simulation studies, indicating that our simple model is a useful and convenient description of ADAF inflow – jet outflow about a spinning black hole for incorporation in semi-analytic modelling as well as cosmological numerical simulation studies focusing on the formation and evolution of galaxies, groups and clusters of galaxies.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 397 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20090821-142820846

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Abstract: A long-range fifth force coupled to dark matter can induce a coupling to ordinary matter if the dark matter interacts with standard model fields. We consider constraints on such a scenario from both astrophysical observations and laboratory experiments. We also examine the case where the dark matter is a weakly interacting massive particle, and derive relations between the coupling to dark matter and the coupling to ordinary matter for different models. Currently, this scenario is most tightly constrained by galactic dynamics, but improvements in Eötvös experiments can probe unconstrained regions of parameter space.

Publication: Physical Review Letters Vol.: 103 No.: 1 ISSN: 0031-9007

ID: CaltechAUTHORS:20090807-114119837

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Abstract: One of the targets of the recently launched Fermi Gamma-ray Space Telescope is a diffuse gamma-ray background from dark-matter annihilation or decay in the Galactic halo. N-body simulations and theoretical arguments suggest that the dark matter in the Galactic halo may be clumped into substructure, rather than smoothly distributed. Here we propose the gamma-ray-flux probability distribution function (PDF) as a probe of substructure in the Galactic halo. We calculate this PDF for a phenomenological model of halo substructure and determine the regions of the substructure parameter space in which the PDF may be distinguished from the PDF for a smooth distribution of dark matter. In principle, the PDF allows a statistical detection of substructure, even if individual halos cannot be detected. It may also allow detection of substructure on the smallest microhalo mass scales, ~ M_⊕, for weakly-interacting massive particles (WIMPs). Furthermore, it may also provide a method to measure the substructure mass function. However, an analysis that assumes a typical halo substructure model and a conservative estimate of the diffuse background suggests that the substructure PDF may not be detectable in the lifespan of Fermi in the specific case that the WIMP is a neutralino. Nevertheless, for a large range of substructure, WIMP annihilation, and diffuse background models, PDF analysis may provide a clear signature of substructure.

Publication: Journal of Cosmology and Astroparticle Physics Vol.: 2009 No.: 07 ISSN: 1475-7516

ID: CaltechAUTHORS:20090901-161606701

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Abstract: This paper is the third in a series investigating the possibility that if we reside in an inflationary “bubble universe,” we might observe the effects of collisions with other such bubbles. Here, we study the interior structure of a bubble collision spacetime, focusing on the issue of where observers can reside. Numerical simulations indicate that if the interbubble domain wall accelerates away, infinite spacelike surfaces of homogeneity develop to the future of the collision; this strongly suggests that observers can have collisions to their past, and previous results then imply that this is very likely. However, for observers at nearly all locations, the restoration of homogeneity relegates any observable effects to a vanishingly small region on the sky. We find that bubble collisions may also play an important role in defining measures in inflation: a potentially infinite relative volume factor arises between two bubble types depending on the sign of the acceleration of the domain wall between them; this may in turn correlate with observables such as the scale or type of inflation.

Publication: Physical Review D Vol.: 79 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:20090804-083645740

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Abstract: We consider a class of inflationary models with small oscillations imprinted on an otherwise smooth inflaton potential. These oscillations are manifest as oscillations in the power spectrum of primordial perturbations, which then give rise to oscillating departures from the standard cosmic microwave background power spectrum. We show that current data from the Wilkinson Microwave Anisotropy Probe constrain the amplitude of a sinusoidal variation in the inflaton potential to have an amplitude less than 3×10^(-5). We anticipate that the smallest detectable such oscillations in Planck will be roughly an order of magnitude smaller, with slight improvements possible with a post-Planck cosmic-variance limited experiment.

Publication: Physical Review D Vol.: 79 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:20090901-094341714

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Abstract: We investigate the stability of theories in which Lorentz invariance is spontaneously broken by fixed-norm vector “aether” fields. Models with generic kinetic terms are plagued either by ghosts or by tachyons, and are therefore physically unacceptable. There are precisely three kinetic terms that are not manifestly unstable: a sigma model (∂_µA_ν)^2, the Maxwell Lagrangian F_µνF^µν, and a scalar Lagrangian (∂_µA^µ)^2. The timelike sigma-model case is well defined and stable when the vector norm is fixed by a constraint; however, when it is determined by minimizing a potential there is necessarily a tachyonic ghost, and therefore an instability. In the Maxwell and scalar cases, the Hamiltonian is unbounded below, but at the level of perturbation theory there are fewer degrees of freedom and the models are stable. However, in these two theories there are obstacles to smooth evolution for certain choices of initial data.

Publication: Physical Review D Vol.: 79 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20090508-120529180

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Abstract: Theories of low-energy Lorentz violation by a fixed-norm “aether” vector field with two-derivative kinetic terms have a globally bounded Hamiltonian and are perturbatively stable only if the vector is timelike and the kinetic term in the action takes the form of a sigma model. Here we investigate the phenomenological properties of this theory. We first consider the propagation of modes in the presence of gravity and show that there is a unique choice of curvature coupling that leads to a theory without superluminal modes. Experimental constraints on this theory come from a number of sources, and we examine bounds in a two-dimensional parameter space. We then consider the cosmological evolution of the aether, arguing that the vector will naturally evolve to be orthogonal to constant-density hypersurfaces in a Friedmann-Robertson-Walker cosmology. Finally, we examine cosmological evolution in the presence of an extra compact dimension of space, concluding that a vector can maintain a constant projection along the extra dimension in an expanding universe only when the expansion is exponential.

Publication: Physical Review D Vol.: 79 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:20090429-113052329

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Abstract: Only by incorporating various forms of feedback can theories of galaxy formation reproduce the present-day luminosity function of galaxies. It has also been argued that such feedback processes might explain the counterintuitive behaviour of 'downsizing' witnessed since redshifts z ≃ 1–2. To examine this question, observations spanning 0.4 < z < 1.4 from the Deep Extragalactic Evolutionary Probe (DEEP)2/Palomar survey are compared with a suite of equivalent mock observations derived from the Millennium Simulation, populated with galaxies using the galform code. Although the model successfully reproduces the observed total mass function and the general trend of 'downsizing', it fails to accurately reproduce the colour distribution and type-dependent mass functions at all redshifts probed. This failure is shared by other semi-analytical models which collectively appear to 'over-quench' star formation in intermediate-mass systems. These mock lightcones are also a valuable tool for investigating the reliability of the observational results in terms of cosmic variance. Using variance estimates derived from the lightcones, we confirm the significance of the decline since z ∼ 1 in the observed number density of massive blue galaxies which, we argue, provides the bulk of the associated growth in the red sequence. We also assess the limitations arising from cosmic variance in terms of our ability to observe mass-dependent growth since z ∼ 1.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 393 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20091006-151109583

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Abstract: We study the effect of a close encounter of a passing star on the shape of the inner Oort Cloud, using the impulse approximation. The deviation of the perturbed Oort Cloud from sphericity adds angular fluctuations to the brightness of the cosmic microwave background (CMB) due to thermal emission by the comets. An encounter with a solar-mass star at an impact parameter of 2100AU, as expected based on the abundance and velocity dispersion of stars in the solar neighborhood, leads to a quadrupole moment C-2 = 4.5(3.5) x 10^(-15) 6.7(1.1) x 10^(-12), 1.1 (0.11) x 10^(-9) at v = 30, 353, 545 GHz, respectively in intensity and (temperature) fluctuations. We also quantify the quadrupole spectral distortions produced by the Scattered Disc, which will exist irregardless of any perturbation and the subsequent shape of the Oort Cloud. For comparison, the temperature fluctuation quadrupole moment predicted by the current cosmological model is C-2 = 1.76 x 10^(-10), which corresponds to fluctuation in the CMB intensity of C-2 = 2.9 x 10^(-10), 6.8 x 10^(-9), 1.6 x 10^(-8) at v = 30, 353, 545 GHz. Finally, we discuss how a measurement of the anisotropic spectral distortions could be used to constrain the trajectory of the closest stellar fly-by.

Publication: New Astronomy Vol.: 14 No.: 2 ISSN: 1384-1076

ID: CaltechAUTHORS:BABna09

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Abstract: We explore the feasibility and astrophysical consequences of a new long-range U(1) gauge field (“dark electromagnetism”) that couples only to dark matter, not to the standard model. The dark matter consists of an equal number of positive and negative charges under the new force, but annihilations are suppressed if the dark-matter mass is sufficiently high and the dark fine-structure constant α^ is sufficiently small. The correct relic abundance can be obtained if the dark matter also couples to the conventional weak interactions, and we verify that this is consistent with particle-physics constraints. The primary limit on alpha^ comes from the demand that the dark matter be effectively collisionless in galactic dynamics, which implies α^<~10^-3 for TeV-scale dark matter. These values are easily compatible with constraints from structure formation and primordial nucleosynthesis. We raise the prospect of interesting new plasma effects in dark-matter dynamics, which remain to be explored.

Publication: Physical Review D Vol.: 79 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:ACKprd09

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Abstract: We use a Press-Schechter-like calculation to study how the abundance of voids changes in models with non-Gaussian initial conditions. While a positive skewness increases the cluster abundance, a negative skewness does the same for the void abundance. We determine the dependence of the void abundance on the non-Gaussianity parameter fnl for the local-model bispectrum—which approximates the bispectrum in some multi-field inflation models—and for the equilateral bispectrum, which approximates the bispectrum in single-field slow-roll inflation and in string-inspired DBI models of inflation. We show that the void abundance in large-scale-structure surveys currently being considered should probe values as small as fnl ≾ 10 and fnleq ≾ 30, over distance scales ~ 10 Mpc.

Publication: Journal of Cosmology and Astroparticle Physics Vol.: 2009 No.: 01 ISSN: 1475-7516

ID: CaltechAUTHORS:KAMjcap09

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Abstract: Recently, the DAMA/LIBRA Collaboration has repeated and reinforced their claim to have detected an annual modulation in their signal rate, and have interpreted this observation as evidence for dark-matter particles at the 8.2σ confidence level. Furthermore, it has also been noted that the effects of channeling may enable a weakly interacting massive particle (WIMP) that scatters elastically via spin-independent interactions from nuclei to produce the signal observed by DAMA/LIBRA without exceeding the limits placed by CDMS, XENON, CRESST, CoGeNT, and other direct-detection experiments. To accommodate this elastic-scattering explanation, however, the mass of the responsible dark-matter particle must be relatively light, mDM ≾ 10 GeV. Such dark-matter particles will become captured by and annihilate in the Sun at very high rates, leading to a potentially large flux of GeV-scale neutrinos. We calculate the neutrino spectrum resulting from WIMP annihilations in the Sun and show that existing limits from Super-Kamiokande can be used to close a significant portion of the DAMA region, especially if the dark-matter particles produce tau leptons or neutrinos in a sizable fraction of their annihilations. We also determine the spin-dependent WIMP-nuclei elastic-scattering parameter space consistent with DAMA. The constraints from Super-Kamiokande on the spin-dependent scenario are even more severe—they exclude any self-annihilating WIMP in the DAMA region that annihilates 1% of the time or more to any combination of neutrinos, tau leptons, or charm or bottom quarks.

Publication: Physical Review D Vol.: 79 No.: 1 ISSN: 2470-0010

ID: CaltechAUTHORS:20090413-094317493

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Abstract: Recent studies have considered modifications to the standard weakly interacting massive particle scenario in which the pair annihilation cross section (times relative velocity v) is enhanced by a factor 1/v~10^-3 in the Galaxy, enough to explain several puzzling Galactic radiation signals. We show that in these scenarios a burst of weakly interacting massive particle annihilation occurs in the first collapsed dark-matter halos. We show that severe constraints to the annihilation cross section derive from measurements of the diffuse extragalactic radiation and from ionization and heating of the intergalactic medium.

Publication: Physical Review Letters Vol.: 101 No.: 26 ISSN: 0031-9007

ID: CaltechAUTHORS:KAMprl08

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Abstract: Measurements of cosmic microwave background temperature fluctuations by the Wilkinson Microwave Anisotropy Probe indicate that the fluctuation amplitude in one half of the sky differs from the amplitude in the other half. We show that such an asymmetry cannot be generated during single-field slow-roll inflation without violating constraints to the homogeneity of the Universe. In contrast, a multifield inflationary theory, the curvaton model, can produce this power asymmetry without violating the homogeneity constraint. The mechanism requires the introduction of a large-amplitude superhorizon perturbation to the curvaton field, possibly a preinflationary remnant or a superhorizon curvaton-web structure. The model makes several predictions, including non-Gaussianity and modifications to the inflationary consistency relation, that will be tested with forthcoming cosmic microwave background experiments.

Publication: Physical Review D Vol.: 78 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:ERIprd08b

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Abstract: We construct domain walls and instantons in a class of models with coupled scalar fields, determining, in agreement with previous studies, that many such solutions contain naked timelike singularities. Vacuum bubble solutions of this type do not contain a region of true vacuum, obstructing the ability of eternal inflation to populate other vacua. We determine a criterion that potentials must satisfy to avoid the existence of such singularities and show that many domain wall solutions in type IIB string theory are singular.

Publication: Physical Review D Vol.: 78 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:JOHprd08b

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Abstract: We present a method for extracting the expected cosmological 21-cm signal from the epoch of re-ionization, taking into account contaminating radiations and random instrumental noise. The method is based on the maximum a posteriori probability (MAP) formalism and employs the coherence of the contaminating radiation along the line-of-sight and the three-dimensional correlations of the cosmological signal. We test the method using a detailed and comprehensive modelling of the cosmological 21-cm signal and the contaminating radiation. The signal is obtained using a high-resolution N-body simulation where the gas is assumed to trace the dark matter and is re-ionized by stellar radiation computed from semi-analytic galaxy formation recipes. We model contaminations to the cosmological signal from synchrotron and free–free galactic foregrounds and extragalactic sources including active galactic nuclei, radio haloes and relics, synchrotron and free–free emission from star-forming galaxies and free–free emission from dark matter haloes and the intergalactic medium. We provide tests of the reconstruction method for several rms values of instrumental noise from σN= 1 to 250 mK. For low instrumental noise, the recovered signal, along individual lines-of-sight, fits the true cosmological signal with a mean rms difference of drms≈ 1.7 ± 0.6 for σN= 1 mK, and drms≈ 4.2 ± 0.4 for σN= 5 mK. The one-dimensional power spectrum is nicely reconstructed for all values of σN considered here, while the reconstruction of the two-dimensional power spectrum and the Minkowski functionals is good only for noise levels of the order of few mK.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 391 No.: 1 ISSN: 0035-8711

ID: CaltechAUTHORS:GLEmnras08

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Abstract: One of the goals of NASA's Fermi Gamma-ray Space Telescope (formerly GLAST) will be the detection of gamma rays from dark-matter annihilation in the Galactic halo. Theoretical arguments suggest that dark matter may be bound into subhalos with masses as small as 10^-4–10^2M⊕. If so, it may be possible to detect individual subhalos as point sources in the Fermi Telescope. It has further been argued that some of these point sources may exhibit proper motions. Here we show that upper limits to the diffuse gamma-ray background constrain the number of subhalos close enough to exhibit proper motions to be less than one.

Publication: Physical Review D Vol.: 78 No.: 10 ISSN: 2470-0010

ID: CaltechAUTHORS:ANDprd08

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Abstract: Only ~10 per cent of baryons in the Universe are in the form of stars, yet most models of luminous structure formation have concentrated on the properties of the luminous stellar matter. Such models are now largely successful at reproducing the observed properties of galaxies, including the galaxy luminosity function and the star formation history of the universe. In this paper we focus on the 'flip side' of galaxy formation and investigate the properties of the material that is not presently locked up in galaxies. This 'by-product' of galaxy formation can be observed as an X-ray emitting plasma [the intracluster medium (ICM)] in groups and clusters. Since much of this material has been processed through galaxies, observations of the ICM represent an orthogonal set of constraints on galaxy formation models. In this paper, we attempt to self-consistently model the formation of galaxies and the heating of the ICM. We set out the challenges for such a combined model and demonstrate a possible means of bringing the model into line with both sets of constraints. In this paper, we present a version of the Durham semi-analytic galaxy formation model GALFORM that allows us to investigate the properties of the ICM. As we would expect on the basis of gravitational scaling arguments, the previous model fails to reproduce even the most basic observed properties of the ICM. We present a simple modification to the model to allow for heat input into the ICM from the active galactic nucleus (AGN) 'radio-mode' feedback. This heating acts to expel gas from the X-ray luminous central regions of the host halo. With this modification, the model reproduces the observed gas mass fractions and luminosity–temperature (L–T) relation of groups and clusters. In contrast to simple 'pre-heating' models of the ICM, the model predicts mildly positive evolution of the L–T relation, particularly at low temperatures. The model is energetically plausible, but seems to exceed the observed heating rates of intermediate-temperature clusters. Introducing the heating process into the model requires changes to a number of model parameters in order to retain a good match to the observed galaxy properties. With the revised parameters, the best-fitting luminosity function is comparable to that presented in Bower et al. The new model makes a fundamental step forward, providing a unified model of galaxy and cluster ICM formation. However, the detailed comparison with the data is not completely satisfactory, and we highlight key areas for improvement.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 390 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:BOWmnras08

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Abstract: We investigate field dynamics and tunneling between metastable minima in a landscape of type IIB flux compactifications, utilizing monodromies of the complex structure moduli space to continuously connect flux vacua. After describing the generic features of a flux-induced potential for the complex structure and type IIB axiodilaton, we specialize to the mirror quintic Calabi-Yau to obtain an example landscape. Studying the cosmological dynamics of the complex structure moduli, we find that the potential generically does not support slow-roll inflation and that in general the landscape separates neatly into basins of attraction of the various minima. We then discuss tunneling, with the inclusion of gravitational effects, in many-dimensional field spaces. A set of constraints on the form of the Euclidean paths through field space are presented, and then applied to construct approximate instantons mediating the transition between de Sitter vacua in the flux landscape. We find that these instantons are generically thick wall and that the tunneling rate is suppressed in the large-volume limit. We also consider examples where supersymmetry is not broken by fluxes, in which case near–Bogomolnyi-Prasad-Sommerfeld thin-wall bubbles can be constructed. We calculate the bubble-wall tension, finding that it scales like a D- or NS-brane bubble, and comment on the implications of this correspondence. Finally, we present a brief discussion of eternal inflation in the flux landscape.

Publication: Physical Review D Vol.: 78 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:JOHprd08a

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Abstract: Inflation predicts a stochastic background of gravitational waves over a broad range of frequencies, from those accessible with cosmic microwave background (CMB) measurements, to those accessible directly with gravitational-wave detectors, like NASA's Big-Bang Observer (BBO), currently under study. In a previous paper [Phys. Rev. D 73, 023504 (2006)] we connected CMB constraints to the amplitude and tensor spectral tilt of the inflationary gravitational-wave background (IGWB) at BBO frequencies for four classes of models of inflation by directly solving the inflationary equations of motion. Here we extend that analysis by including results obtained in the Wilkinson Microwave Anisotropy Probe third-year data release as well as by considering two additional classes of inflationary models. As often noted in the literature, the recent indication that the primordial density power spectrum has a red spectral index implies (with some caveats) that the amplitude of the IGWB may be large enough to be observable in the CMB polarization. Here we also explore the implications for the direct detection of the IGWB.

Publication: Physical Review D Vol.: 78 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:SMIprd08b

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Abstract: Superhorizon perturbations induce large-scale temperature anisotropies in the cosmic microwave background (CMB) via the Grishchuk-Zel'dovich effect. We analyze the CMB temperature anisotropies generated by a single-mode adiabatic superhorizon perturbation. We show that an adiabatic superhorizon perturbation in a LambdaCDM universe does not generate a CMB temperature dipole, and we derive constraints to the amplitude and wavelength of a superhorizon potential perturbation from measurements of the CMB quadrupole and octupole. We also consider constraints to a superhorizon fluctuation in the curvaton field, which was recently proposed as a source of the hemispherical power asymmetry in the CMB.

Publication: Physical Review D Vol.: 78 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:ERIprd08

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Abstract: Current models of galaxy formation predict satellite galaxies in groups and clusters that are redder than observed. We investigate the effect on the colours of satellite galaxies produced by the ram-pressure stripping of their hot-gaseous atmospheres as the satellites orbit within their parent halo. We incorporate a model of the stripping process based on detailed hydrodynamic simulations within the Durham semi-analytic model of galaxy formation. The simulations show that the environment in groups and clusters is less aggressive than previously assumed. The main uncertainty in the model is the treatment of gas expelled by supernovae. With reasonable assumptions for the stripping of this material, we find that satellite galaxies are able to retain a significant fraction of their hot gas for several Gyr, thereby replenishing their reservoirs of cold, star-forming gas and remaining blue for a relatively long period of time. A bimodal distribution of galaxy colours, similar to that observed in Sloan Digital Sky Survey data, is established and the colours of the satellite galaxies are in good agreement with the data. In addition, our model naturally accounts for the observed dependence of satellite colours on environment, from small groups to high-mass clusters.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 389 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:FONmnras08

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Abstract: Coherent oscillations of a scalar field can mimic the behavior of a perfect fluid with an equation-of-state parameter determined by the properties of the potential, possibly driving accelerated expansion in the early Universe (inflation) and/or in the Universe today (dark energy) or behaving as dark matter. We consider the growth of inhomogeneities in such a field, mapping the problem to that of two coupled anharmonic oscillators. We provide a simple physical argument that oscillating fields with a negative equation-of-state parameter possess a large-scale dynamical instability to growth of inhomogeneities. This instability renders these models unsuitable for explaining cosmic acceleration. We then consider the gravitational instability of oscillating fields in potentials that are close to, but not precisely, harmonic. We use these results to show that if axions make up the dark matter, then the small-scale cutoff in the matter power spectrum is around 10^-15M⊕.

Publication: Physical Review D Vol.: 78 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:JOHprd08

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Abstract: We place additional constraints on the three parameters of the dark matter halo merger rate function recently proposed by Parkinson, Cole & Helly by utilizing Smoluchowski's coagulation equation, which must be obeyed by any binary merging process which conserves mass. We find that the constraints from Smoluchowski's equation are degenerate, limiting to a thin plane in the three-dimensional parameter space. This constraint is consistent with those obtained from fitting to N-body measures of progenitor mass functions, and provides a better match to the evolution of the overall dark matter halo mass function, particularly for the most massive haloes. We demonstrate that the proposed merger rate function does not permit an exact solution of Smoluchowski's equation and, therefore, the choice of parameters must reflect a compromise between fitting various parts of the mass function. The techniques described herein are applicable to more general merger rate functions, which may permit a more accurate solution of Smoluchowski's equation. The current merger rate solutions are most probably sufficiently accurate for the vast majority of applications.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 388 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:BENmnras08

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Abstract: In this paper we analyze the biasing effect of point sources, either thermal Sunyaev-Zeldovich clusters or standard radio sources, on the estimated strength of the non-Gaussianity in the cosmic microwave background (CMB). We show that the biggest contribution comes from the cross correlation of the CMB with the matter density rather than from the Poisson term which is conventionally assumed in these calculations. For the three year WMAP data, we estimate that point sources could produce a non-Gaussian signature equivalent to a bias in fNL of 0.35, 0.24, -0.097, -0.13 in the Ka, Q, V, and W bands, respectively. The level of bias we find is largely insufficient to explain the very high fNL values recently detected by Yadav and Wandelt. For Planck, we estimate the point source bispectra to contaminate the fNL estimator with a bias of 1.3, 0.34, -0.25, -0.48 at 30, 44, 70, 100 GHz, respectively. These results depend on the assumed redshift distribution of the point sources. However, given the projected Planck sensitivity of DeltafNL~=5 (95% C.L.), a good estimate of point sources' properties including their number density and redshift distribution is essential before deriving strong conclusions on primordial non-Gaussianity.

Publication: Physical Review D Vol.: 77 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:BABprd08

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Abstract: We assess the effects of a collision between two vacuum bubbles in the thin-wall limit. After describing the outcome of a generic collision possessing the expected hyperbolic symmetry, we focus on collisions experienced by a bubble containing positive vacuum energy, which could in principle contain our observable universe. We provide criteria governing whether the post-collision domain wall accelerates towards or away from this observation bubble, and discuss the implications for observers located at various positions inside of the bubble. Then, we identify the class of solutions which have minimal impact on the interior of the observation bubble, and derive a simple formula for the energy density of a shell of radiation emitted from such a collision. In the context of a universe undergoing false-vacuum eternal inflation, these solutions are perhaps the most promising candidates for collisions that could exist within our past light cone, and therefore in principle be observable.

Publication: Physical Review D Vol.: 77 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:AGUprd08

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Abstract: Luminous red galaxies (LRGs) are much rarer and more massive than L* galaxies. Coupled with their extreme colours, LRGs therefore provide a demanding testing ground for the physics of massive galaxy formation. We present the first self-consistent predictions for the abundance and properties of LRGs in hierarchical structure formation models. We test two published models which use quite different mechanisms to suppress the formation of massive galaxies: the Bower et al. model which invokes 'active galactic nuclei (AGN) feedback' to prevent gas from cooling in massive haloes and the Baugh et al. model which relies upon a 'superwind' to eject gas before it is turned into stars. Without adjusting any parameters, the Bower et al. model gives an excellent match to the observed luminosity function of LRGs in the Sloan Digital Sky Survey (with a median redshift of z=0.24 ) and to their clustering; the Baugh et al. model is less successful in these respects. Both models fail to match the observed abundance of LRGs at z= 0.5 to better than a factor of ≈2. In the models, LRGs are typically bulge-dominated systems with stellar masses of ≈2 × 10^11 h^−1 M⊙ and velocity dispersions of σ∼ 250 km s^−1 . Around half of the stellar mass in the model LRGs is already formed by z∼ 2.2 and is assembled into one main progenitor by z∼ 1.5; on average, only 25 per cent of the mass of the main progenitor is added after z∼ 1. LRGs are predicted to be found in a wide range of halo masses, a conclusion which relies on properly taking into account the scatter in the formation histories of haloes. Remarkably, we find that the correlation function of LRGs is predicted to be a power law down to small pair separations, in excellent agreement with observational estimates. Neither the Bower et al. nor the Baugh et al. model is able to reproduce the observed radii of LRGs.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 386 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:ALMmnras08

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Abstract: We study the effects of substructure in the Galactic halo on direct detection of dark matter, on searches for energetic neutrinos from weakly interacting massive particles (WIMP) annihilation in the Sun and Earth, and on the enhancement in the WIMP annihilation rate in the halo. Our central result is a probability distribution function (PDF) P(rho) for the local dark-matter density. This distribution must be taken into account when using null dark-matter searches to constrain the properties of dark-matter candidates. We take two approaches to calculating the PDF. The first is an analytic model that capitalizes on the scale-invariant nature of the structure-formation hierarchy in order to address early stages in the hierarchy (very small scales; high densities). Our second approach uses simulation-inspired results to describe the PDF that arises from lower-density larger-scale substructures which formed in more recent stages in the merger hierarchy. The distributions are skew positive, and they peak at densities lower than the mean density. The local dark-matter density may be as small as 1/10th the canonical value of ~=0.4 GeV cm^-3, but it is probably no less than 0.2 GeV cm^-3.

Publication: Physical Review D Vol.: 77 No.: 10 ISSN: 2470-0010

ID: CaltechAUTHORS:KAMprd08

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Abstract: It is usually assumed that dark matter is produced during the radiation-dominated era. There is, however, no direct evidence for radiation domination prior to big-bang nucleosynthesis. Two nonstandard thermal histories are considered. In one, the low-temperature-reheating scenario, radiation domination begins as late as ~1 MeV, and is preceded by significant entropy generation. Thermal axion relic abundances are then suppressed, and cosmological limits to axions are loosened. For reheating temperatures Trh<~35 MeV, the large-scale structure limit to the axion mass is lifted. The remaining constraint from the total density of matter is significantly relaxed. Constraints are also relaxed for higher reheating temperatures. In a kination scenario, a more modest change to cosmological axion constraints is obtained. Future possible constraints to axions and low-temperature reheating from the helium abundance and next-generation large-scale-structure surveys are discussed.

Publication: Physical Review D Vol.: 77 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:GRIprd08c

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Abstract: We study the implications of recent indications from the Wilkinson Microwave Anisotropy Probe (WMAP) and other cosmological data for a red spectrum of primordial density perturbations for the detection of inflationary gravitational waves (IGWs) with forthcoming cosmic microwave background experiments. We consider a variety of single-field power-law, chaotic, spontaneous symmetry-breaking and Coleman–Weinberg inflationary potentials which are expected to provide a sizable tensor component and quantify the expected tensor-to-scalar ratio given existing constraints from WMAP on the tensor-to-scalar ratio and the power spectrum tilt. We discuss the ability of the near-future Planck satellite to detect the IGW background in the framework of those models. We find that the proposed satellite missions of the Cosmic Vision and Inflation Probe programs will be able to detect IGWs from all the models we have surveyed at better than 5σ confidence level. We also provide an example of what is required if the IGW background is to remain undetected even by these latter experiments.

Publication: Journal of Cosmology and Astroparticle Physics Vol.: 2008 No.: 04 ISSN: 1475-7516

ID: CaltechAUTHORS:PAGjcap08

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Abstract: There has been growing interest in the possibility of testing more precisely the assumption of statistical isotropy of primordial density perturbations. If it is to be tested with galaxy surveys at distance scales <~10 Mpc, then nonlinear evolution of anisotropic power must be understood. To this end, we calculate the angular dependence of the power spectrum to third order in perturbation theory for a primordial power spectrum with a quadrupole dependence on the wave vector direction. Our results suggest that primordial power anisotropies will be suppressed by <~7% in the quasilinear regime. We also show that the skewness in the statistically anisotropic theory differs by no more than 1% from that in the isotropic theory.

Publication: Physical Review Letters Vol.: 100 No.: 7 ISSN: 0031-9007

ID: CaltechAUTHORS:ANDprl08

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Abstract: One of the possible low-energy consequences of string theory is the addition of a Chern-Simons term to the standard Einstein-Hilbert action of general relativity. It can be argued that the quintessence field should couple to this Chern-Simons term, and if so, it drives in the linearized theory a parity-violating interaction between the gravito-electric and gravitomagnetic fields. In this paper, the linearized spacetime for Chern-Simons gravity around a massive spinning body is found to include new modifications to the gravitomagnetic field that have not appeared in previous work. The orbits of test bodies and the precession of gyroscopes in this spacetime are calculated, leading to new constraints on the Chern-Simons parameter space due to current satellite experiments.

Publication: Physical Review D Vol.: 77 No.: 2 ISSN: 2470-0010

ID: CaltechAUTHORS:SMIprd08

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Abstract: The formation of galactic discs and the efficiency of star formation within them are issues central to our understanding of galaxy formation. We have developed a detailed and versatile model of disc formation which combines the strengths of previous studies of isolated discs with those of hierarchical galaxy formation models. Disc structure is inferred from the distribution of angular momentum in hot halo gas and the hierarchical build-up of dark matter, leading to theoretically generated systems where the evolution of surface density, rotation, velocity dispersion, stability and metallicity is predicted for annular regions of width 20-100 pc. The model will be used to establish whether the accepted theory of large-scale structure formation in the universe is consistent with observed trends in the properties of disc galaxies. This first paper explicitly examines the importance of embedding such calculations within a merging hierarchy of dark matter haloes, finding that this leads to dramatically different formation histories compared to models in which discs grow in isolation. Different models of star formation are explored, and are found to have only a secondary influence on the properties of the resulting galaxy discs, the main governing factor being the infalling gas supply from the hot halo.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 382 No.: 2 ISSN: 0035-8711

ID: CaltechAUTHORS:STRImnras07

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Abstract: Statistical isotropy of primordial perturbations is a common assumption in cosmology, but it is an assumption that should be tested. To this end, we develop cosmic microwave background statistics for a primordial power spectrum that depends on the direction, as well as the magnitude, of the Fourier wave vector. We first consider a simple estimator that searches in a model-independent way for anisotropy in the square of the temperature (and/or polarization) fluctuation. We then construct the minimum-variance estimators for the coefficients of a spherical-harmonic expansion of the directional dependence of the primordial power spectrum. To illustrate, we apply these statistics to an inflation model with a quadrupole dependence of the primordial power spectrum on direction and find that a power quadrupole as small as 2.0% can be detected with the Planck satellite.

Publication: Physical Review D Vol.: 76 No.: 10 ISSN: 2470-0010

ID: CaltechAUTHORS:PULprd07b

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Abstract: We discuss two new observational techniques that use observations of the cosmic microwave background (CMB) to place constraints on the mass, distance, and size distribution of small objects in the Kuiper Belt and inner Oort Cloud, collectively known as trans-Neptunian objects (TNOs). The first new technique considers the spectral distortion of the isotropic, or monopole, CMB by TNOs that have been heated by solar radiation to temperatures above that of the CMB. We apply this technique to the spectral measurements of the CMB by the Far Infrared Absolute Spectrophotometer on the Cosmic Background Explorer. The second technique utilizes the change in amplitude of the TNO signal due to the orbital motion of the observer to separate the TNO signal from the invariant extragalactic CMB and construct a map of the mass distribution in the outer solar system. We estimate the ability of future CMB experiments to create such a map.

Publication: Astrophysical Journal Vol.: 669 No.: 2 ISSN: 0004-637X

ID: CaltechAUTHORS:20091014-105356620

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Abstract: If dark matter decays to electromagnetically interacting particles, it can inject energy into the baryonic gas and thus affect the processes of recombination and reionization. This leaves an imprint on the cosmic microwave background (CMB): the large-scale polarization is enhanced, and the small-scale temperature fluctuation is damped. We use the Wilkinson Microwave Anisotropy Probe (WMAP) three-year data combined with galaxy surveys to constrain radiatively decaying dark matter. Our new limits to the dark-matter decay width are about 10 times stronger than previous limits. For dark-matter lifetimes that exceed the age of the Universe, a limit of zetaGammachi<1.7×10^-25 s^-1 (95% C.L.) is derived, where zeta is the efficiency of converting decay energy into ionization energy. Limits for lifetimes short compared with the age of the Universe are also derived. We forecast improvements expected from the Planck satellite.

Publication: Physical Review D Vol.: 76 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:ZHAprd07

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Abstract: We search data from the Energetic Gamma Ray Experiment Telescope (EGRET) for a gamma-ray line in the energy range 0.1–10 GeV from the 10°×10° region around the Galactic center. Our null results lead to upper limits to the line flux from the Galactic center. Such lines may have appeared if the dark matter in the Galactic halo is composed of weakly interacting massive particles (WIMPs) in the mass range 0.1–10 GeV. For a given dark-matter-halo model, our null search translates to upper limits to the WIMP two-photon annihilation cross section as a function of WIMP mass. We show that, for a toy model in which Majorana WIMPs in this mass range annihilate only to electron-positron pairs, these upper limits supersede those derived from measurements of the 511-keV line and continuum photons from internal bremsstrahlung at the Galactic center.

Publication: Physical Review D Vol.: 76 No.: 6 ISSN: 2470-0010

ID: CaltechAUTHORS:PULprd07c

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Abstract: We present the redshift distribution of the Submillimetre Common-User Bolometer Array (SCUBA) Half Degree Survey (SHADES) galaxy population based on the rest-frame radio–mm–far-infrared (FIR) colours of 120 robustly detected 850 μm sources in the Lockman Hole East (LH) and Subaru XMM–Newton Deep Field (SXDF). The redshift distribution derived from the full spectral energy distribution (SED) information is shown to be narrower than that determined from the radio–sub-mm spectral index, as more photometric bands contribute to a higher redshift accuracy. The redshift distribution of sources derived from at least two photometric bands peaks at z ≈ 2.4 and has a near-Gaussian distribution, with 50 per cent (interquartile range) of sources at z = 1.8–3.1. We find a statistically significant difference between the measured redshift distributions in the two fields; the SXDF peaking at a slightly lower redshift (median z ≈ 2.2 ) than the LH (median z ≈ 2.7 ), which we attribute to the noise properties of the radio observations. We demonstrate, however, that there could also be field-to-field variations that are consistent with the measured differences in the redshift distributions and, hence, that the incomplete area observed by SHADES with SCUBA, despite being the largest sub-mm survey to date, may still be too small to fully characterize the bright sub-mm galaxy population. Finally, we present a brief comparison with the predicted, or assumed, redshift distributions of sub-mm galaxy formation and evolution models, and we derive the contribution of these SHADES sources and the general sub-mm galaxy population to the star formation rate density at different epochs.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 379 No.: 4 ISSN: 0035-8711

ID: CaltechAUTHORS:20090410-112320222

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Abstract: Using the galactica (GALaxy Automated ComponenT Image Construction Algorithm) code of Benson et al., we obtain quantitative measurements of spheroid-to-disc ratios for a sample of 8839 galaxies observed in the Sloan Digital Sky Survey (SDSS). We carry out extensive tests of this code and of gim2d, finding that they perform similarly in all respects. From the spheroid and disc luminosities, we construct luminosity and stellar mass functions for each component and estimate the relative luminosity and stellar mass densities of discs and spheroids in the local Universe. Assuming a simple one-to-one mapping between spheroid mass and the mass of a central supermassive black hole, we provide the most accurate determination so far of the black hole mass function in the local universe. From this, we infer a cosmological mass density of black holes of ρ_• = (3.77 ± 0.97) × 10^5 h M_⊙ Mpc^(−3) . We compare our results to predictions from current hierarchical models of galaxy formation and these are found to fare well in predicting the qualitative trends observed. We find that stars in discs contribute 35–51 per cent of the local stellar mass density.

Publication: Monthly Notices of the Royal Astronomical Society Vol.: 379 No.: 3 ISSN: 0035-8711

ID: CaltechAUTHORS:20091016-133255351

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Abstract: While the accelerated expansion of the Universe is by now well established, an underlying scalar-field potential possibly responsible for this acceleration remains unconstrained. We present an attempt to reconstruct this potential using recent SN data, under the assumption that the acceleration is driven by a single scalar field. Current approaches to such reconstructions are based upon simple parametric descriptions of either the luminosity distance or the dark energy equation of state. We find that these various approximations lead to a range of derived evolutionary histories of the dark energy equation of state (although there is considerable overlap between the different potential shapes allowed by the data). Instead of these indirect reconstruction schemes, we discuss a technique to determine the potential directly from the data by expressing it in terms of a binned scalar field. We apply this technique to a recent SN data set, and compare the results with model-dependent approaches. In a similar fashion to direct estimates of the dark energy equation of state, we advocate direct reconstruction of the scalar-field potential as a way to minimize prior assumptions on the shape, and thus minimize the introduction of bias in the derived potential.

Publication: Physical Review D Vol.: 75 No.: 10 ISSN: 2470-0010

ID: CaltechAUTHORS:LICprd07b

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Abstract: A search for optical line emission from the two-photon decay of relic axions was conducted in the galaxy clusters Abell 2667 and 2390, using spectra from the VIMOS (Visible MultiObject Spectrograph) integral field unit at the Very Large Telescope. New upper limits to the two-photon coupling of the axion are derived, and are at least a factor of 3 more stringent than previous upper limits in this mass window. The improvement follows from a larger collecting area, integration time, and spatial resolution, as well as from improvements in signal to noise and sky subtraction made possible by strong-lensing mass models of these clusters. The new limits either require that the two-photon coupling of the axion be extremely weak or that the axion mass window between 4.5 eV and 7.7 eV be closed. Implications for sterile-neutrino dark matter are discussed briefly also.

Publication: Physical Review D Vol.: 75 No.: 10 ISSN: 2470-0010

ID: CaltechAUTHORS:GRIprd07

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Abstract: Shortly after the addition of a 1/R term to the Einstein-Hilbert action was proposed as a solution to the cosmic-acceleration puzzle, Chiba showed that such a theory violates Solar System tests of gravity. A flurry of recent papers have called Chiba's result into question. They argue that the spherically-symmetric vacuum spacetime in this theory is the Schwarzschild-de Sitter solution, making this theory consistent with Solar System tests. We point out that although the Schwarzschild-de Sitter solution exists in this theory, it is not the unique spherically-symmetric vacuum solution, and it is not the solution that describes the spacetime in the Solar System. The solution that correctly matches onto the stellar-interior solution differs from Schwarzschild-de Sitter in a way consistent with Chiba's claims. Thus, 1/R gravity is ruled out by Solar System tests.

Publication: Physical Review D Vol.: 74 No.: 12 ISSN: 2470-0010

ID: CaltechAUTHORS:ERIprd06

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Abstract: Satellite galaxies currently undergoing tidal disruption offer a unique opportunity to constrain an effective violation of the equivalence principle in the dark sector. While dark matter in the standard scenario interacts solely through gravity on large scales, a new long-range force between dark-matter particles may naturally arise in theories in which the dark matter couples to a light scalar field. An inverse-square-law force of this kind would manifest itself as a violation of the equivalence principle in the dynamics of dark matter compared to baryons in the form of gas or stars. In a previous paper, we showed that an attractive force would displace stars outwards from the bottom of the satellite's gravitational potential well, leading to a higher fraction of stars being disrupted from the tidal bulge further from the Galactic center. Since stars disrupted from the far (near) side of the satellite go on to form the trailing (leading) tidal stream, an attractive dark-matter force will produce a relative enhancement of the trailing stream compared to the leading stream. This distinctive signature of a dark-matter force might be detected through detailed observations of the tidal tails of a disrupting satellite, such as those recently performed by the Two-Micron All-Sky Survey (2MASS) and Sloan Digital Sky Survey (SDSS) on the Sagittarius (Sgr) dwarf galaxy. Here we show that this signature is robust to changes in our models for both the satellite and Milky Way, suggesting that we might hope to search for a dark-matter force in the tidal features of other recently discovered satellite galaxies in addition to the Sgr dwarf.

Publication: Physical Review D Vol.: 74 No.: 8 ISSN: 2470-0010

ID: CaltechAUTHORS:KESprd06

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Abstract: Satellite galaxies are tidally disrupted as they orbit the Milky Way. If dark matter (DM) experiences a stronger self-attraction than baryons, stars will preferentially gain rather than lose energy during tidal disruption, leading to an enhancement in the trailing compared to the leading tidal stream. The Sgr dwarf galaxy is seen to have roughly equal streams, challenging models in which DM and baryons accelerate differently by more than 10%. Future observations and a better understanding of DM distribution should allow detection of equivalence violation at the percent level.

Publication: Physical Review Letters Vol.: 97 No.: 13 ISSN: 0031-9007

ID: CaltechAUTHORS:KESprl06

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