CaltechAUTHORS: Combined
https://feeds.library.caltech.edu/people/Kamionkowski-M/combined.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenFri, 09 Aug 2024 19:03:55 -0700Cosmic-Ray Energy Spectra Between Ten and Several Hundred GeV/amu for Elements from _(18)Ar to _(25)Ni--Results from HEAO-3
https://resolver.caltech.edu/CaltechAUTHORS:20140512-120447856
Year: 1987
Using the relativistic rise of energy loss as a measure of energy, we have
determined the energy dependence of the abundances relative to _(26)Fe of the elements _(18)Ar,
_(19)K, _(20)Ca, _(21)Sc, _(22)Ti, _(23)V, and _(28)Ni, from 10 to several hundred Ge V /amu. From the energy
dependence of the observed Ar/Fe and Ca/Fe ratios we infer primary source ratios for these
elements.https://resolver.caltech.edu/CaltechAUTHORS:20140512-120447856Unitarity limits on the mass and radius of dark-matter particles
https://resolver.caltech.edu/CaltechAUTHORS:GRIprl90
Year: 1990
DOI: 10.1103/PhysRevLett.64.615
Using partial-wave unitarity and the observed density of the Universe, we show that a stable elementary particle which was once in thermal equilibrium cannot have a mass greater than 340 TeV. An extended object which was once in thermal equilibrium cannot have a radius less than 7.5×10-7 fm. A lower limit to the relic abundance of such particles is also found.https://resolver.caltech.edu/CaltechAUTHORS:GRIprl90Searching for CP violation in "charge-blind" jets
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd90a
Year: 1990
DOI: 10.1103/PhysRevD.41.1672
Although it is difficult to determine the identity of the parent parton of hadronic jets produced in high-energy particle-antiparticle collisions, ''charge-blind'' jets, those where such information is not determined, may still be used to obtain CP-violating correlations. In this paper, I provide a general method for determining which correlations may be used as signatures of CP violation.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd90aSupersymmetric dark matter above the W mass
https://resolver.caltech.edu/CaltechAUTHORS:GRIprd90
Year: 1990
DOI: 10.1103/PhysRevD.41.3565
In this paper we study the cosmological consequences of the minimal supersymmetric extension of the standard model in the case that the neutralino is heavier than the W. We calculate the cross section for annihilation of heavy neutralinos into final states containing gauge and Higgs bosons (χ̃χ̃→WW,ZZ,HH,HW,HZ) and combine these results with those previously obtained for annihilation into fermions to find the relic cosmological abundance for the most general neutralino. The new channels are particularly important for Higgsino-like and mixed-state neutralinos, but are subdominant (to the fermion-antifermion annihilation channels) in the case that the neutralino is mostly a gaugino. The effect of the top-quark mass is also considered. Using these cross sections and the cosmological constraint Ωχ̃h2 ~< 1 we map the entire range of cosmologically acceptable supersymmetric parameter space and find a very general bound on the neutralino mass. For a top-quark mass of less than 180 GeV, neutralinos heavier than 3200 GeV are cosmologically inconsistent, and if the top-quark mass is less than 120 GeV, the bound is lowered to 2600 GeV. We find that a "heavy" neutralino (mχ̃>mW) that contributes Ωχ̃∼ 1 arises for a very wide range of model parameters and makes, therefore, a very natural and attractive dark-matter candidate.https://resolver.caltech.edu/CaltechAUTHORS:GRIprd90Thermal relics: Do we know their abundances?
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd90b
Year: 1990
DOI: 10.1103/PhysRevD.42.3310
The relic abundance of a particle species that was once in thermal equilibrium in the expanding Universe depends upon a competition between the annihilation rate of the species and the expansion rate of the Universe. Assuming that the Universe is radiation dominated at early times the relic abundance is easy to compute and well known. At times earlier than about 1 sec after the bang there is little or no evidence that the Universe had to be radiation dominated, although that is the simplest–and standard–assumption. Because early-Universe relics are of such importance both to particle physics and to cosmology, we consider in detail three nonstandard possibilities for the Universe at the time a species' abundance froze in: energy density dominated by shear (i.e., anisotropic expansion), energy density dominated by some other nonrelativistic species, and energy density dominated by the kinetic energy of the scalar field that sets the gravitational constant in a Brans-Dicke-Jordan cosmological model. In the second case the relic abundance is less than the standard value, while in the other two cases it can be enhanced by a significant factor. We also mention two other more exotic possibilities for enhancing the relic abundance of a species–a larger value of Newton's constant at early times (e.g., as might occur in superstring or Kaluza-Klein theories) or a component of the energy density at early times with a very stiff equation of state (p>ρ/3), e.g., a scalar field φ with potential V(φ)=β‖φ‖n with n>4. Our results have implications for dark-matter searches and searches for particle relics in general.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd90bDistinctive positron feature from particle dark-matter annihilations in the galatic halo
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd91a
Year: 1991
DOI: 10.1103/PhysRevD.43.1774
If the dark matter in our galactic halo consists of weakly interacting massive particles (WIMP's) heavier than the W± boson which have a significant annihilation branch into W± and Z0 pairs, e.g., a Higgsino-like neutralino, a very distinctive feature in the cosmic-ray positron spectrum arises from W+ and Z0 decays. Because of inherent astrophysical uncertainties such a signal is by no means guaranteed even if heavy WIMP's do comprise the galactic halo. However, the positron signature is virtually a "smoking gun" for particle dark matter in the halo and thus worthy of note.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd91aEnergetic neutrinos from heavy-neutralino annihilation in the Sun
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd91b
Year: 1991
DOI: 10.1103/PhysRevD.44.3021
Neutralinos may be captured in the Sun and annihilate therein producing high-energy neutrinos. Present limits on the flux of such neutrinos from underground detectors such as Irvine-Michigan-Brookhaven (IMB) and Kamiokande II may be used to rule out certain supersymmetric dark-matter candidates, while in many other supersymmetric models the rates are large enough that if neutralinos do reside in the galactic halo, observation of a neutrino signal may be possible in the near future. Neutralinos that are either nearly pure Higgsino or a Higgsino-gaugino combination are generally captured in the Sun by a scalar interaction with nuclei in which a virtual lightest Higgs boson is exchanged. If the squark mass is not much greater than the neutralino mass then capture of neutralinos that are primarily gaugino occurs predominantly by spin-dependent scattering off hydrogen in the Sun. Although only neutrinos from annihilation in the Sun are considered here, the neutrino signal from weakly interacting massive particle annihilation in the Earth should be of comparable strength. Detection rates for mixedstate neutralinos are generally higher than those for Higgsinos or gauginos.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd91bSignatures of dark matter in underground detectors
https://resolver.caltech.edu/CaltechAUTHORS:HALprd92
Year: 1992
DOI: 10.1103/PhysRevD.45.4439
The neutralino, the lightest superpartner in many supersymmetric theories, is arguably the leading dark-matter candidate from both the cosmological and particle-physics points of view. Its mass is bracketed by a minimum value of tens of GeV, determined from unsuccessful accelerator searches, and a maximum value of several TeV, above which neutralinos "overclose" the Universe. If neutralinos exist in our galactic halo, they will be gravitationally captured by scattering off elements in the Sun. Annihilation of neutralinos in the Sun will produce a neutrino flux which can be detected on Earth and thus provide indirect evidence for galactic dark matter. We show that a 1-km2 area is the natural scale of a neutrino telescope capable of probing the GeV-TeV neutralino mass range by searching for high-energy neutrinos produced by their annihilation in the Sun.https://resolver.caltech.edu/CaltechAUTHORS:HALprd92Are textures natural?
https://resolver.caltech.edu/CaltechAUTHORS:KAMprl92a
Year: 1992
DOI: 10.1103/PhysRevLett.69.1485
We make the simple observation that, because of global-symmetry-violating higher-dimension operators, as for instance might be expected to be induced by Planck-scale physics, textures are generically much too short lived to be of use for large-scale structure formation.https://resolver.caltech.edu/CaltechAUTHORS:KAMprl92aInstability and subsequent evolution of electroweak bubbles
https://resolver.caltech.edu/CaltechAUTHORS:KAMprl92b
Year: 1992
DOI: 10.1103/PhysRevLett.69.2743
Bubbles in a first-order electroweak phase transition are nucleated with radii R0 and expand with velocity v. If v is subsonic, a bubble becomes unstable to nonspherical perturbations when its radius is roughly 10^4R0. These perturbations acclerate the transition, and the effective velocity of bubble growth rapidly becomes supersonic. The transition should subsequently proceed spherically via detonation. If for some reason the onset of detonation is postponed, the surface area of the bubbles may be enhanced by 10^5. We discuss consequences for electroweak baryogenesis.https://resolver.caltech.edu/CaltechAUTHORS:KAMprl92bVacuum-polarization corrections to solar-fusion rates
https://resolver.caltech.edu/CaltechAUTHORS:KAMprc94
Year: 1994
DOI: 10.1103/PhysRevC.49.545
The vacuum-polarization (VP) corrections to rates for nuclear-fusion reactions in the pp chain and in the CNO cycle are calculated. For the reactions of particular importance to the solar-neutrino problem, the 3He(3He,2p)4He, 3He(α,γ)7Be, 7Be(p,γ)8B, and 14N(p,γ)15O reactions, we find the magnitude of the effect to be less than 2%. The effect of VP on all the other reaction rates is expected to be of a similar order of magnitude. We discuss how these results affect the predicted fluxes of solar neutrinos.https://resolver.caltech.edu/CaltechAUTHORS:KAMprc94Cosmic-ray antiprotons from neutralino annihilation into gluons
https://resolver.caltech.edu/CaltechAUTHORS:JUNprd94
Year: 1994
DOI: 10.1103/PhysRevD.49.2316
We estimate the flux of cosmic-ray antiprotons expected from the annihilation of neutralinos in the galactic halo. The antiproton signal may offer an important alternative detection scheme in the case that neutralino annihilation proceeds mainly to the two-gluon final state.https://resolver.caltech.edu/CaltechAUTHORS:JUNprd94Gravitational radiation from first-order phase transitions
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd94b
Year: 1994
DOI: 10.1103/PhysRevD.49.2837
We consider the stochastic background of gravity waves produced by first-order cosmological phase transitions from two types of sources: colliding bubbles and hydrodynamic turbulence. First we discuss the fluid mechanics of relativistic spherical combustion. We then numerically collide many bubbles expanding at a velocity v and calculate the resulting spectrum of gravitational radiation in the linearized gravity approximation. Our results are expressed as simple functions of the mean bubble separation, the bubble expansion velocity, the latent heat, and the efficiency of converting latent heat to kinetic energy of the bubble walls. A first-order phase transition is also likely to excite a Kolmogoroff spectrum of turbulence. We estimate the gravity waves produced by such a spectrum of turbulence and find that the characteristic amplitude of the gravity waves produced is comparable to that from bubble collisions. Finally, we apply these results to the electroweak transition. Using the one-loop effective potential for the minimal electroweak model, the characteristic amplitude of the gravity waves produced is h≃1.5×10^-27 at a characteristic frequency of 4.1 × 10^-3 Hz corresponding to Ω∼10^-22 in gravity waves, far too small for detection. Gravity waves from more strongly first-order phase transitions, including the electroweak transition in nonminimal models, have better prospects for detection, though probably not by LIGO.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd94bNeutrinos from particle decay in the Sun and Earth
https://resolver.caltech.edu/CaltechAUTHORS:JUNprd95a
Year: 1995
DOI: 10.1103/PhysRevD.51.328
Weakly interacting massive particles (WIMP's) may be indirectly detected by observation of upward muons induced by energetic neutrinos from annihilation of WIMP's that have accumulated in the Sun and/or Earth. Energetic muon neutrinos come from the decays of τ leptons, c, b, and t quarks, gauge bosons, and Higgs bosons produced by WIMP annihilation. We provide analytic expressions, suitable for computing the flux of upward muons, for the neutrino energy spectra from decays of all these particles in the center of the Sun and Earth. These analytic expressions should obviate the need for Monte Carlo calculations of the upward-muon flux. We investigate the effects of polarization of the gauge bosons on the neutrino spectra, and find that they are small. We also present simple expressions for the second moments of the neutrino distributions which can be used to estimate the rates for observation of neutrino-induced muons from WIMP annihilation.https://resolver.caltech.edu/CaltechAUTHORS:JUNprd95aγ rays from neutralino annihilation
https://resolver.caltech.edu/CaltechAUTHORS:JUNprd95b
Year: 1995
DOI: 10.1103/PhysRevD.51.3121
We calculate the flux of cosmic γ rays expected from the annihilation of neutralinos in the galactic halo. Our calculation of the annihilation cross section to two photons improves the existing calculations by inclusion of exact one-loop diagrams for the amplitudes involving Higgs boson and chargino states as well as those involving fermion and sfermion states. A survey of supersymmetric parameter space shows that numerous models would be observable at the 3σ level with an air Cherenkov telescope with an exposure of 10^4 m^2 yr.https://resolver.caltech.edu/CaltechAUTHORS:JUNprd95bSolar neutrinos: Radiative corrections in neutrino-electron scattering experiments
https://resolver.caltech.edu/CaltechAUTHORS:BAHprd95a
Year: 1995
DOI: 10.1103/PhysRevD.51.6146
Radiative corrections to the electron recoil-energy spectra and to total cross sections are computed for neutrino-electron scattering by solid neutrinos. Radiative corrections change monotonically the electron recoil spectrum for incident 8B neutrinos, with the relative probability of observing recoil electrons being reduced by about 4% at the highest electron energies. For p-p and 7Be neutrinos, the recoil spectra are not affected significantly. Total cross sections for solar neutrino-electron scattering are reduced by about 2% compared to previously computed values. We also calculate the recoil spectra from 13N and 15O neutrinos including radiative corrections.https://resolver.caltech.edu/CaltechAUTHORS:BAHprd95aModel-Independent Comparison of Direct versus Indirect Detection of Supersymmetric Dark Matter
https://resolver.caltech.edu/CaltechAUTHORS:KAMprl95
Year: 1995
DOI: 10.1103/PhysRevLett.74.5174
We compare the rate for scattering of neutralinos from nuclei with the flux of muons induced by energetic neutrinos from neutralino annihilation in the Sun and Earth. We consider scalar and spin interactions of neutralinos. We find that the event rate in a kg of Ge is roughly equivalent to that in a 10^5–1^07-m^2 muon detector for a neutralino with primarily scalar coupling to nuclei. For a spin-coupled neutralino, the event rate in a 50-g H detector is roughly that in a 10–500-m^2 muon detector. Expected backgrounds favor forthcoming elastic-scattering detectors for scalar couplings while neutrino detectors are favored for spin couplings.https://resolver.caltech.edu/CaltechAUTHORS:KAMprl95Weighing the Universe with the Cosmic Microwave Background
https://resolver.caltech.edu/CaltechAUTHORS:JUNprl96
Year: 1996
DOI: 10.1103/PhysRevLett.76.1007
Variations in Ω, the total density of the Universe, leave an imprint on the power spectrum of temperature fluctuations in the cosmic microwave background (CMB). We evaluate the precision with which Ω can be determined by a CMB map as a function of sky coverage, pixel noise, and beam size. Assuming only that the primordial density perturbations were adiabatic and with no prior information on the values of any other cosmological parameters, a full-sky CMB map at 0.5° angular resolution and a noise level of 15 μK per pixel can determine Ω with a standard error of 5%. If all other cosmological parameters are fixed, Ω can be measured to better than 1%.https://resolver.caltech.edu/CaltechAUTHORS:JUNprl96Cosmological-parameter determination with microwave background maps
https://resolver.caltech.edu/CaltechAUTHORS:JUNprd96
Year: 1996
DOI: 10.1103/PhysRevD.54.1332
The angular power spectrum of the cosmic microwave background (CMB) contains information on virtually all cosmological parameters of interest, including the geometry of the Universe (Ω), the baryon density, the Hubble constant (h), the cosmological constant (Λ), the number of light neutrinos, the ionization history, and the amplitudes and spectral indices of the primordial scalar and tensor perturbation spectra. We review the imprint of each parameter on the CMB. Assuming only that the primordial perturbations were adiabatic, we use a covariance-matrix approach to estimate the precision with which these parameters can be determined by a CMB temperature map as a function of the fraction of sky mapped, the level of pixel noise, and the angular resolution. For example, with no prior information about any of the cosmological parameters, a full-sky CMB map with 0.5° angular resolution and a noise level of 15 μK per pixel can determine Ω, h, and Λ with standard errors of ±0.1 or better, and provide determinations of other parameters which are inaccessible with traditional observations. Smaller beam sizes or prior information on some of the other parameters from other observations improves the sensitivity. The dependence on the underlying cosmological model is discussed.https://resolver.caltech.edu/CaltechAUTHORS:JUNprd96A Low-Density Closed Universe
https://resolver.caltech.edu/CaltechAUTHORS:KAMprl96
Year: 1996
DOI: 10.1103/PhysRevLett.77.587
Matter with an equation of state p = -ρ/3 may arise in certain scalar field theories, and the energy density of this matter decreases as a-2 with the scale factor a of the Universe. In this case, the Universe could be closed but still have a nonrelativistic-matter density Ω0<1. Furthermore, the cosmic microwave background could come from a causally connected region at the other side of the Universe. This model is currently viable and might be tested by a host of forthcoming observations.https://resolver.caltech.edu/CaltechAUTHORS:KAMprl96Matter-microwave correlations in an open universe
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd96
Year: 1996
DOI: 10.1103/PhysRevD.54.4169
In an intriguing recent paper, Crittenden and Turok proposed cross-correlating the cosmic microwave background with tracers of the matter density to probe the existence of a cosmological constant. Here I emphasize that a similar cross correlation arises in an open universe and, depending on the redshift distribution of the tracer population and the matter density, may be comparable to or stronger than that in a flat cosmological-constant universe with the same matter density. The two cases can be distinguished through cross correlation with tracer populations with different redshift distributions.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd96Indirect detection of a light Higgsino motivated by collider data
https://resolver.caltech.edu/CaltechAUTHORS:FREprd97
Year: 1997
DOI: 10.1103/PhysRevD.55.1771
Kane and Wells recently argued that collider data point to a Higgsino-like lightest supersymmetric partner which would explain the dark matter in our Galactic halo. They discuss direct detection of such dark-matter particles in laboratory detectors. Here, we argue that such a particle, if it is indeed the dark matter, might alternatively be accessible in experiments which search for energetic neutrinos from dark-matter annihilation in the Sun. We provide accurate analytic estimates for the rates which take into account all relevant physical effects. Currently, the predicted signal falls roughly one to three orders of magnitude below experimental bounds, depending on the mass and coupling of the particle; however, detectors such as MACRO, super-Kamiokande, and AMANDA will continue to take data and should be able to rule out or confirm an interesting portion of the possible mass range for such a dark-matter particle within the next five years.https://resolver.caltech.edu/CaltechAUTHORS:FREprd97Comment on "Dispersion velocity of galactic dark matter particles"
https://resolver.caltech.edu/CaltechAUTHORS:GATprl97
Year: 1997
DOI: 10.1103/PhysRevLett.78.2261
A Comment on the Letter by R. Cowsiki Charu Ratnam, and P. Bhattacharjee, Phys. Rev. Lett. 76, 3886 (1996). The authors of the Letter offer a Reply.https://resolver.caltech.edu/CaltechAUTHORS:GATprl97A Probe of Primordial Gravity Waves and Vorticity
https://resolver.caltech.edu/CaltechAUTHORS:KAMprl97
Year: 1997
DOI: 10.1103/PhysRevLett.78.2058
A formalism for describing an all-sky map of the polarization of the cosmic microwave background is presented. The polarization pattern on the sky can be decomposed into two geometrically distinct components. One of these components is not coupled to density inhomogeneities. A nonzero amplitude for this component of polarization can only be caused by tensor or vector metric perturbations. This allows unambiguous identification of long-wavelength gravity waves or large-scale vortical flows at the time of last scattering.https://resolver.caltech.edu/CaltechAUTHORS:KAMprl97Statistics of cosmic microwave background polarization
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd97a
Year: 1997
DOI: 10.1103/PhysRevD.55.7368
We present a formalism for analyzing a full-sky temperature and polarization map of the cosmic microwave background. Temperature maps are analyzed by expanding over the set of spherical harmonics to give multipole moments of the two-point correlation function. Polarization, which is described by a second-rank tensor, can be treated analogously by expanding in the appropriate tensor spherical harmonics. We provide expressions for the complete set of temperature and polarization multipole moments for scalar and tensor metric perturbations. Four sets of multipole moments completely describe isotropic temperature and polarization correlations; for scalar metric perturbations one set is identically zero, giving the possibility of a clean determination of the vector and tensor contributions. The variance with which the multipole moments can be measured in idealized experiments is evaluated, including the effects of detector noise, sky coverage, and beam width. Finally, we construct coordinate-independent polarization two-point correlation functions, express them in terms of the multipole moments, and derive small-angle limits.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd97aInstability of the one-texture universe
https://resolver.caltech.edu/CaltechAUTHORS:CHEprd97
Year: 1997
DOI: 10.1103/PhysRevD.56.2051
The one-texture universe, introduced by Davis in 1987, is a homogeneous mapping of a scalar field with an S^3 vacuum into a closed universe. It has long been known to mathematicians that such solutions, although static, are unstable. We show by explicit construction that there are four degenerate lowest modes which are unstable, corresponding to collapse of the texture towards a single point, in the case where gravitational back reaction is neglected. We discuss the instability time scale in both static and expanding space-times; in the latter case it is of order of the present age of the universe, suggesting that, though unstable, the one-texture universe could survive to the present. The cosmic microwave background constrains the initial magnitude of this unstable perturbation to be less than ∼10^-3.https://resolver.caltech.edu/CaltechAUTHORS:CHEprd97Getting around cosmic variance
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd97b
Year: 1997
DOI: 10.1103/PhysRevD.56.4511
Cosmic microwave background (CMB) anisotropies probe the primordial density field at the edge of the observable Universe. There is a limiting precision ("cosmic variance") with which anisotropies can determine the amplitude of primordial mass fluctuations. This arises because the surface of last scatter (SLS) probes only a finite two-dimensional slice of the Universe. Probing other SLS's observed from different locations in the Universe would reduce the cosmic variance. In particular, the polarization of CMB photons scattered by the electron gas in a cluster of galaxies provides a measurement of the CMB quadrupole moment seen by the cluster. Therefore, CMB polarization measurements toward many clusters would probe the anisotropy on a variety of SLS's within the observable Universe, and hence reduce the cosmic-variance uncertainty.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd97bDetectability of inflationary gravitational waves with microwave background polarization
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd98a
Year: 1998
DOI: 10.1103/PhysRevD.57.685
Inflation predicts specific relations between the amplitudes and spectral indices of the primordial spectrum of density (scalar metric) perturbations and gravitational waves (tensor metric perturbations). Detection of a stochastic gravitational-wave background is essential for identifying this unmistakable signature. Polarization of the cosmic microwave background can isolate these tensor modes in a model-independent way and thereby circumvent the cosmic-variance limit to detecting a small tensor signal with only a temperature map. Here we assess the detectability of a gravity-wave background with a temperature and polarization map. For detector sensitivities better than 10–20 μK sqrt[sec], the sensitivity to a tensor signal is always dominated by the polarization map. With a detector sensitivity of order 1 μK sqrt[sec], polarization could improve on a temperature-map sensitivity to tensor modes by two to three orders of magnitude. Even a small amount of reionization substantially enhances tensor-mode detectability. We also argue that the sensitivity of the Planck Surveyor to tensor modes is significantly improved with polarization, even taking into account the resulting degradation of the intensity determination in the high-frequency channels.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd98aGalactic halo models and particle dark-matter detection
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd98b
Year: 1998
DOI: 10.1103/PhysRevD.57.3256
Rates for detection of weakly interacting massive-particle (WIMP) dark matter are usually carried out assuming the Milky Way halo is an isothermal sphere. However, it is possible that our halo is not precisely spherical; it may have some bulk rotation; and the radial profile may differ from that of an isothermal sphere. In this paper, we calculate detection rates in observationally consistent alternative halo models that produce the same halo contributions to the local and asymptotic rotation speeds to investigate the effects of the theoretical uncertainty of the WIMP spatial and velocity distribution. We use self-consistent models to take into account the effects of various mass distributions on the local velocity distribution. The local halo density may be increased up to a factor of 2 by flattening or by an alternative radial profile (which may also decrease the density slightly). However, changes in the WIMP velocity distribution in these models produce only negligible changes in the WIMP detection rate. Reasonable bulk rotations lead to only an O(10%) effect on event rates. We also show how the nuclear recoil spectrum in a direct-detection experiment could provide information on the shape and rotation of the halo.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd98bElectron-screening correction for the proton-proton reaction
https://resolver.caltech.edu/CaltechAUTHORS:BAHprc98a
Year: 1998
DOI: 10.1103/PhysRevC.57.2756
We test the Salpeter formalism for the electron screening of the solar proton-proton fusion reaction by solving numerically the relevant Schrödinger equation. We evaluate exactly the square of the overlap integral of the two-proton wave function and the deuteron wave function and compare with the usual analytic approximation. The usual WKB solution agrees with the numerical result to O(10^-4). The WKB approximation should be even more precise for the other nuclear fusion reactions in the pp chain and CNO cycles.https://resolver.caltech.edu/CaltechAUTHORS:BAHprc98aThree-body annihilation of neutralinos below two-body thresholds
https://resolver.caltech.edu/CaltechAUTHORS:CHEjhep98
Year: 1998
DOI: 10.1088/1126-6708/1998/07/001
We calculate the cross section for s-wave neutralino annihilation to three-body final states below the W+W- and tbar t thresholds. Such three-body channels may dominate the annihilation cross section if the neutralino mass is not too much less than mt and mW respectively. Furthermore, because neutrinos produced in these channels are much more energetic than those from the bbar b or τ+τ- channels, they can dominate the energetic-neutrino fluxes from neutralino annihilation in the Sun or Earth far below these thresholds and significantly enhance the neutrino signal in certain regions of the supersymmetric parameter space.https://resolver.caltech.edu/CaltechAUTHORS:CHEjhep98Calculation of the Ostriker-Vishniac effect in cold dark matter models
https://resolver.caltech.edu/CaltechAUTHORS:JAFprd98
Year: 1998
DOI: 10.1103/PhysRevD.58.043001
We present a new derivation of the cosmic microwave background anisotropy spectrum from the Ostriker-Vishniac effect for an open, flat, or closed universe, and calculate the anisotropy expected in cold dark matter (CDM) models. We provide simple semi-analytic fitting formulas for the Vishniac power spectrum that can be used to evaluate the expected anisotropy in CDM models for any arbitrary ionization history. In a flat universe, CDM models normalized to cluster abundances produce rms temperature anisotropies of 0.8–2.4 μK on arcminute angular scales for a constant ionization fraction of unity, whereas an ionization fraction of 0.2 yields rms anisotropies of 0.3–0.8 μK. In an open and/or high-baryon-density universe, the level of anisotropy is somewhat higher. The signal in some of these models may be detectable with planned interferometry experiments. The damping of the acoustic peaks in the primary-anisotropy spectrum at degree angular scales depends primarily on the optical depth and only secondarily on the epoch of reionization. On the other hand, the amplitude of Ostriker-Vishniac anisotropies depends sensitively on the epoch of reionization. Therefore, when combined with the estimate of the reionization optical depth provided by maps of degree-scale anisotropies, the Ostriker-Vishniac effect can provide a unique probe of the epoch of reionization.https://resolver.caltech.edu/CaltechAUTHORS:JAFprd98Solar fusion cross sections
https://resolver.caltech.edu/CaltechAUTHORS:ADErmp98
Year: 1998
DOI: 10.1103/RevModPhys.70.1265
We review and analyze the available information on the nuclear-fusion cross sections that are most important for solar energy generation and solar neutrino production. We provide best values for the low-energy cross-section factors and, wherever possible, estimates of the uncertainties. We also describe the most important experiments and calculations that are required in order to improve our knowledge of solar fusion rates.https://resolver.caltech.edu/CaltechAUTHORS:ADErmp98Theory and statistics of weak lensing from large-scale mass inhomogeneities
https://resolver.caltech.edu/CaltechAUTHORS:20210116-163404370
Year: 1998
DOI: 10.1046/j.1365-8711.1998.02054.x
Weak lensing by large-scale mass inhomogeneities in the Universe induces correlations in the observed ellipticities of distant sources. We first review the harmonic analysis and statistics required of these correlations and discuss calculations for the predicted signal. We consider the ellipticity correlation function, the mean-square ellipticity, the ellipticity power spectrum and a global maximum-likelihood analysis to isolate a weak-lensing signal from the data. Estimates for the sensitivity of a survey of a given area, surface density, and mean intrinsic source ellipticity are presented. We then apply our results to the FIRST radio-source survey. We predict an rms ellipticity of roughly 0.011 in 1 × 1 deg² pixels and 0.018 in 20 × 20 arcmin² pixels if the power spectrum is normalized to σ₈Ω⁰.⁵³ = 0.6, as indicated by the cluster abundance. The signal is significantly larger in some models if the power spectrum is normalized instead to the COBE anisotropy. The uncertainty in the predictions from imprecise knowledge of the FIRST redshift distribution is about 25 per cent in the rms ellipticity. We show that FIRST should be able to make a statistically significant detection of a weak-lensing signal for cluster-abundance-normalized power spectra.https://resolver.caltech.edu/CaltechAUTHORS:20210116-163404370The first space-based gravitational-wave detectors
https://resolver.caltech.edu/CaltechAUTHORS:CALprd99
Year: 1999
DOI: 10.1103/PhysRevD.59.027101
Gravitational waves provide a laboratory for general relativity and a window to energetic astrophysical phenomena invisible with electromagnetic radiation. Several terrestrial detectors are currently under construction, and a space-based interferometer is envisioned for launch early next century to detect test-mass motions induced by waves of relatively short wavelength. Very-long-wavelength gravitational waves can be detected using the plasma in the early Universe as test masses; the motion induced in the plasma by a wave is imprinted onto the cosmic microwave background (CMB). While the signature of gravitational waves on the CMB temperature fluctuations is not unique, the polarization pattern can be used to unambiguously detect gravitational radiation. Thus, forthcoming CMB polarization experiments, such as the Microwave Anisotropy Probe and Planck, will be the first space-based gravitational-wave detectors.https://resolver.caltech.edu/CaltechAUTHORS:CALprd99New constraint on open cold-dark-matter models
https://resolver.caltech.edu/CaltechAUTHORS:KINprl99
Year: 1999
DOI: 10.1103/PhysRevLett.82.4172
We calculate the large-angle cross correlation between the cosmic-microwave-background temperature and the x-ray-background intensity expected in an open universe with cold dark matter (CDM) and a nearly scale-invariant spectrum of adiabatic density perturbations. Results are presented as a function of the nonrelativistic-matter density Ω0 and the x-ray bias bx for both an open universe and a flat cosmological-constant universe. Recent experimental upper limits to the amplitude of this cross correlation provide a new constraint to the Ω0-bx parameter space that open-CDM models (and the open-inflation models that produce them) must satisfy.https://resolver.caltech.edu/CaltechAUTHORS:KINprl99Cosmological Signature of New Parity-Violating Interactions
https://resolver.caltech.edu/CaltechAUTHORS:LUEprl99
Year: 1999
DOI: 10.1103/PhysRevLett.83.1506
Are there any manifestations of parity violation other than those observed in weak interactions? A map of the cosmic microwave background (CMB) temperature and polarization will provide a new signature of P violation. We examine two classes of P-violating interactions that would give rise to such a signature. The first interaction leads to a cosmological birefringence, possibly driven by quintessence. The other interaction leads to an asymmetry in the amplitude of right- versus left-handed gravitational waves produced during inflation. The Planck Surveyor should improve upon the current sensitivity to birefringence. While the primordial effect would most likely elude detection by MAP and Planck, it may be detectable with a future dedicated CMB polarization experiment.https://resolver.caltech.edu/CaltechAUTHORS:LUEprl99Cosmic microwave background temperature and polarization anisotropy in Brans-Dicke cosmology
https://resolver.caltech.edu/CaltechAUTHORS:CHEprd99
Year: 1999
DOI: 10.1103/PhysRevD.60.104036
We develop a formalism for calculating cosmic microwave background (CMB) temperature and polarization anisotropies in cosmological models with Brans-Dicke gravity. We then modify publicly available Boltzmann codes to calculate numerically the temperature and polarization power spectra. Results are illustrated with a few representative models. Comparing with the general-relativistic model of the same cosmological parameters, both the amplitude and the width of the acoustic peaks are different in the Brans-Dicke models. We use a covariance-matrix calculation to investigate whether the effects of Brans-Dicke gravity are degenerate with those of variation in other cosmological parameters and to simultaneously determine whether forthcoming CMB maps might be able to distinguish Brans-Dicke and general-relativistic cosmology. Although the predicted power spectra for plausible Brans-Dicke models differ from those in general relativity only slightly, we find that MAP and/or the Planck Surveyor may in principle provide a test of Brans-Dicke theory that is competitive to solar-system tests. For example, if all other parameters except for the CMB normalization are fixed, a value of the Brans-Dicke parameter ω as large as 500 could be identified (at the 2σ level) with MAP, and for Planck, values as large as ω≃3000 could be identified; these sensitivities are decreased roughly by a factor of 3 if we marginalize over the baryon density, Hubble constant, spectral index, and re-ionization optical depth. In more general scalar-tensor theories, ω may evolve with time, and in this case, the CMB probe would be complementary to that from solar-system tests.https://resolver.caltech.edu/CaltechAUTHORS:CHEprd99The cosmic microwave background and particle physics
https://resolver.caltech.edu/CaltechAUTHORS:KAMarnps99
Year: 1999
DOI: 10.1146/annurev.nucl.49.1.77
In forthcoming years, connections between cosmology and particle physics will become increasingly important with the advent of a new generation of cosmic microwave background (CMB) experiments. Here, we review a number of these links. Our primary focus is on new CMB tests of inflation. We explain how the inflationary predictions for the geometry of the Universe and primordial density perturbations will be tested by CMB temperature fluctuations, and how the gravitational waves predicted by inflation can be pursued with the CMB polarization. The CMB signatures of topological defects and primordial magnetic fields from cosmological phase transitions are also discussed. Furthermore, we review current and future CMB constraints on various types of dark matter (e.g. massive neutrinos, weakly interacting massive particles, axions, vacuum energy), decaying particles, the baryon asymmetry of the Universe, ultra-high-energy cosmic rays, exotic cosmological topologies, and other new physics.https://resolver.caltech.edu/CaltechAUTHORS:KAMarnps99Testing linear-theory predictions of galaxy formation
https://resolver.caltech.edu/CaltechAUTHORS:20210116-163404669
Year: 2000
DOI: 10.1046/j.1365-8711.2000.03107.x
The angular momentum of galaxies is routinely ascribed to a process of tidal torques acting during the early stages of gravitational collapse, and is predicted from the initial mass distribution using second-order perturbation theory and the Zel'dovich approximation. We test this theory for a flat hierarchical cosmogony using a large N-body simulation with sufficient dynamic range to include tidal fields, allow resolution of individual galaxies, and thereby expand on previous studies. The predictions of linear collapse, linear tidal torque, and biased-peaks galaxy formation are applied to the initial conditions and compared with results for evolved bound objects. We find relatively good correlation between the predictions of linear theory and actual galaxy evolution. Collapse is well described by an ellipsoidal model within a shear field, which results primarily in triaxial objects that do not map directly to the initial density field. While structure formation from early times is a complex history of hierarchical merging, salient features are well described by the simple spherical-collapse model. Most notably, we test several methods for determining the turnaround epoch, and find that turnaround is successfully described by the spherical-collapse model. The angular momentum of collapsing structures grows linearly until turnaround, as predicted, and continues quasi-linearly until shell crossing. The predicted angular momentum for well-resolved galaxies at turnaround overestimates the true turnaround and final values by a factor of ∼3, with a scatter of ∼70 per cent, and only marginally yields the correct direction of the angular momentum vector. We recover the prediction that final angular momentum scales as mass to the 5/3 power. We find that mass and angular momentum also vary proportionally with peak height. In view of the fact that the observed galaxy collapse is a stochastic hierarchical and non-linear process, it is encouraging that the linear theory can serve as an effective predictive and analytic tool.https://resolver.caltech.edu/CaltechAUTHORS:20210116-163404669Cosmic microwave background bispectrum and inflation
https://resolver.caltech.edu/CaltechAUTHORS:WANprd00
Year: 2000
DOI: 10.1103/PhysRevD.61.063504
We derive an expression for the non-Gaussian cosmic microwave background (CMB) statistic Il3 defined recently by Ferreira, Magueijo, and Górski in terms of the slow-roll-inflation parameters ε and η. This result shows that a nonzero value of Il3 in COBE would rule out single-field slow-roll inflation. A sharp change in the slope of the inflaton potential could increase the predicted value of Il3, but not significantly. This further suggests that it will be difficult to account for such a detection in multiple-field models in which density perturbations are produced by quantum fluctuations in the scalar field driving inflation. An Appendix shows how to evaluate an integral that is needed in our calculation as well as in more general calculations of CMB bispectra.https://resolver.caltech.edu/CaltechAUTHORS:WANprd00Large-scale structure, the cosmic microwave background and primordial non-Gaussianity
https://resolver.caltech.edu/CaltechAUTHORS:20210116-163404791
Year: 2000
DOI: 10.1046/j.1365-8711.2000.03191.x
Cosmic microwave background and large-scale structure data will shortly improve dramatically with the Microwave Anisotropy Probe and Planck Surveyor, and the Anglo-Australian 2-Degree Field and Sloan Digital Sky Survey. It is therefore timely to ask which of the microwave background and large-scale structure will provide a better probe of primordial non-Gaussianity. In this paper we consider this question, using the bispectrum as a discriminating statistic. We consider several non-Gaussian models and find that in each case the microwave background will provide a better probe of primordial non-Gaussianity. Our results suggest that if microwave background maps appear Gaussian, then apparent deviations from Gaussian initial conditions in galaxy surveys can be attributed with confidence to the effects of biasing. We demonstrate this precisely for the spatial bispectrum induced by local non-linear biasing.https://resolver.caltech.edu/CaltechAUTHORS:20210116-163404791Polarization pursuers' guide
https://resolver.caltech.edu/CaltechAUTHORS:JAFprd00
Year: 2000
DOI: 10.1103/PhysRevD.61.083501
We calculate the detectability of the polarization of the cosmic microwave background (CMB) as a function of the sky coverage, angular resolution, and instrumental sensitivity for a hypothetical experiment. We consider the gradient component of the polarization from density perturbations (scalar modes) and the curl component from gravitational waves (tensor modes). We show that the amplitude (and thus the detectability) of the polarization from density perturbations is roughly the same in any model as long as the model fits the big-bang-nucleosynthesis (BBN) baryon density and degree-scale anisotropy measurements. The degree-scale polarization is smaller (and accordingly more difficult to detect) if the baryon density is higher. We show that the sensitivity to the polarization from density perturbations and gravitational waves is improved (by a factor of 30) in a fixed-time experiment with a deeper survey of a smaller region of sky.https://resolver.caltech.edu/CaltechAUTHORS:JAFprd00The Dearth of Halo Dwarf Galaxies: Is There Power on Short Scales?
https://resolver.caltech.edu/CaltechAUTHORS:KAMprl00
Year: 2000
DOI: 10.1103/PhysRevLett.84.4525
N-body simulations of structure formation with scale-invariant primordial perturbations show significantly more virialized objects of dwarf-galaxy mass in a typical galactic halo than are observed around the Milky Way. We show that the dearth of observed dwarf galaxies could be explained by a dramatic downturn in the power spectrum at small distance scales. This suppression of small-scale power might also help mitigate the disagreement between cuspy simulated halos and smooth observed halos, while remaining consistent with Lyman-alpha-forest constraints on small-scale power. Such a spectrum could arise in inflationary models with broken-scale invariance.https://resolver.caltech.edu/CaltechAUTHORS:KAMprl00Two ways of biasing galaxy formation
https://resolver.caltech.edu/CaltechAUTHORS:20200519-170818755
Year: 2000
DOI: 10.1046/j.1365-8711.2000.04023.x
We calculate the galaxy bispectrum in both real and redshift space, adopting the most common prescriptions for local Eulerian biasing and the Lagrangian evolving-bias model. We show that the two biasing schemes make measurably different predictions for these clustering statistics. The Eulerian prescription implies that the galaxy distribution depends only on the present-day local mass distribution, while its Lagrangian counterpart relates the current galaxy distribution to the mass distribution at an earlier epoch when galaxies first formed. Detailed measurement of the galaxy bispectrum (of its reduced amplitude) can help establish whether galaxy positions are determined by the current mass distribution or an earlier mass distribution.https://resolver.caltech.edu/CaltechAUTHORS:20200519-170818755The Second Peak: The Dark-Energy Density and the Cosmic Microwave Background
https://resolver.caltech.edu/CaltechAUTHORS:20200205-144743826
Year: 2001
DOI: 10.1007/978-3-662-04587-9_11
Supernova evidence for a negative-pressure dark energy (e. g., cosmological constant or quintessence) that contributes a fraction Ω_Λ ≃ 0.7 of closure density has been bolstered by the discrepancy between the total density, Ω_(tot) ≃ 1, suggested by the location of the first peak in the cosmic microwave background (CMB) power spectrum and the nonrelativistic-matter density Ω_m ≃ 0.3 obtained from dynamical measurements. Here we show that the impending identification of the location of the second peak in the CMB power spectrum will provide an immediate and independent probe of the cosmological constant. As an aside, we show how the measured height of the first peak probably already points toward a low matter density and places upper limits to the reionization optical depth and gravitational-wave amplitude.https://resolver.caltech.edu/CaltechAUTHORS:20200205-144743826Intrinsic and extrinsic galaxy alignment
https://resolver.caltech.edu/CaltechAUTHORS:20170409-072654766
Year: 2001
DOI: 10.1046/j.1365-8711.2001.04105.x
We show with analytic models that the assumption of uncorrelated intrinsic ellipticities of target sources that is usually made in searches for weak gravitational lensing arising from large-scale mass inhomogeneities ('field lensing') is unwarranted. If the orientation of the galaxy image is determined either by the angular momentum or by the shape of the halo in which it forms, then the image should be aligned preferentially with the component of the tidal gravitational field perpendicular to the line of sight. Long-range correlations in the tidal field will thus lead to long-range ellipticity–ellipticity correlations that mimic the shear correlations arising from weak gravitational lensing. We calculate the ellipticity–ellipticity correlation expected if halo shapes determine the observed galaxy shape, and we discuss uncertainties (which are still considerable) in the predicted amplitude of this correlation. The ellipticity–ellipticity correlation induced by angular momenta should be smaller. We consider several methods for discriminating between the weak-lensing (extrinsic) and intrinsic correlations, including the use of redshift information. An ellipticity–tidal-field correlation also implies the existence of an alignment of images of galaxies near clusters. Although the intrinsic alignment may complicate the interpretation of field-lensing results, it is inherently interesting as it may shed light on galaxy formation as well as on structure formation.https://resolver.caltech.edu/CaltechAUTHORS:20170409-072654766On galaxy cluster sizes and temperatures
https://resolver.caltech.edu/CaltechAUTHORS:20200519-170818941
Year: 2001
DOI: 10.1046/j.1365-8711.2001.04185.x
We show that the distribution of the sizes and temperatures of clusters can be used to constrain cosmological models. The size-temperature (ST) distribution predicted in a flat Gaussian cluster-abundance-normalized Ω₀ = 0.3 model agrees well with the fairly tight ST relation observed. A larger power-spectrum amplitude σ₈ would give rise to a larger scatter about the ST relation as would a larger value of Ω₀ and/or long non-Gaussian high-density tails in the probability density function. For Gaussian initial conditions, the ST distribution suggests a constraint σ₈Ω₀⁰·²⁶ ≃ 0.76. The ST relation is expected to get tighter at high redshifts. In the process, we derive a simple formula for the halo formation-redshift distribution for non-Gaussian models. We also suggest that the discrepancy between the naive zero-redshift ST relation and that observed may be owing, at least in part, to the fact that lower-mass clusters form over a wider range of redshifts. An Appendix derives an equation for the formation-redshift distribution of haloes.https://resolver.caltech.edu/CaltechAUTHORS:20200519-170818941Velocity distributions and annual-modulation signatures of weakly-interacting massive particles
https://resolver.caltech.edu/CaltechAUTHORS:ULLjhep01b
Year: 2001
DOI: 10.1088/1126-6708/2001/03/049
An annual modulation in the event rate of the NaI detector of the DAMA collaboration has been used to infer the existence of particle dark matter in the Galactic halo. Bounds on the WIMP mass and WIMP-nucleon cross section have been derived. These analyses have assumed that the local dark-matter velocity distribution is either isotropic or has some bulk rotation. Here we consider the effects of possible structure in the WIMP velocity distribution on the annual-modulation amplitude. We show that if we allow for a locally anisotropic velocity dispersion tensor, the interpretation of direct detection experiments could be altered significantly. We also show that uncertainties in the velocity distribution function that arise from uncertainties in the radial density profile are less important if the velocity dispersion is assumed to be isotropic.https://resolver.caltech.edu/CaltechAUTHORS:ULLjhep01bGalactosynthesis: halo histories, star formation and discs
https://resolver.caltech.edu/CaltechAUTHORS:20200602-124838359
Year: 2001
DOI: 10.1046/j.1365-8711.2001.04031.x
We investigate the effects of a variety of ingredients that must enter into a realistic model for disc galaxy formation, focusing primarily on the Tully–Fisher (TF) relation and its scatter in several wavebands. In particular, we employ analytic distributions for halo formation redshifts and halo spins, empirical star formation rates and initial mass functions, realistic stellar populations, and chemical evolution of the gas. Our main findings are as follows. (a) The slope, normalization and scatter of the TF relation across various wavebands are determined largely by the parent halo properties as dictated by the initial conditions, but are also influenced by star formation in the disc. (b) TF scatter in this model is due primarily to the spread in formation redshifts. The scatter can be measurably reduced by chemical evolution, and also by the weak anticorrelation between peak height and spin. (c) Multiwavelength constraints can be important in distinguishing between models that appear to fit the TF relation in I or K. (d) Assuming passive disc evolution, successful models seem to require that the bulk of disc formation cannot occur too early (z>2–3) or too late (z<0.2), and are inconsistent with high values of Ω₀. (e) A simple, realistic model with the above ingredients, and fewer free parameters than typical semi-analytic models, can reasonably reproduce the observed z=0 TF relation in all bands (B, R, I and K), as well as the observed B-band surface brightness–magnitude relation. In such a model, the near-infrared TF relation at z=1 is similar to that at z=0, while bluer bands show a markedly steeper TF slope at high redshift, consistent with limited current data. The remarkable agreement with observations suggests that the amount of gas that is expelled or poured into a disc galaxy may be small (though small fluctuations might serve to align B-band predictions better with observations), and that the specific angular momentum of the baryons should roughly equal that of the halo; there is little room for angular momentum transfer. In Appendix A we present analytic fits to stellar population synthesis models.https://resolver.caltech.edu/CaltechAUTHORS:20200602-124838359Spin-dependent WIMPs in DAMA?
https://resolver.caltech.edu/CaltechAUTHORS:ULLjhep01a
Year: 2001
We investigate whether the annual modulation observed in the DAMA experiment can be due to a weakly-interacting massive particle (WIMP) with an axial-vector (spin-dependent; SD) coupling to nuclei. We evaluate the SD WIMP-proton cross section under the assumption that such scattering accounts for the DAMA modulation, and we do the same for a SD WIMP-neutron cross section. We show that SD WIMP-proton scattering is ruled out in a model-independent fashion by null searches for energetic neutrinos from WIMP annihilation in the Sun, and that SD WIMP-neutron scattering is ruled out for WIMP masses gtrsim20 GeV by the null result with the DAMA Xe detector. A SD WIMP with mass lesssim20 GeV is still compatible, but only if the SD WIMP-neutron interaction is four orders of magnitude greater than the WIMP-proton interaction.https://resolver.caltech.edu/CaltechAUTHORS:ULLjhep01aTests for primordial non-Gaussianity
https://resolver.caltech.edu/CaltechAUTHORS:20200602-124838541
Year: 2001
DOI: 10.1046/j.1365-8711.2001.04459.x
We investigate the relative sensitivities of several tests for deviations from Gaussianity in the primordial distribution of density perturbations. We consider models for non-Gaussianity that mimic that which comes from inflation as well as that which comes from topological defects. The tests we consider involve the cosmic microwave background (CMB), large-scale structure, high-redshift galaxies, and the abundances and properties of clusters. We find that the CMB is superior at finding non-Gaussianity in the primordial gravitational potential (as inflation would produce), while observations of high-redshift galaxies are much better suited to find non-Gaussianity that resembles that expected from topological defects. We derive a simple expression that relates the abundance of high-redshift objects in non-Gaussian models to the primordial skewness.https://resolver.caltech.edu/CaltechAUTHORS:20200602-124838541Kinetic decoupling of neutralino dark matter
https://resolver.caltech.edu/CaltechAUTHORS:CHEprd01
Year: 2001
DOI: 10.1103/PhysRevD.64.021302
After neutralinos cease annihilating in the early Universe, they may still scatter elastically from other particles in the primordial plasma. At some point in time, however, they will eventually stop scattering. We calculate the cross sections for neutralino elastic scattering from standard-model particles to determine the time at which this kinetic decoupling occurs. We show that kinetic decoupling occurs above a temperature T∼MeV. Thereafter, neutralinos act as collisionless cold dark matter.https://resolver.caltech.edu/CaltechAUTHORS:CHEprd01Dark-matter spike at the galactic center?
https://resolver.caltech.edu/CaltechAUTHORS:ULLprd01
Year: 2001
DOI: 10.1103/PhysRevD.64.043504
The past growth of the central black hole (BH) might have enhanced the density of cold dark matter halo particles at the galactic center. We compute this effect in realistic growth models of the present (2-3)×106M⊙ BH from a low-mass seed BH, with special attention to dynamical modeling in a realistic galaxy environment with merger and orbital decay of a seed BH formed generally outside the exact center of the halo. An intriguing "very-dense spike" of dark matter has been claimed in models of Gondolo and Silk with a density high enough to contradict with experimental upper bounds of neutralino annihilation radiation. This "spike" disappears completely or is greatly weakened when we include important dynamical processes neglected in their idealized or restrictive picture with cold particles surrounding an at-the-center zero-seed adiabatically growing BH. For the seed BH to spiral in and settle to the center within a Hubble time by dynamical friction, the seed mass must be at least a significant fraction of the present BH. Any subsequent at-the-center growth of the BH and steepening of the central Keplerian potential well can squeeze the halo density distribution only mildly, whether the squeezing happens adiabatically or instantaneously.https://resolver.caltech.edu/CaltechAUTHORS:ULLprd01Galactosynthesis predictions at high redshift
https://resolver.caltech.edu/CaltechAUTHORS:20200527-135432922
Year: 2001
DOI: 10.1046/j.1365-8711.2001.04895.x
We predict the Tully–Fisher (TF) and surface-brightness–magnitude relations for disc galaxies at z = 3 and discuss the origin of these scaling relations and their scatter. We find that both halo dynamics and the star formation history play important roles, and we show that the variation of the TF relation with redshift can be a potentially powerful discriminator of galaxy-formation models. In particular, the TF relation at high redshift might be used to break parameter degeneracies among galactosynthesis models at z = 0, as well as to constrain the redshift distribution of collapsing dark-matter haloes, the star formation history and baryon fraction in the disc and the distribution of halo spins.https://resolver.caltech.edu/CaltechAUTHORS:20200527-135432922Statistics of Sunyaev-Zel'dovich cluster surveys
https://resolver.caltech.edu/CaltechAUTHORS:20200602-124838660
Year: 2002
DOI: 10.1046/j.1365-8711.2002.05139.x
We describe a detailed analytic model for predicting statistical quantities (such as number counts, redshift distributions and sizes) of clusters detected in blank-field, thermal Sunyaev-Zel'dovich effect experiments. We include in this model the possibility of non-Gaussian density perturbations in the early Universe and also describe a simple model for the effects of preheating on cluster Sunyaev-Zel'dovich effect fluxes. We use this model to explore the current state of the theoretical uncertainties present in this type of analytic modelling, highlighting where further improvement will be necessary to exploit forthcoming surveys fully. We then go on to explore the constraints on cosmological parameters, the presence of any non-Gaussianity and the degree of cluster preheating that may be obtained from both the Bolocam and Planck experiments. We find that, providing redshifts can be measured for all detected clusters, the Bolocam experiment may provide detections of non-Gaussianity or preheating and could give approximate measurements of these effects if prior knowledge of the various cosmological parameters is taken into account. The Planck experiment Sunyaev-Zel'dovich effect cluster survey is predicted to provide highly accurate (∼5 per cent) measurements of the degree of non-Gaussianity and preheating while also providing measurements of several cosmological parameters to accuracies of a few per cent independently of those constraints that will be derived from its detections of primordial cosmic microwave background anisotropies.https://resolver.caltech.edu/CaltechAUTHORS:20200602-124838660A Hawking-eye View of the Universe
https://resolver.caltech.edu/CaltechAUTHORS:20141119-092534759
Year: 2002
DOI: 10.1126/science.1070877
In the world of Newton and Galileo, space and time were the flat, featureless canvas on which were painted the then-known celestial objects. Einstein's canvas was curved and it had bumps and wiggles caused by the presence of matter. During the past few decades, it is the canvas itself that has increasingly become the focus of study, as the dimensionality, shape, and nature of space and time have become intertwined with subatomic particles, black holes, and superstrings in a unified quest by physicists and cosmologists to understand the laws of physics and how the Universe came to be.https://resolver.caltech.edu/CaltechAUTHORS:20141119-092534759Theoretical estimates of intrinsic galaxy alignment
https://resolver.caltech.edu/CaltechAUTHORS:20200603-094959825
Year: 2002
DOI: 10.1046/j.1365-8711.2002.05337.x
It has recently been argued that the observed ellipticities of galaxies may be determined at least in part by the primordial tidal gravitational field in which the galaxy formed. Long-range correlations in the tidal field could thus lead to an ellipticity–ellipticity correlation for widely separated galaxies. We present a new model relating ellipticity to angular momentum, which can be calculated in linear theory. We use this model to calculate the angular power spectrum of intrinsic galaxy shape correlations. We show that, for low-redshift galaxy surveys, our model predicts that intrinsic correlations will dominate correlations induced by weak lensing, in good agreement with previous theoretical work and observations. We find that our model produces 'E-mode' correlations enhanced by a factor of 3.5 over B-modes on small scales, making it harder to disentangle intrinsic correlations from those induced by weak gravitational lensing.https://resolver.caltech.edu/CaltechAUTHORS:20200603-094959825Separation of Gravitational-Wave and Cosmic-Shear Contributions to Cosmic Microwave Background Polarization
https://resolver.caltech.edu/CaltechAUTHORS:KESprl02
Year: 2002
DOI: 10.1103/PhysRevLett.89.011304
Inflationary gravitational waves (GW) contribute to the curl component in the polarization of the cosmic microwave background (CMB). Cosmic shear—gravitational lensing of the CMB—converts a fraction of the dominant gradient polarization to the curl component. Higher-order correlations can be used to map the cosmic shear and subtract this contribution to the curl. Arcminute resolution will be required to pursue GW amplitudes smaller than those accessible by the Planck surveyor mission. The blurring by lensing of small-scale CMB power leads with this reconstruction technique to a minimum detectable GW amplitude corresponding to an inflation energy near 10^15 GeV.https://resolver.caltech.edu/CaltechAUTHORS:KESprl02X-rays from isolated black holes in the Milky Way
https://resolver.caltech.edu/CaltechAUTHORS:20200602-124838791
Year: 2002
DOI: 10.1046/j.1365-8711.2002.05523.x
Galactic stellar-population-synthesis models, chemical-enrichment models, and possibly gravitational microlensing indicate that about N_(tot) = 10⁸–10⁹ stellar-mass black holes reside in our Galaxy. We study X-ray emission from accretion from the interstellar medium on to isolated black holes. Although black holes may be fewer in number than neutron stars, N_(ɴꜱ)∼10⁹, their higher masses, 〈M〉∼9 M_⊙, and smaller space velocities, σ_υ∼40 km s⁻¹, result in Bondi–Hoyle accretion rates ∼4×10³ times higher than for neutron stars. Given a total number of black holes N_(tot) = N₉10⁹ within the Milky Way, we estimate that ∼10³N₉ should accrete at Ṁ > 10¹⁵ g s⁻¹, comparable to accretion rates inferred for black hole X-ray binaries. If black holes accrete at the Bondi–Hoyle rate with efficiencies only ∼10⁻⁴(N_(ɴꜱ)/N_(tot))^(0.8) of the neutron-star accretion efficiency, a comparable number of each may be detectable. We make predictions for the number of isolated accreting black holes in our Galaxy that can be detected with X-ray surveys as a function of efficiency, concluding that all-sky surveys at a depth of F = F₋₁₅10⁻¹⁵ erg cm⁻² s⁻¹ dex⁻¹ can find N(> F)∼10⁴N⁹(F₋₁₅/ε₋₅)^(1.2) isolated accreting black holes for a velocity dispersion of 40 km s⁻¹ and an X-ray accretion efficiency of ε = ε-₅10⁻⁵. Deeper surveys of the Galactic plane with Chandra or XMM-Newton may find tens of these objects per year, depending on the efficiency. We argue that a mass estimate can be derived for microlensing black hole candidates with an X-ray detection.https://resolver.caltech.edu/CaltechAUTHORS:20200602-124838791Weak lensing of the CMB: Cumulants of the probability distribution function
https://resolver.caltech.edu/CaltechAUTHORS:KESprd02
Year: 2002
DOI: 10.1103/PhysRevD.66.083007
We discuss the real-space moments of temperature anisotropies in the cosmic microwave background (CMB) due to weak gravitational lensing by intervening large-scale structure. We show that if the probability distribution function of primordial temperature anisotropies is Gaussian, then it remains unchanged after gravitational lensing. With finite resolution, however, nonzero higher-order cumulants are generated both by lensing autocorrelations and by cross-correlations between the lensing potential and secondary anisotropies in the CMB such as the Sunayev-Zel'dovich (SZ) effect. Skewness is produced by these lensing-SZ correlations, while kurtosis receives contributions from both lensing alone and lensing-SZ correlations. We show that if the projected lensing potential is Gaussian, all cumulants of higher order than the kurtosis vanish. While recent results raise the possibility of detection of the skewness in upcoming data, the kurtosis will likely remain undetected.https://resolver.caltech.edu/CaltechAUTHORS:KESprd02The contribution of the first stars to the cosmic infrared background
https://resolver.caltech.edu/CaltechAUTHORS:20200602-124838918
Year: 2002
DOI: 10.1046/j.1365-8711.2002.05895.x
We calculate the contribution to the cosmic infrared background from very massive metal-free stars at high redshift. We explore two plausible star formation models and two limiting cases for the reprocessing of the ionizing stellar emission. We find that Population III stars may contribute significantly to the cosmic near-infrared background if the following conditions are met. (i) The first stars were massive, with M≳ 100 M⊙. (ii) Molecular hydrogen can cool baryons in low-mass haloes. (iii) Population III star formation is ongoing, and not shut off through negative feedback effects. (iv) Virialized haloes form stars at ∼40 per cent efficiency up to the redshift of reionization, z∼ 7. (v) The escape fraction of the ionizing radiation into the intergalactic medium is small. (vi) Nearly all of the stars end up in massive black holes without contributing to the metal enrichment of the Universe.https://resolver.caltech.edu/CaltechAUTHORS:20200602-124838918A New Window to the Early Universe
https://resolver.caltech.edu/CaltechAUTHORS:20141114-151439594
Year: 2002
DOI: 10.1126/science.1078736
Small temperature variations in the cosmic microwave background (CMB), the radiation left over from the big bang, have shed much light on the early universe. According to predictions, the CMB should also be polarized, providing a better test for early-universe theories than temperature alone. In their Perspective, Hivon and Kamionkowski highlight the recent detection of the CMB polarization. The new results end a 34-year quest to verify the predictions, but are only the first of many upcoming studies of the CMB polarization.https://resolver.caltech.edu/CaltechAUTHORS:20141114-151439594Strange matters: Undiscovered ideas at the frontiers of space and time [Book Review]
https://resolver.caltech.edu/CaltechAUTHORS:20150325-150151172
Year: 2002
DOI: 10.1038/420362a
For almost 30 years, physicists have been
facing a long haul across the almost interminable
desert to the grand unified theory
(GUT) of strong, weak and electromagnetic
forces. This theory postulates an almost
unimaginably huge gap between the energies
of current particle accelerators and those
at which the manifestations of unification
become most apparent. If that's not sufficiently
daunting, string theorists have
speculated further, suggesting that gravity
is tied to the other three interactions at an
even higher energy scale.https://resolver.caltech.edu/CaltechAUTHORS:20150325-150151172Weak gravitational lensing by dark clusters
https://resolver.caltech.edu/CaltechAUTHORS:20200527-135433015
Year: 2002
DOI: 10.1046/j.1365-8711.2002.05977.x
We calculate the abundance of dark matter concentrations that are sufficiently overdense to produce a detectable weak-gravitational-lensing signal. Most of these overdensities are virialized haloes containing identifiable X-ray and/or optical clusters. However, a significant fraction are non-virialized, cluster-mass overdensities still in the process of gravitational collapse — these should produce significantly weaker or no X-ray emission. Our predicted abundance of such dark clusters is consistent with the abundance implied by the detection by Erben et al. of an apparent dark lens. Weak lensing by these non-virialized objects will need to be considered when determining cosmological parameters with the lens abundance in future weak-lensing surveys. Such weak lenses should also help shed light on the process of cluster formation.https://resolver.caltech.edu/CaltechAUTHORS:20200527-135433015Aspects of the cosmic microwave background dipole
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd03
Year: 2003
DOI: 10.1103/PhysRevD.67.063001
Cosmic microwave background (CMB) experiments generally infer a temperature fluctuation from a measured intensity fluctuation through the first term in the Taylor expansion of the Planck function, the relation between the intensity in a given frequency and the temperature. However, with the forthcoming Planck satellite, and perhaps even with the Microwave Anisotropy Probe, the CMB-dipole amplitude will be large enough to warrant inclusion of the next higher order term. To quadratic order in the dipole amplitude, there is an intensity quadrupole induced by the dipole with a frequency dependence given by the second derivative of the Planck function. The Planck satellite should be able to detect this dipole-induced intensity quadrupole and distinguish it through its frequency dependence from the intrinsic CMB temperature and foreground quadrupoles. This higher-order effect provides a robust pre-determined target that may provide tests of Planck's and MAP's large-angle-fluctuation measurements and of their techniques for multifrequency foreground subtraction.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd03Constraining dark energy from the abundance of weak gravitational lenses
https://resolver.caltech.edu/CaltechAUTHORS:20200610-122805835
Year: 2003
DOI: 10.1046/j.1365-8711.2003.06421.x
We examine the prospect of using the observed abundance of weak gravitational lenses to constrain the equation-of-state parameter w = p/ρ of dark energy. Dark energy modifies the distance-redshift relation, the amplitude of the matter power spectrum, and the rate of structure growth. As a result, it affects the efficiency with which dark-matter concentrations produce detectable weak-lensing signals. Here we solve the spherical-collapse model with dark energy, clarifying some ambiguities found in the literature. We also provide fitting formulae for the non-linear overdensity at virialization and the linear-theory overdensity at collapse. We then compute the variation in the predicted weak-lens abundance with w. We find that the predicted redshift distribution and number count of weak lenses are highly degenerate in w and the present matter density Ω₀. If we fix Ω₀ the number count of weak lenses for w = −2/3 is a factor of ∼2 smaller than for the Λ cold dark matter (CDM) model w = −1. However, if we allow Ω₀ to vary with w such that the amplitude of the matter power spectrum as measured by the Cosmic Background Explorer (COBE) matches that obtained from the X-ray cluster abundance, the decrease in the predicted lens abundance is less than 25 per cent for −1 ⩽ w < −0.4. We show that a more promising method for constraining dark energy - one that is largely unaffected by the Ω₀−w degeneracy as well as uncertainties in observational noise - is to compare the relative abundance of virialized X-ray lensing clusters with the abundance of non-virialized, X-ray underluminous, lensing haloes. For aperture sizes of ∼15 arcmin, the predicted ratio of the non-virialized to virialized lenses is greater than 40 per cent and varies by ∼20 per cent between w = −1 and −0.6. Overall, we find that, if all other weak-lensing parameters are fixed, a survey must cover at least ∼40 deg² in order for the weak-lens number count to differentiate a ΛCDM cosmology from a dark-energy model with w = −0.9 at the 3σ level. If, on the other hand, we take into account uncertainties in the lensing parameters, then the non-virialized lens fraction provides the most robust constraint on w, requiring ∼50 deg² of sky coverage in order to differentiate a ΛCDM model from a w = −0.6 model to 3σ.https://resolver.caltech.edu/CaltechAUTHORS:20200610-122805835Sunyaev-Zeldovich fluctuations from the first stars?
https://resolver.caltech.edu/CaltechAUTHORS:20200610-122805615
Year: 2003
DOI: 10.1046/j.1365-8711.2003.06708.x
WMAP's detection of high electron-scattering optical depth τe suggests substantial star formation at high redshift z∼ 17 ± 5. On the other hand, the recovered σ8∼ 0.84 ± 0.04 argues against a cluster Sunyaev–Zeldovich (SZ) origin for the observed small-scale cosmic microwave background (CMB) fluctuation excess, which generally requires σ8∼ 1.1. Here we consider the effects of high-redshift star formation on the CMB. We derive a fairly model-independent relation between τₑ and the number of ionizing photons emitted per baryon Nᵧ, and use this to calibrate the amount of high-redshift supernova activity. The resulting supernova remnants Compton cool against the CMB creating a Compton y distortion y∼ few × 10⁻⁶ within observational bounds. However they also create small-scale SZ fluctuations, which could be comparable with SZ fluctuations from unresolved galaxy clusters. This raises the exciting possibility that we have already detected signatures of the first stars not just once, but twice, in the CMB.https://resolver.caltech.edu/CaltechAUTHORS:20200610-122805615Lensing reconstruction with CMB temperature and polarization
https://resolver.caltech.edu/CaltechAUTHORS:KESprd03
Year: 2003
DOI: 10.1103/PhysRevD.67.123507
Weak gravitational lensing by an intervening large-scale structure induces a distinct signature in the cosmic microwave background (CMB) that can be used to reconstruct the weak-lensing displacement map. Estimators for individual Fourier modes of this map can be combined to produce an estimator for the lensing-potential power spectrum. The naive estimator for this quantity will be biased upwards by the uncertainty associated with reconstructing individual modes; we present an iterative scheme for removing this bias. The variance and covariance of the lensing-potential power spectrum estimator are calculated and evaluated numerically in a LambdaCDM universe for Planck and future polarization-sensitive CMB experiments.https://resolver.caltech.edu/CaltechAUTHORS:KESprd03Phantom Energy: Dark Energy with w<–1 Causes a Cosmic Doomsday
https://resolver.caltech.edu/CaltechAUTHORS:CALprl03
Year: 2003
DOI: 10.1103/PhysRevLett.91.071301
We explore the consequences that follow if the dark energy is phantom energy, in which the sum of the pressure and energy density is negative. The positive phantom-energy density becomes infinite in finite time, overcoming all other forms of matter, such that the gravitational repulsion rapidly brings our brief epoch of cosmic structure to a close. The phantom energy rips apart the Milky Way, solar system, Earth, and ultimately the molecules, atoms, nuclei, and nucleons of which we are composed, before the death of the Universe in a "big rip."https://resolver.caltech.edu/CaltechAUTHORS:CALprl03Spatial variation of the fine-structure parameter and the cosmic microwave background
https://resolver.caltech.edu/CaltechAUTHORS:SIGprd03
Year: 2003
DOI: 10.1103/PhysRevD.68.103509
We study the effects on cosmic microwave background (CMB) temperature and polarization anisotropies of spatial fluctuations of the fine-structure parameter between causally disconnected regions of the Universe at the time of recombination. Analogous to weak gravitational lensing, in addition to modifying the mean power spectra and inducing a curl component (B mode) to the polarization, spatial fluctuations of the fine-structure parameter induce higher-order (non-Gaussian) temperature and polarization correlations in the CMB. We calculate these effects for the general case of arbitrary correlation between temperature fluctuations and fine-structure parameter fluctuations, and show the results for a model where these two types of fluctuations are uncorrelated. The formalism we present here may also be applied to other modifications of recombination physics that do not significantly alter the evolution of the dominant density perturbations. We discuss the constraints on the effective Lagrangian for the variable fine-structure parameter necessary to realize this scenario.https://resolver.caltech.edu/CaltechAUTHORS:SIGprd03Can Cosmic Shear Shed Light on Low Cosmic Microwave Background Multipoles?
https://resolver.caltech.edu/CaltechAUTHORS:KESprl03
Year: 2003
DOI: 10.1103/PhysRevLett.91.221302
The lowest multipole moments of the cosmic microwave background (CMB) are smaller than expected for a scale-invariant power spectrum. One possible explanation is a cutoff in the primordial power spectrum below a comoving scale of kc~=5.0×10^–4 Mpc^–1. Such a cutoff would increase significantly the cross correlation between the large-angle CMB and cosmic-shear patterns. The cross correlation may be detectable at >2sigma which, combined with the low CMB moments, may tilt the balance between a 2sigma result and a firm detection of a large-scale power-spectrum cutoff. The cutoff also increases the large-angle cross correlation between the CMB and the low-redshift tracers of the mass distribution.https://resolver.caltech.edu/CaltechAUTHORS:KESprl03New Contribution to Scattering of Weakly Interacting Massive Particles on Nuclei
https://resolver.caltech.edu/CaltechAUTHORS:PREprl03
Year: 2003
DOI: 10.1103/PhysRevLett.91.231301
A weakly interacting massive particle (WIMP) is perhaps the most promising candidate for the dark matter in the Galactic halo. The WIMP detection rate in laboratory searches is fixed by the cross section for elastic WIMP-nucleus scattering. Here we calculate the contribution to this cross section from two-nucleon currents from pion exchange in the nucleus and show that it may, in some cases, be comparable to the one-nucleon current that has been considered in prior work and perhaps help resolve the discrepancies between the various direct dark-matter search experiments. We provide simple expressions that allow these new contributions to be included in current calculations.https://resolver.caltech.edu/CaltechAUTHORS:PREprl03Generalized analysis of the direct weakly interacting massive particle searches
https://resolver.caltech.edu/CaltechAUTHORS:KURprd04
Year: 2004
DOI: 10.1103/PhysRevD.69.063503
We perform a generalized analysis of data from WIMP search experiments for pointlike WIMPs of arbitrary spin and general Lorenz-invariant WIMP-nucleus interaction. We explicitly show that in the nonrelativistic limit only spin-independent (SI) and spin-dependent (SD) WIMP-nucleon interactions survive, which can be parametrized by only five independent parameters. We explore this five-dimensional parameter space to determine whether the annual modulation observed in the DAMA experiment can be consistent with all other experiments which reported null results. The pure SI interaction is ruled out except for a very small region of parameter space with the WIMP mass close to 50 GeV and the ratio of the WIMP-neutron to WIMP-proton SI couplings –0.77<=fn/fp<=–0.75. For the predominantly SD interaction, we find an upper limit on the WIMP mass of about 18 GeV, which can only be weakened if the constraint stemming from null searches for energetic neutrinos from WIMP annihilation the Sun is evaded. None of the regions of the parameter space that can reconcile all WIMP search results can be easily accommodated in the minimal supersymmetric standard model.https://resolver.caltech.edu/CaltechAUTHORS:KURprd04Charged-particle decay and suppression of primordial power on small scales
https://resolver.caltech.edu/CaltechAUTHORS:SIGprl04
Year: 2004
DOI: 10.1103/PhysRevLett.92.171302
We study the suppression of the small-scale power spectrum due to the decay of charged matter to dark matter prior to recombination. Prior to decay, the charged particles couple to the photon-baryon fluid and participate in its acoustic oscillations. However, after these charged particles decay to neutral dark matter, the photon-baryon fluid is coupled only gravitationally to the newly created dark matter. This generically leads to suppression of power on length scales that enter the horizon prior to decay. For decay times of ~3.5 yr this leads to suppression of power on subgalactic scales, bringing the observed number of galactic substructures in line with observation. Decay times of a few years are possible if the dark matter is purely gravitationally interacting, such as the gravitino in supersymmetric models or a massive Kaluza-Klein graviton in models with universal extra dimensions.https://resolver.caltech.edu/CaltechAUTHORS:SIGprl04Particle decays during the cosmic dark ages
https://resolver.caltech.edu/CaltechAUTHORS:CHEprd04
Year: 2004
DOI: 10.1103/PhysRevD.70.043502
We consider particle decays during the cosmic dark ages with two aims: (1) to explain the high optical depth reported by the Wilkinson Microwave Anisotropy Probe (WMAP), and (2) to provide new constraints to the parameter space for decaying particles. We delineate the decay channels in which most of the decay energy ionizes and heats the intergalactic medium gas [and thus affects the cosmic microwave background (CMB)], and those in which most of the energy is carried away—e.g. photons with energies 100 keV<~E<~1 TeV—and thus appears as a contribution to diffuse x-ray and gamma-ray backgrounds. The new constraints to the decay-particle parameters from the CMB power spectrum thus complement those from the cosmic x-ray and gamma-ray backgrounds. Although decaying particles can indeed produce an optical depth consistent with that reported by WMAP, in so doing they produce new fluctuations in the CMB temperature and polarization power spectra. For decay lifetimes less than the age of the Universe, the induced power spectra generally violate current constraints, while the power spectra are usually consistent if the lifetime is longer than the age of the Universe.https://resolver.caltech.edu/CaltechAUTHORS:CHEprd04Expansion, geometry, and gravity
https://resolver.caltech.edu/CaltechAUTHORS:CALjcap04
Year: 2004
DOI: 10.1088/1475-7516/2004/09/009
In general-relativistic cosmological models, the expansion history, matter content, and geometry are closely intertwined. In this brief paper, we clarify the distinction between the effects of geometry and expansion history on the luminosity distance. We show that the cubic correction to the Hubble law, measured recently with high-redshift supernovae, is the first cosmological measurement, apart from the cosmic microwave background, that probes directly the effects of spatial curvature. We illustrate the distinction between geometry and expansion with a toy model for which the supernova results already indicate a curvature radius larger than the Hubble distance.https://resolver.caltech.edu/CaltechAUTHORS:CALjcap04Dark-matter electric and magnetic dipole moments
https://resolver.caltech.edu/CaltechAUTHORS:SIGprd04
Year: 2004
DOI: 10.1103/PhysRevD.70.083501
We consider the consequences of a neutral dark-matter particle with a nonzero electric and/or magnetic dipole moment. Theoretical constraints, as well as constraints from direct searches, precision tests of the standard-model, the cosmic microwave background and matter power spectra, and cosmic gamma rays, are included. We find that a relatively light particle with mass between an MeV and a few GeV and an electric or magnetic dipole as large as ~3 x 10 to the -16 e cm (roughly 1.6 x 10 to the -5 μB) satisfies all experimental and observational bounds. Some of the remaining parameter space may be probed with forthcoming more sensitive direct searches and with the Gamma-Ray Large Area Space Telescope.https://resolver.caltech.edu/CaltechAUTHORS:SIGprd04Dynamical-friction galaxy-gas coupling and cluster cooling flows
https://resolver.caltech.edu/CaltechAUTHORS:20200608-110444545
Year: 2004
DOI: 10.1111/j.1365-2966.2004.08175.x
We revisit the notion that galaxy motions can efficiently heat intergalactic gas in the central regions of clusters through dynamical friction. For plausible values of the galaxy mass‐to‐light ratio, the heating rate is comparable with the cooling rate due to X‐ray emission. Heating occurs only for supersonic galaxy motions, so the mechanism is self‐regulating: it becomes efficient only when the gas sound speed is smaller than the galaxy velocity dispersion. We illustrate with the Perseus cluster, assuming a stellar mass‐to‐light ratio for galaxies in the very central region with the dark matter contribution becoming comparable with this at some radius rₛ. For rₛ≲ 400 kpc ∼ 3r_(cool) – corresponding to an average mass‐to‐light ratio of ∼10 inside that radius – the dynamical‐friction coupling is strong enough to provide the required rate of gas heating. Such values of rₛ are associated with total mass attached to galaxies that is about 10 per cent of the mass of the cluster – consistent with values inferred from numerical simulations and observations. The measured sound speed is smaller than the galaxy velocity dispersion, as required by this mechanism. With this smaller gas temperature and the observed distribution of galaxies and gas, the energy reservoir in galactic motions is sufficient to sustain the required heating rate for the lifetime of the cluster. The galaxies also lose a smaller amount of energy through dynamical friction to the dark matter implying that non‐cooling‐flow clusters should have flat‐cored dark matter density distributions.https://resolver.caltech.edu/CaltechAUTHORS:20200608-110444545A running spectral index in supersymmetric dark-matter models with quasistable charged particles
https://resolver.caltech.edu/CaltechAUTHORS:PROprd05
Year: 2005
DOI: 10.1103/PhysRevD.71.023518
We show that charged particles decaying in the early Universe can induce a scale-dependent or running spectral index in the small-scale linear and nonlinear matter power spectrum and discuss examples of this effect in minimal supersymmetric models in which the lightest neutralino is a viable cold-dark-matter candidate. We find configurations in which the neutralino relic density is set by coannihilations with a long-lived stau and the late decay of staus partially suppresses the linear matter power spectrum. Nonlinear evolution on small scales then causes the modified linear power spectrum to evolve to a nonlinear power spectrum similar to (but different in detail) models parametrized by a constant running alphas = dns/dlnk by redshifts of 2 to 4. Thus, Lyman-alpha forest observations, which probe the matter power spectrum at these redshifts, might not discriminate between the two effects. However, a measurement of the angular power spectrum of primordial 21-cm radiation from redshift z[approximate]30–200 might distinguish between this charged-decay model and a primordial running spectral index. The direct production of a long-lived charged particle at future colliders is a dramatic prediction of this model.https://resolver.caltech.edu/CaltechAUTHORS:PROprd05Gravitational-wave signature of an inspiral into a supermassive horizonless object
https://resolver.caltech.edu/CaltechAUTHORS:KESprd05
Year: 2005
DOI: 10.1103/PhysRevD.71.044015
Event horizons are among the most intriguing of general relativity's predictions. Although on firm theoretical footing, direct indications of their existence have yet to be observed. With this motivation in mind, we explore here the possibility of finding a signature for event horizons in the gravitational waves (GWs) produced during the inspiral of stellar-mass compact objects (COs) into the supermassive (~10 (to the 6th) M[sun]) objects that lie at the center of most galaxies. Such inspirals will be a major source for LISA, the future space-based GW observatory. We contrast supermassive black holes with models in which the central object is a supermassive boson star (SMBS). Provided the COs interact only gravitationally with the SMBS, stable orbits exist not just outside the Schwarzschild radius but also inside the surface of the SMBS as well. The absence of an event horizon allows GWs from these orbits to be observed. Here we solve for the metric in the interior of a fairly generic class of SMBS and evolve the trajectory of an inspiraling CO from the Schwarzschild exterior through the plunge into the exotic SMBS interior. We calculate the approximate waveforms for GWs emitted during this inspiral. Geodesics within the SMBS surface will exhibit extreme pericenter precession and other features making the emitted GWs readily distinguishable from those emitted during an inspiral into a black hole.https://resolver.caltech.edu/CaltechAUTHORS:KESprd05Self-consistent theory of halo mergers
https://resolver.caltech.edu/CaltechAUTHORS:20200608-110444618
Year: 2005
DOI: 10.1111/j.1365-2966.2005.08679.x
The rate of merging of dark matter haloes is an absolutely essential ingredient for studies of both structure and galaxy formation. Remarkably, however, our quantitative understanding of the halo merger rate is still quite limited, and current analytic descriptions based upon the extended Press–Schechter formalism are fundamentally flawed. We show that a mathematically self-consistent merger rate must be consistent with the evolution of the halo abundance in the following sense. The merger rate must, when inserted into the Smoluchowski coagulation equation, yield the correct evolution of the halo abundance. We then describe a numerical technique to find merger rates that are consistent with this evolution. We present Results from a preliminary study in which we find merger rates that reproduce the evolution of the halo abundance according to the Press–Schechter formalism for power-law power spectra. We discuss the limitations of the current approach and outline the questions that must still be answered before we have a fully consistent and correct theory of halo merger rates.https://resolver.caltech.edu/CaltechAUTHORS:20200608-110444618Cosmic shear of the microwave background: The curl diagnostic
https://resolver.caltech.edu/CaltechAUTHORS:COOprd05
Year: 2005
DOI: 10.1103/PhysRevD.71.123527
Weak-lensing distortions of the cosmic-microwave-background (CMB) temperature and polarization patterns can reveal important clues to the intervening large-scale structure. The effect of lensing is to deflect the primary temperature and polarization signal to slightly different locations on the sky. Deflections due to density fluctuations, gradient-type for the gradient of the projected gravitational potential, give a direct measure of the mass distribution. Curl-type deflections can be induced by, for example, a primordial background of gravitational waves from inflation or by second-order effects related to lensing by density perturbations. Whereas gradient-type deflections are expected to dominate, we show that curl-type deflections can provide a useful test of systematics and serve to indicate the presence of confusing secondary and foreground non-Gaussian signals.https://resolver.caltech.edu/CaltechAUTHORS:COOprd05Direct detection of the inflationary gravitational-wave background
https://resolver.caltech.edu/CaltechAUTHORS:SMIprd06a
Year: 2006
DOI: 10.1103/PhysRevD.73.023504
Inflation generically 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) or Japan's Deci-Hertz Interferometer Gravitational-wave Observer (DECIGO), both currently under study. Here we investigate the detectability of the inflationary gravitational-wave background at BBO/DECIGO frequencies. To do so, we survey a range of slow-roll inflationary models consistent with constraints from the CMB and large-scale structure (LSS). We go beyond the usual assumption of power-law power spectra, which may break down given the 16 orders of magnitude in frequency between the CMB and direct detection, and solve instead the inflationary dynamics for four classes of inflaton potentials. Direct detection is possible in a variety of inflationary models, although probably not in any in which the gravitational-wave signal does not appear in the CMB polarization. However, direct detection by BBO/DECIGO can help discriminate between inflationary models that have the same slow-roll parameters at CMB/LSS scales.https://resolver.caltech.edu/CaltechAUTHORS:SMIprd06aPair correlations and merger bias
https://resolver.caltech.edu/CaltechAUTHORS:20200608-110444711
Year: 2006
DOI: 10.1111/j.1365-2966.2005.09880.x
We analytically study the possibility that mergers of haloes are more highly clustered than the general population of haloes of comparable masses. We begin by investigating predictions for merger bias within the extended Press-Schechter formalism and discuss the limitations and ambiguities of this approach. We then postulate that mergers occur whenever two objects form within a (small) fixed distance of each other. We therefore study the clustering of pairs of points for a highly biased population in the linear regime, for the overall mass distribution in the quasi-linear regime and (using the halo model of clustering) in the non-linear regime. Biasing, quasi-linear evolution and non-linear clustering all lead to non-zero reduced (or connected) three- and four-point correlation functions. These higher order correlation functions can in many cases enhance the clustering of close pairs of points relative to the clustering of individual points. If close pairs are likely to merge, then the clustering of mergers may be enhanced. We discuss implications for the observed clustering of luminous z = 3 galaxies and for correlations of active galactic nuclei and galaxy clusters.https://resolver.caltech.edu/CaltechAUTHORS:20200608-110444711Dark matter and the CACTUS gamma-ray excess from Draco
https://resolver.caltech.edu/CaltechAUTHORS:PROjcap06
Year: 2006
DOI: 10.1088/1475-7516/2006/03/003
The CACTUS atmospheric Cherenkov telescope collaboration recently reported a gamma-ray excess from the Draco dwarf spheroidal galaxy. Draco features a very low gas content and a large mass-to-light ratio, suggesting as a possible explanation annihilation of weakly interacting massive particles (WIMPs) in the Draco dark-matter halo. We show that with improved angular resolution, future measurements can determine whether the halo is cored or cuspy, as well as its scale radius. We find the relevant WIMP masses and annihilation cross sections and show that supersymmetric models can account for the required gamma-ray flux. The annihilation cross section range is found to be not compatible with a standard thermal relic dark-matter production. We compute for these supersymmetric models the resulting Draco gamma-ray flux in the GLAST energy range and the rates for direct neutralino detection and for the flux of neutrinos from neutralino annihilation in the Sun. We also discuss the possibility that the bulk of the signal detected by CACTUS comes from direct WIMP annihilation to two photons and point out that a decaying-dark-matter scenario for Draco is not compatible with the gamma-ray flux from the Galactic centre and in the diffuse gamma-ray background.https://resolver.caltech.edu/CaltechAUTHORS:PROjcap06Cluster magnetic fields from large-scale structure and galaxy cluster shocks
https://resolver.caltech.edu/CaltechAUTHORS:MEDapj06
Year: 2006
The origin of the microgauss magnetic fields in galaxy clusters is one of the outstanding problems of modern cosmology. We have performed three-dimensional particle-in-cell simulations of the nonrelativistic Weibel instability in an electron-proton plasma, in conditions typical of cosmological shocks. These simulations indicate that cluster fields could have been produced by shocks propagating through the intergalactic medium during the formation of large-scale structure or by shocks within the cluster. The strengths of the shock-generated fields range from a few nanogauss in the intercluster medium to a fraction of a microgauss inside galaxy clusters. Further amplification of these fields by sheared turbulent motions and gas accretion in clusters may be expected. Thus, cluster fields may be explained without resorting to the amplification of a primordial field.https://resolver.caltech.edu/CaltechAUTHORS:MEDapj06New Cosmic Microwave Background Constraint to Primordial Gravitational Waves
https://resolver.caltech.edu/CaltechAUTHORS:SMIprl06
Year: 2006
DOI: 10.1103/PhysRevLett.97.021301
Primordial gravitational waves (GWs) with frequencies >~10^-15 Hz contribute to the radiation density of the Universe at the time of decoupling of the cosmic microwave background (CMB). This affects the CMB and matter power spectra in a manner identical to massless neutrinos, unless the initial density perturbation for the GWs is nonadiabatic, as may occur if such GWs are produced during inflation or some post-inflation phase transition. In either case, current observations provide a constraint to the GW amplitude that competes with that from big-bang nucleosynthesis (BBN), although it extends to much lower frequencies (~10^-15 Hz rather than the ~10^-10 Hz from BBN): at 95% confidence level, Omegagwh2<~8.4×10^-6 for homogeneous (i.e., nonadiabatic) initial conditions. Future CMB experiments, like Planck and CMBPol, should allow sensitivities to Omegagwh2<~1.4×10^-6 and Omegagwh2<~5×10^-7, respectively.https://resolver.caltech.edu/CaltechAUTHORS:SMIprl06What Mass Are the Smallest Protohalos?
https://resolver.caltech.edu/CaltechAUTHORS:PROprl06
Year: 2006
We calculate the kinetic-decoupling temperature for weakly interacting massive particles (WIMPs) in supersymmetric (SUSY) and extra dimensional models that can account for the cold-dark-matter abundance determined from cosmic microwave background measurements. Depending on the parameters of the particle-physics model, a wide variety of decoupling temperatures is possible, ranging from several MeV to a few GeV. These decoupling temperatures imply a range of masses for the smallest protohalos much larger than previously thought—ranging from 10-6M[direct-sum] to 102M[direct-sum]. We expect the range of protohalos masses derived here to be characteristic of most particle-physics models that can thermally accommodate the required relic abundance of WIMP dark matter, even beyond SUSY and extra dimensions.https://resolver.caltech.edu/CaltechAUTHORS:PROprl06Cosmological bounds on dark-matter-neutrino interactions
https://resolver.caltech.edu/CaltechAUTHORS:MANprd06c
Year: 2006
DOI: 10.1103/PhysRevD.74.043517
We investigate the cosmological effects of a neutrino interaction with cold dark-matter. We postulate a neutrino that interacts with a "neutrino-interacting dark-matter" (NIDM) particle with an elastic-scattering cross section that either decreases with temperature as T2 or remains constant with temperature. The neutrino-dark-matter interaction results in a neutrino-dark-matter fluid with pressure, and this pressure results in diffusion-damped oscillations in the matter power spectrum, analogous to the acoustic oscillations in the baryon-photon fluid. We discuss the bounds from the Sloan Digital Sky Survey on the NIDM opacity (ratio of cross section to NIDM-particle mass) and compare with the constraint from observation of neutrinos from supernova 1987A. If only a fraction of the dark matter interacts with neutrinos, then NIDM oscillations may affect current cosmological constraints from measurements of galaxy clustering. We discuss how detection of NIDM oscillations would suggest a particle-antiparticle asymmetry in the dark-matter sector.https://resolver.caltech.edu/CaltechAUTHORS:MANprd06cGalilean Equivalence for Galactic Dark Matter
https://resolver.caltech.edu/CaltechAUTHORS:KESprl06
Year: 2006
DOI: 10.1103/PhysRevLett.97.131303
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.https://resolver.caltech.edu/CaltechAUTHORS:KESprl06Supermassive black hole merger rates: uncertainties from halo merger theory
https://resolver.caltech.edu/CaltechAUTHORS:20110408-133955916
Year: 2006
DOI: 10.1111/j.1365-2966.2006.10838.x
The merger of two supermassive black holes is expected to produce a gravitational-wave signal detectable by the Laser Interferometer Space Antenna(LISA). The rate of supermassive-black-hole mergers is intimately connected to the halo merger rate, and the extended Press-Schechter (EPS) formalism is often employed when calculating the rate at which these events will be observed by LISA. This merger theory is flawed and provides two rates for the merging of the same pair of haloes. We show that the two predictions for the LISA supermassive-black-hole-merger event rate from EPS merger theory are nearly equal because mergers between haloes of similar masses dominate the event rate. An alternative merger rate may be obtained by inverting the Smoluchowski coagulation equation to find the merger rate that preserves the Press–Schechter halo abundance, but these rates are only available for power-law power spectra. We compare the LISA event rates derived from the EPS merger formalism to those derived from the merger rates obtained from the coagulation equation and find that the EPS LISA event rates are 30 per cent higher for a power spectrum spectral index that approximates the full Λ cold dark matter result of the EPS theory.https://resolver.caltech.edu/CaltechAUTHORS:20110408-133955916Tidal tails test the equivalence principle in the dark-matter sector
https://resolver.caltech.edu/CaltechAUTHORS:KESprd06
Year: 2006
DOI: 10.1103/PhysRevD.74.083007
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.https://resolver.caltech.edu/CaltechAUTHORS:KESprd06Solar system tests do rule out 1/R gravity
https://resolver.caltech.edu/CaltechAUTHORS:ERIprd06
Year: 2006
DOI: 10.1103/PhysRevD.74.121501
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.https://resolver.caltech.edu/CaltechAUTHORS:ERIprd06Galaxy surveys, inhomogeneous re-ionization and dark energy
https://resolver.caltech.edu/CaltechAUTHORS:PRImnras07a
Year: 2007
DOI: 10.1111/j.1365-2966.2006.11131.x
We examine the effect of inhomogeneous re-ionization on the galaxy power spectrum and the consequences for probing dark energy. To model feedback during re-ionization, we apply an ansatz setting the galaxy overdensity proportional to the underlying ionization field. Thus, inhomogeneous re-ionization may leave an imprint in the galaxy power spectrum. We evolve this imprint to low redshift and use the Fisher-matrix formalism to assess the effect on parameter estimation. We show that a combination of low-redshift (z= 0.3) and high-redshift (z= 3) galaxy surveys can constrain the size of cosmological H ii regions during re-ionization. This imprint can also cause confusion when using baryon oscillations or other features of the galaxy power spectrum to probe the dark energy. We show that when bubbles are large, and hence detectable, our ability to constrain w can be degraded by up to 50 per cent. When bubbles are small, the imprint has little or no effect on measuring dark energy parameters.https://resolver.caltech.edu/CaltechAUTHORS:PRImnras07aTelescope search for decaying relic axions
https://resolver.caltech.edu/CaltechAUTHORS:GRIprd07
Year: 2007
DOI: 10.1103/PhysRevD.75.105018
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.https://resolver.caltech.edu/CaltechAUTHORS:GRIprd07Cluster magnetic fields from large scale structure shocks
https://resolver.caltech.edu/CaltechAUTHORS:MEDaipcp07
Year: 2007
DOI: 10.1063/1.2778953
The origin of the micro-Gauss magnetic fields in galaxy clusters is one of the outstanding problem of modern cosmology. We suggest that these fields could have been produced via the Weibel instability operating at shocks during the Large-Scale Structure formation and at accretion shocks in Galaxy Clusters. We have performed three-dimensional particle-in-cell simulations of the nonrelativistic Weibel instability in an electron-proton plasma (with the ion-to-electron mass ratio of 100), in conditions typical of cosmological shocks. These simulations indicate that cluster fields could have been produced by shocks propagating through the intergalactic medium during the formation of large-scale structure or by shocks within the cluster. The strengths of the shock-generated fields range from tens of nano-Gauss in the intercluster medium to a few micro-Gauss inside galaxy clusters. We discuss whether and how our results may change with the change the mass ratio to the realistic value of 1836. We stress that even if the Weibel-generated small-scale magnetic fields decay with time, they can serve as seed fields that can be further amplified and inverse-cascade to larger scales by turbulent motions of post-shock MHD turbulence.https://resolver.caltech.edu/CaltechAUTHORS:MEDaipcp07Search with EGRET for a gamma ray line from the Galactic center
https://resolver.caltech.edu/CaltechAUTHORS:PULprd07c
Year: 2007
DOI: 10.1103/PhysRevD.76.063006
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.https://resolver.caltech.edu/CaltechAUTHORS:PULprd07cConstraints on radiative dark-matter decay from the cosmic microwave background
https://resolver.caltech.edu/CaltechAUTHORS:ZHAprd07
Year: 2007
DOI: 10.1103/PhysRevD.76.061301
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.https://resolver.caltech.edu/CaltechAUTHORS:ZHAprd07Cosmic microwave background statistics for a direction-dependent primordial power spectrum
https://resolver.caltech.edu/CaltechAUTHORS:PULprd07b
Year: 2007
DOI: 10.1103/PhysRevD.76.103529
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.https://resolver.caltech.edu/CaltechAUTHORS:PULprd07bEffects of Chern-Simons gravity on bodies orbiting the Earth
https://resolver.caltech.edu/CaltechAUTHORS:SMIprd08
Year: 2008
DOI: 10.1103/PhysRevD.77.024015
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.https://resolver.caltech.edu/CaltechAUTHORS:SMIprd08Nonlinear Evolution of Anisotropic Cosmological Power
https://resolver.caltech.edu/CaltechAUTHORS:ANDprl08
Year: 2008
DOI: 10.1103/PhysRevLett.100.071301
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.https://resolver.caltech.edu/CaltechAUTHORS:ANDprl08Red density perturbations and inflationary gravitational waves
https://resolver.caltech.edu/CaltechAUTHORS:PAGjcap08
Year: 2008
DOI: 10.1088/1475-7516/2008/04/009
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.https://resolver.caltech.edu/CaltechAUTHORS:PAGjcap08Axion constraints in nonstandard thermal histories
https://resolver.caltech.edu/CaltechAUTHORS:GRIprd08c
Year: 2008
DOI: 10.1103/PhysRevD.77.085020
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.https://resolver.caltech.edu/CaltechAUTHORS:GRIprd08cGalactic substructure and direct detection of dark matter
https://resolver.caltech.edu/CaltechAUTHORS:KAMprd08
Year: 2008
DOI: 10.1103/PhysRevD.77.103509
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.https://resolver.caltech.edu/CaltechAUTHORS:KAMprd08Very broad [O III] λλ4959, 5007 emission from the NGC 4472 globular cluster RZ 2109 and implications for the mass of its black hole X-ray source
https://resolver.caltech.edu/CaltechAUTHORS:ZEPapjl08
Year: 2008
DOI: 10.1086/591937
We present Keck LRIS spectroscopy of the black hole–hosting globular cluster RZ 2109 in the Virgo elliptical galaxy NGC 4472. We find that this object has extraordinarily broad [O III] λ5007 and [O III] λ4959 emission lines, with velocity widths of approximately 2000 km s^-1. This result has significant implications for the nature of this accreting black hole system and the mass of the globular cluster black hole. We show that the broad [O III] λ5007 emission must arise from material driven at high velocity from the black hole system. This is because the volume available near the black hole is too small by many orders of magnitude to have enough [O III]-emitting atoms to account for the observed L([O III] λ5007) at high velocities, even if this volume is filled with oxygen at the critical density for [O III] λ5007. The Balmer emission is also weak, indicating the observed [O III] is not due to shocks. We therefore conclude that the [O III] λλ4959, 5007 is produced by photoionization of material driven across the cluster. The only known way to drive significant material at high velocity is for a system accreting mass near or above its Eddington limit, which indicates a stellar-mass black hole. Since it is dynamically implausible to form an accreting stellar-mass black hole system in a globular cluster with an intermediate-mass black hole (IMBH), it appears this massive globular cluster does not have an IMBH. We discuss further tests of this conclusion, and its implications for the MBH-Mstellar and MBH-σ relations.https://resolver.caltech.edu/CaltechAUTHORS:ZEPapjl08Dynamical and gravitational instability of an oscillating-field dark energy and dark matter
https://resolver.caltech.edu/CaltechAUTHORS:JOHprd08
Year: 2008
DOI: 10.1103/PhysRevD.78.063010
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⊕.https://resolver.caltech.edu/CaltechAUTHORS:JOHprd08Inflationary gravitational-wave background and measurements of the scalar spectral index
https://resolver.caltech.edu/CaltechAUTHORS:SMIprd08b
Year: 2008
DOI: 10.1103/PhysRevD.78.083525
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.https://resolver.caltech.edu/CaltechAUTHORS:SMIprd08bSuperhorizon perturbations and the cosmic microwave background
https://resolver.caltech.edu/CaltechAUTHORS:ERIprd08
Year: 2008
DOI: 10.1103/PhysRevD.78.083012
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.https://resolver.caltech.edu/CaltechAUTHORS:ERIprd08Can proper motions of dark-matter subhalos be detected?
https://resolver.caltech.edu/CaltechAUTHORS:ANDprd08
Year: 2008
DOI: 10.1103/PhysRevD.78.101301
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.https://resolver.caltech.edu/CaltechAUTHORS:ANDprd08A hemispherical power asymmetry from inflation
https://resolver.caltech.edu/CaltechAUTHORS:ERIprd08b
Year: 2008
DOI: 10.1103/PhysRevD.78.123520
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.https://resolver.caltech.edu/CaltechAUTHORS:ERIprd08bEarly Annihilation and Diffuse Backgrounds in Models of Weakly Interacting Massive Particles in Which the Cross Section for Pair Annihilation Is Enhanced by 1/v
https://resolver.caltech.edu/CaltechAUTHORS:KAMprl08
Year: 2008
DOI: 10.1103/PhysRevLett.101.261301
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.https://resolver.caltech.edu/CaltechAUTHORS:KAMprl08New DAMA dark-matter window and energetic-neutrino searches
https://resolver.caltech.edu/CaltechAUTHORS:20090413-094317493
Year: 2009
DOI: 10.1103/PhysRevD.79.015010
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.https://resolver.caltech.edu/CaltechAUTHORS:20090413-094317493The void abundance with non-gaussian primordial perturbations
https://resolver.caltech.edu/CaltechAUTHORS:KAMjcap09
Year: 2009
DOI: 10.1088/1475-7516/2009/01/010
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.https://resolver.caltech.edu/CaltechAUTHORS:KAMjcap09Dark matter and dark radiation
https://resolver.caltech.edu/CaltechAUTHORS:ACKprd09
Year: 2009
DOI: 10.1103/PhysRevD.79.023519
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.https://resolver.caltech.edu/CaltechAUTHORS:ACKprd09How to Derotate the Cosmic Microwave Background Polarization
https://resolver.caltech.edu/CaltechAUTHORS:20090908-103552819
Year: 2009
DOI: 10.1103/PhysRevLett.102.111302
If the linear polarization of the cosmic microwave background is rotated in a frequency-independent
manner as it propagates from the surface of last scatter, it may introduce a B-mode polarization. Here I
show that measurement of higher-order TE, EE, EB, and TB correlations induced by this rotation can be
used to reconstruct the rotation angle as a function of position on the sky. This technique can be used to
distinguish primordial B modes from those induced by rotation. The effects of rotation can be
distinguished geometrically from similar effects due to cosmic shear.https://resolver.caltech.edu/CaltechAUTHORS:20090908-103552819Cosmology: Dark matter and dark energy
https://resolver.caltech.edu/CaltechAUTHORS:20090812-112103742
Year: 2009
DOI: 10.1038/458587a
Observations continue to indicate that the Universe is dominated by invisible components — dark matter and dark energy. Shedding light on this cosmic darkness is a priority for astronomers and physicists.https://resolver.caltech.edu/CaltechAUTHORS:20090812-112103742Oscillations in the inflaton potential?
https://resolver.caltech.edu/CaltechAUTHORS:20090901-094341714
Year: 2009
DOI: 10.1103/PhysRevD.79.083503
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.https://resolver.caltech.edu/CaltechAUTHORS:20090901-094341714The gamma-ray-flux PDF from galactic halo substructure
https://resolver.caltech.edu/CaltechAUTHORS:20090901-161606701
Year: 2009
DOI: 10.1088/1475-7516/2009/07/007
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.https://resolver.caltech.edu/CaltechAUTHORS:20090901-161606701Derotation of the cosmic microwave background polarization: Full-sky formalism
https://resolver.caltech.edu/CaltechAUTHORS:20090817-144813013
Year: 2009
DOI: 10.1103/PhysRevD.80.023510
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.https://resolver.caltech.edu/CaltechAUTHORS:20090817-144813013Gravity ripples chased
https://resolver.caltech.edu/CaltechAUTHORS:20090828-231032542
Year: 2009
DOI: 10.1038/460964a
Discovering gravitational waves would not only validate Einstein's theory of gravitation but also reveal aspects of the Universe's earliest moments. The hunt for these elusive ripples is now well under way.https://resolver.caltech.edu/CaltechAUTHORS:20090828-231032542Axion Constraints in Non-standard Thermal Histories
https://resolver.caltech.edu/CaltechAUTHORS:20100804-142905281
Year: 2009
DOI: 10.1063/1.3232168
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.https://resolver.caltech.edu/CaltechAUTHORS:20100804-142905281Galactic Substructure and Energetic Neutrinos from the Sun and Earth
https://resolver.caltech.edu/CaltechAUTHORS:20091013-091141467
Year: 2009
DOI: 10.1103/PhysRevLett.103.121301
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.https://resolver.caltech.edu/CaltechAUTHORS:20091013-091141467A scale-dependent power asymmetry from isocurvature perturbations
https://resolver.caltech.edu/CaltechAUTHORS:20091124-151956283
Year: 2009
DOI: 10.1103/PhysRevD.80.083507
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.https://resolver.caltech.edu/CaltechAUTHORS:20091124-151956283The Physics of Cosmic Acceleration
https://resolver.caltech.edu/CaltechAUTHORS:20100111-100032725
Year: 2009
DOI: 10.1146/annurev-nucl-010709-151330
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.https://resolver.caltech.edu/CaltechAUTHORS:20100111-100032725Neutrino oscillations, Lorentz/CPT violation, and dark energy
https://resolver.caltech.edu/CaltechAUTHORS:20100120-140505994
Year: 2009
DOI: 10.1103/PhysRevD.80.123522
If dark energy (DE) couples to neutrinos, then there may be apparent violations of Lorentz/CPT invariance in neutrino oscillations. The DE-induced Lorentz/CPT violation takes a specific form that introduces neutrino oscillations that are energy independent, differ for particles and antiparticles, and can lead to novel effects for neutrinos propagating through matter. We show that ultra-high-energy neutrinos may provide one avenue to seek this type of Lorentz/CPT violation in ν_μ-ν_τ oscillations, improving the current sensitivity to such effects by 7 orders of magnitude. Lorentz/CPT violation in electron-neutrino oscillations may be probed with the zenith-angle dependence for high-energy atmospheric neutrinos. More compelling evidence for a DE-neutrino coupling would be provided by a dependence of neutrino oscillations on the direction of the neutrino momentum relative to our peculiar velocity with respect to the cosmic microwave background rest frame. While the amplitude of this directional dependence is expected to be small, it may nevertheless be worth seeking in current data and may be a target for future neutrino experiments.https://resolver.caltech.edu/CaltechAUTHORS:20100120-140505994Dark matter and dark radiation
https://resolver.caltech.edu/CaltechAUTHORS:20100715-111557235
Year: 2010
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)https://resolver.caltech.edu/CaltechAUTHORS:20100715-111557235Cosmic chronometers: constraining the equation of state of dark energy. I: H(z) measurements
https://resolver.caltech.edu/CaltechAUTHORS:20100603-113559863
Year: 2010
DOI: 10.1088/1475-7516/2010/02/008
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.https://resolver.caltech.edu/CaltechAUTHORS:20100603-113559863Galactic substructure and dark-matter annihilation in the Milky Way halo
https://resolver.caltech.edu/CaltechAUTHORS:20100414-091218940
Year: 2010
DOI: 10.1103/PhysRevD.81.043532
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.https://resolver.caltech.edu/CaltechAUTHORS:20100414-091218940Dark-matter decays and self-gravitating halos
https://resolver.caltech.edu/CaltechAUTHORS:20100624-142330707
Year: 2010
DOI: 10.1103/PhysRevD.81.103501
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.https://resolver.caltech.edu/CaltechAUTHORS:20100624-142330707Non-Gaussianity from self-ordering scalar fields
https://resolver.caltech.edu/CaltechAUTHORS:20100625-080640927
Year: 2010
DOI: 10.1103/PhysRevD.81.123504
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.https://resolver.caltech.edu/CaltechAUTHORS:20100625-080640927Testing parity-violating mechanisms with cosmic microwave background experiments
https://resolver.caltech.edu/CaltechAUTHORS:20100713-100910867
Year: 2010
DOI: 10.1103/PhysRevD.81.123529
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.https://resolver.caltech.edu/CaltechAUTHORS:20100713-100910867Light gravitinos at colliders and implications for cosmology
https://resolver.caltech.edu/CaltechAUTHORS:20100816-154924446
Year: 2010
DOI: 10.1103/PhysRevD.82.015012
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.https://resolver.caltech.edu/CaltechAUTHORS:20100816-154924446Nonuniform cosmological birefringence and active galactic nuclei
https://resolver.caltech.edu/CaltechAUTHORS:20100913-095859358
Year: 2010
DOI: 10.1103/PhysRevD.82.047302
The CMB constrains the cosmological-birefringence rotation angle to be |α|≲1° (1σ) out to redshifts z≃1100 for a rotation that is uniform across the sky. However, the rotation angle α(θ,ϕ) may vary with position (θ,ϕ) on the sky, and if so, then it can be sought in current and future future active galactic nuclei data. An upper limit ^(1/2)≲3.7° to the scatter in the position-angle–polarization offsets in a sample of only N=9 active galactic nuclei already constrains the rotation spherical-harmonic coefficients to (4π)^(-1/2)α_(lm)≲3.7° and constrains the power spectrum for α in models where it is a stochastic field. Future constraints can be improved with more sources and by analyzing well-mapped sources with a tensor-harmonic decomposition of the polarization.https://resolver.caltech.edu/CaltechAUTHORS:20100913-095859358Thermal axion constraints in non-standard thermal histories
https://resolver.caltech.edu/CaltechAUTHORS:20110318-145420435
Year: 2010
DOI: 10.1063/1.3489561
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.https://resolver.caltech.edu/CaltechAUTHORS:20110318-145420435Halo clustering with nonlocal non-Gaussianity
https://resolver.caltech.edu/CaltechAUTHORS:20101207-091750993
Year: 2010
DOI: 10.1103/PhysRevD.82.103002
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.https://resolver.caltech.edu/CaltechAUTHORS:20101207-091750993Odd-parity cosmic microwave background bispectrum
https://resolver.caltech.edu/CaltechAUTHORS:20110302-101406467
Year: 2011
DOI: 10.1103/PhysRevD.83.027301
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.https://resolver.caltech.edu/CaltechAUTHORS:20110302-101406467CMB bispectrum, trispectrum, non-Gaussianity, and the Cramer-Rao bound
https://resolver.caltech.edu/CaltechAUTHORS:20110302-093713796
Year: 2011
DOI: 10.1103/PhysRevD.83.023007
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.https://resolver.caltech.edu/CaltechAUTHORS:20110302-093713796Metals at the surface of last scatter
https://resolver.caltech.edu/CaltechAUTHORS:20110429-142624622
Year: 2011
DOI: 10.1103/PhysRevD.83.083508
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.https://resolver.caltech.edu/CaltechAUTHORS:20110429-142624622Cross-correlation of cosmological birefringence with CMB temperature
https://resolver.caltech.edu/CaltechAUTHORS:20110822-092055341
Year: 2011
DOI: 10.1103/PhysRevD.84.043504
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.https://resolver.caltech.edu/CaltechAUTHORS:20110822-092055341Probability distribution for non-Gaussianity estimators
https://resolver.caltech.edu/CaltechAUTHORS:20111018-073820503
Year: 2011
DOI: 10.1103/PhysRevD.84.063013
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.https://resolver.caltech.edu/CaltechAUTHORS:20111018-073820503Compensated isocurvature perturbations and the cosmic microwave background
https://resolver.caltech.edu/CaltechAUTHORS:20120202-112949558
Year: 2011
DOI: 10.1103/PhysRevD.84.123003
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%.https://resolver.caltech.edu/CaltechAUTHORS:20120202-112949558Do Baryons Trace Dark Matter in the Early Universe?
https://resolver.caltech.edu/CaltechAUTHORS:20120130-152321521
Year: 2011
DOI: 10.1103/PhysRevLett.107.261301
Baryon-density perturbations of large amplitude may exist if they are compensated by dark-matter perturbations such that the total density is unchanged. Primordial abundances and galaxy clusters allow these compensated isocurvature perturbations (CIPs) to have amplitudes as large as ∼10%. CIPs will modulate the power spectrum of cosmic microwave background (CMB) fluctuations—those due to the usual adiabatic perturbations—as a function of position on the sky. This leads to correlations between different spherical-harmonic coefficients of the temperature and/or polarization maps, and induces polarization B modes. Here, the magnitude of these effects is calculated and techniques to measure them are introduced. While a CIP of this amplitude can be probed on large scales with existing data, forthcoming CMB experiments should improve the sensitivity to CIPs by at least an order of magnitudehttps://resolver.caltech.edu/CaltechAUTHORS:20120130-152321521Correlation of inflation-produced magnetic fields with scalar fluctuations
https://resolver.caltech.edu/CaltechAUTHORS:20120210-112540018
Year: 2011
DOI: 10.1103/PhysRevD.84.123525
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.https://resolver.caltech.edu/CaltechAUTHORS:20120210-112540018Odd-parity bipolar spherical harmonics
https://resolver.caltech.edu/CaltechAUTHORS:20121011-091507881
Year: 2012
DOI: 10.1103/PhysRevD.85.023010
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.https://resolver.caltech.edu/CaltechAUTHORS:20121011-091507881Charged-particle decay at finite temperature
https://resolver.caltech.edu/CaltechAUTHORS:20120306-065630130
Year: 2012
DOI: 10.1103/PhysRevD.85.025018
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.https://resolver.caltech.edu/CaltechAUTHORS:20120306-065630130Lensing of 21-cm Fluctuations by Primordial Gravitational Waves
https://resolver.caltech.edu/CaltechAUTHORS:20120619-131243430
Year: 2012
DOI: 10.1103/PhysRevLett.108.211301
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.https://resolver.caltech.edu/CaltechAUTHORS:20120619-131243430Clustering Fossils from the Early Universe
https://resolver.caltech.edu/CaltechAUTHORS:20120720-103630429
Year: 2012
DOI: 10.1103/PhysRevLett.108.251301
Many inflationary theories introduce new scalar, vector, or tensor degrees of freedom that may then affect the generation of primordial density perturbations. Here we show how to search a galaxy (or 21-cm) survey for the imprint of primordial scalar, vector, and tensor fields. These new fields induce local departures to an otherwise statistically isotropic two-point correlation function, or equivalently, nontrivial four-point correlation functions (or trispectra, in Fourier space), that can be decomposed into scalar, vector, and tensor components. We write down the optimal estimators for these various components and show how the sensitivity to these modes depends on the galaxy-survey parameters. New probes of parity-violating early-Universe physics are also presented.https://resolver.caltech.edu/CaltechAUTHORS:20120720-103630429Probability distribution for non-Gaussianity estimators constructed from the CMB trispectrum
https://resolver.caltech.edu/CaltechAUTHORS:20121112-141406024
Year: 2012
DOI: 10.1103/PhysRevD.86.063009
Considerable recent attention has focussed on the prospects to use the cosmic microwave background (CMB) trispectrum to probe the physics of the early universe. Here we evaluate the probability distribution function (PDF) for the standard estimator τ̂_nl for the amplitude τ_nl of the CMB trispectrum both for the null hypothesis (i.e., for Gaussian maps with τnl=0) and for maps with a nonvanishing trispectrum (τ_nl≠0). We find these PDFs to be highly non-Gaussian in both cases. We also evaluate the variance with which the trispectrum amplitude can be measured, ⟨Δτ̂^(2)_(nl)⟩, as a function of its underlying value, τnl. We find a strong dependence of this variance on τ_nl. We also find that the variance does not, given the highly non-Gaussian nature of the PDF, effectively characterize the distribution. Detailed knowledge of these PDFs will therefore be imperative in order to properly interpret the implications of any given trispectrum measurement. For example, if a CMB experiment with a maximum multipole of l_max=1500 (such as the Planck satellite) measures τ̂_nl=0 then at the 95% confidence level our calculations show that we can conclude τ_nl≤1005; assuming a Gaussian PDF but with the correct τnl-dependent variance we would incorrectly conclude τ_nl≤4225; further neglecting the τ_nl-dependence in the variance we would incorrectly conclude τ_nl≤361.https://resolver.caltech.edu/CaltechAUTHORS:20121112-141406024Dark matter detection with polarized detectors
https://resolver.caltech.edu/CaltechAUTHORS:20170409-080034277
Year: 2012
DOI: 10.1016/j.dark.2012.10.003
We consider the prospects to use polarized dark-matter detectors to discriminate between various dark-matter models. If WIMPs are fermions and participate in parity-violating interactions with ordinary matter, then the recoil-direction and recoil-energy distributions of nuclei in detectors will depend on the orientation of the initial nuclear spin with respect to the velocity of the detector through the Galactic halo. If, however, WIMPS are scalars, the only possible polarization-dependent interactions are extremely velocity-suppressed and, therefore, unobservable. Since the amplitude of this polarization modulation is fixed by the detector speed through the halo, in units of the speed of light, exposures several times larger than those of current experiments will be required to be probe this effect.https://resolver.caltech.edu/CaltechAUTHORS:20170409-080034277First CMB constraints on direction-dependent cosmological birefringence from WMAP-7
https://resolver.caltech.edu/CaltechAUTHORS:20130115-101234907
Year: 2012
DOI: 10.1103/PhysRevD.86.103529
A Chern-Simons coupling of a new scalar field to electromagnetism may give rise to cosmological birefringence, a rotation of the linear polarization of electromagnetic waves as they propagate over cosmological distances. Prior work has sought this rotation, assuming the rotation angle to be uniform across the sky, by looking for the parity-violating TB and EB correlations that a uniform rotation produces in the cosmic microwave background temperature/polarization. However, if the scalar field that gives rise to cosmological birefringence has spatial fluctuations, then the rotation angle may vary across the sky. Here we search for direction-dependent cosmological birefringence in the WMAP-7 data. We report the first cosmic microwave background constraint on the rotation-angle power spectrum C^(αα)_L for multipoles between L=0 and L=512. We also obtain a 68% confidence-level upper limit of √(C^(αα)_(2)/(4π))≲1° on the quadrupole of a scale-invariant rotation-angle power spectrum.https://resolver.caltech.edu/CaltechAUTHORS:20130115-101234907Patchy screening of the cosmic microwave background by inhomogeneous reionization
https://resolver.caltech.edu/CaltechAUTHORS:20130328-145351641
Year: 2013
DOI: 10.1103/PhysRevD.87.047303
We derive a constraint on patchy screening of the cosmic microwave background from inhomogeneous reionization using off-diagonal TB and TT correlations in WMAP-7 temperature/polarization data. We interpret this as a constraint on the rms optical-depth fluctuation Δτ as a function of a coherence multipole L_C. We relate these parameters to a comoving coherence scale, of bubble size R_C, in a phenomenological model where reionization is instantaneous but occurs on a crinkly surface, and also to the bubble size in a model of "Swiss cheese" reionization where bubbles of fixed size are spread over some range of redshifts. The current WMAP data are still too weak, by several orders of magnitude, to constrain reasonable models, but forthcoming Planck and future EPIC data should begin to approach interesting regimes of parameter space. We also present constraints on the parameter space imposed by the recent results from the EDGES experiment.https://resolver.caltech.edu/CaltechAUTHORS:20130328-145351641Baryons do trace dark matter 380,000 years after the big bang: Search for compensated isocurvature perturbations with WMAP 9-year data
https://resolver.caltech.edu/CaltechAUTHORS:20140402-134822563
Year: 2014
DOI: 10.1103/PhysRevD.89.023006
Primordial isocurvature fluctuations between photons and either neutrinos or nonrelativistic species such as baryons or dark matter are known to be subdominant to adiabatic fluctuations. Perturbations in the relative densities of baryons and dark matter (known as compensated isocurvature perturbations or CIPs), however, are surprisingly poorly constrained. CIPs leave no imprint in the cosmic microwave background (CMB) on observable scales, at least at linear order in their amplitude and zeroth order in the amplitude of adiabatic perturbations. It is thus not yet empirically known if baryons trace dark matter at the surface of last scattering. If CIPs exist, they would spatially modulate the Silk damping scale and acoustic horizon, causing distinct fluctuations in the CMB temperature/polarization power spectra across the sky: this effect is first order in both the CIP and adiabatic mode amplitudes. Here, temperature data from the Wilkinson Microwave Anisotropy Probe (WMAP) are used to conduct the first CMB-based observational search for CIPs, using off-diagonal correlations and the CMB trispectrum. Reconstruction noise from weak lensing and point sources is shown to be negligible for this data set. No evidence for CIPs is observed, and a 95% confidence upper limit of 1.1×10^(−2) is imposed to the amplitude of a scale-invariant CIP power spectrum. This limit agrees with CIP sensitivity forecasts for WMAP and is competitive with smaller-scale constraints from measurements of the baryon fraction in galaxy clusters. It is shown that the root-mean-squared CIP amplitude on 5–100° scales is smaller than ∼0.07–0.17 (depending on the scale) at the 95% confidence level. Temperature data from the Planck satellite will provide an even more sensitive probe for the existence of CIPs, as will the upcoming ACTPol and SPTPol experiments on smaller angular scales.https://resolver.caltech.edu/CaltechAUTHORS:20140402-134822563