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https://feeds.library.caltech.edu/people/Carroll-S-M/combined.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 16 Apr 2024 15:05:28 +0000The Cosmological Constant
https://resolver.caltech.edu/CaltechAUTHORS:20170213-122552673
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2001
DOI: 10.12942/lrr-2001-1
This is a review of the physics and cosmology of the cosmological constant. Focusing on recent developments, I present a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/mtjbn-swz60The Cosmological Constant
https://resolver.caltech.edu/CaltechAUTHORS:CARlrl01
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2001
PMCID: PMC5256042
This is a review of the physics and cosmology of the cosmological constant. Focusing on recent developments, I present a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy.
NB: The author will not update this review anymore, however, some of its topics are subject of other reviews. In May 2008, the article was republished in the revised Living Reviews layout, therefore the pagination has changed. The publication number lrr-2001-1 has not been altered.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/d1xnp-b0697The Universe, Too Quickly Toured [Book Review]
https://resolver.caltech.edu/CaltechAUTHORS:20150310-152233365
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2006
DOI: 10.1126/science.1130369
There's no reason why everyone shouldn't understand the basics of quantum mechanics and relativity. These two cornerstones of 20th-century physics have become a basis for our deepest understanding of reality, as well as of great practical importance to familiar technologies from lasers to the global positioning system. And, despite their reputations for being somewhat abstruse and inaccessible, the basic points of each theory can be stated simply enough that an interested person with no technical background in physics should be able to understand them. At a time when science seems both more central than ever and more removed from our everyday world, it is certainly worth the effort to share what we've learned about the workings of nature with interested nonscientists.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/hr1ev-ptp62Dark matter is real
https://resolver.caltech.edu/CaltechAUTHORS:20150323-150812283
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2006
DOI: 10.1038/nphys428
The combined data from four systems of telescopes offer the strongest evidence yet that a modification of gravity cannot do away with the need for dark matter.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/mjrqb-qqz63Imprints of a primordial preferred direction on the microwave background
https://resolver.caltech.edu/CaltechAUTHORS:ACKprd07
Authors: {'items': [{'id': 'Ackerman-L', 'name': {'family': 'Ackerman', 'given': 'Lotty'}}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Wise-M-B', 'name': {'family': 'Wise', 'given': 'Mark B.'}}]}
Year: 2007
DOI: 10.1103/PhysRevD.75.083502
Rotational invariance is a well-established feature of low-energy physics. Violations of this symmetry must be extremely small today, but could have been larger in earlier epochs. In this paper we examine the consequences of a small breaking of rotational invariance during the inflationary era when the primordial density fluctuations were generated. Assuming that a fixed-norm vector picked out a preferred-direction during the inflationary era, we explore the imprint it would leave on the cosmic microwave background anisotropy, and provide explicit formulas for the expected amplitudes of the spherical-harmonic coefficients. We suggest that it is natural to expect that the imprint on the primordial power spectrum of a preferred spatial direction is approximately scale-invariant, and examine a simple model in which this is true.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/aqdae-dc009Aether compactification
https://resolver.caltech.edu/CaltechAUTHORS:CARprd08
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Tam-H', 'name': {'family': 'Tam', 'given': 'Heywood'}}]}
Year: 2008
DOI: 10.1103/PhysRevD.78.044047
We propose a new way to hide large extra dimensions without invoking branes, based on Lorentz-violating tensor fields with expectation values along the extra directions. We investigate the case of a single vector aether field on a compact circle. In such a background, interactions of other fields with the aether can lead to modified dispersion relations, increasing the mass of the Kaluza-Klein excitations. The mass scale characterizing each Kaluza-Klein tower can be chosen independently for each species of scalar, fermion, or gauge boson. No small-scale deviations from the inverse square law for gravity are predicted, although light graviton modes may exist.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/8224f-4vz06Superhorizon perturbations and the cosmic microwave background
https://resolver.caltech.edu/CaltechAUTHORS:ERIprd08
Authors: {'items': [{'id': 'Erickcek-A-L', 'name': {'family': 'Erickcek', 'given': 'Adrienne L.'}}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Kamionkowski-M', 'name': {'family': 'Kamionkowski', 'given': 'Marc'}, 'orcid': '0000-0001-7018-2055'}]}
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://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/496r2-6mf91A hemispherical power asymmetry from inflation
https://resolver.caltech.edu/CaltechAUTHORS:ERIprd08b
Authors: {'items': [{'id': 'Erickcek-A-L', 'name': {'family': 'Erickcek', 'given': 'Adrienne L.'}}, {'id': 'Kamionkowski-M', 'name': {'family': 'Kamionkowski', 'given': 'Marc'}, 'orcid': '0000-0001-7018-2055'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
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://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/h9ysv-9f289Dark matter and dark radiation
https://resolver.caltech.edu/CaltechAUTHORS:ACKprd09
Authors: {'items': [{'id': 'Ackerman-L', 'name': {'family': 'Ackerman', 'given': 'Lotty'}}, {'id': 'Buckley-M-R', 'name': {'family': 'Buckley', 'given': 'Matthew R.'}}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Kamionkowski-M', 'name': {'family': 'Kamionkowski', 'given': 'Marc'}, 'orcid': '0000-0001-7018-2055'}]}
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://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/3e9d0-dhc95Sigma-model aether
https://resolver.caltech.edu/CaltechAUTHORS:20090429-113052329
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Dulaney-T-R', 'name': {'family': 'Dulaney', 'given': 'Timothy R.'}}, {'id': 'Gresham-M-I', 'name': {'family': 'Gresham', 'given': 'Moira I.'}}, {'id': 'Tam-H', 'name': {'family': 'Tam', 'given': 'Heywood'}}]}
Year: 2009
DOI: 10.1103/PhysRevD.79.065012
Theories of low-energy Lorentz violation by a fixed-norm "aether" vector field with two-derivative kinetic terms have a globally bounded Hamiltonian and are perturbatively stable only if the vector is timelike and the kinetic term in the action takes the form of a sigma model. Here we investigate the phenomenological properties of this theory. We first consider the propagation of modes in the presence of gravity and show that there is a unique choice of curvature coupling that leads to a theory without superluminal modes. Experimental constraints on this theory come from a number of sources, and we examine bounds in a two-dimensional parameter space. We then consider the cosmological evolution of the aether, arguing that the vector will naturally evolve to be orthogonal to constant-density hypersurfaces in a Friedmann-Robertson-Walker cosmology. Finally, we examine cosmological evolution in the presence of an extra compact dimension of space, concluding that a vector can maintain a constant projection along the extra dimension in an expanding universe only when the expansion is exponential.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/gcqf1-zr861Instabilities in the aether
https://resolver.caltech.edu/CaltechAUTHORS:20090508-120529180
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Dulaney-T-R', 'name': {'family': 'Dulaney', 'given': 'Timothy R.'}}, {'id': 'Gresham-M-I', 'name': {'family': 'Gresham', 'given': 'Moira I.'}}, {'id': 'Tam-Heywood', 'name': {'family': 'Tam', 'given': 'Heywood'}}]}
Year: 2009
DOI: 10.1103/PhysRevD.79.065011
We investigate the stability of theories in which Lorentz invariance is spontaneously broken by fixed-norm vector "aether" fields. Models with generic kinetic terms are plagued either by ghosts or by tachyons, and are therefore physically unacceptable. There are precisely three kinetic terms that are not manifestly unstable: a sigma model (∂_µA_ν)^2, the Maxwell Lagrangian F_µνF^µν, and a scalar Lagrangian (∂_µA^µ)^2. The timelike sigma-model case is well defined and stable when the vector norm is fixed by a constraint; however, when it is determined by minimizing a potential there is necessarily a tachyonic ghost, and therefore an instability. In the Maxwell and scalar cases, the Hamiltonian is unbounded below, but at the level of perturbation theory there are fewer degrees of freedom and the models are stable. However, in these two theories there are obstacles to smooth evolution for certain choices of initial data.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/8q626-vh745Dark-matter-induced violation of the weak equivalence principle
https://resolver.caltech.edu/CaltechAUTHORS:20090807-114119837
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Mantry-S', 'name': {'family': 'Mantry', 'given': 'Sonny'}}, {'id': 'Ramsey-Musolf-M-J', 'name': {'family': 'Ramsey-Musolf', 'given': 'Michael J.'}, 'orcid': '0000-0001-8110-2479'}, {'id': 'Stubbs-C-W', 'name': {'family': 'Stubbs', 'given': 'Christopher W.'}, 'orcid': '0000-0003-0347-1724'}]}
Year: 2009
DOI: 10.1103/PhysRevLett.103.011301
A long-range fifth force coupled to dark matter can induce a coupling to ordinary matter if the dark matter interacts with standard model fields. We consider constraints on such a scenario from both astrophysical observations and laboratory experiments. We also examine the case where the dark matter is a weakly interacting massive particle, and derive relations between the coupling to dark matter and the coupling to ordinary matter for different models. Currently, this scenario is most tightly constrained by galactic dynamics, but improvements in Eötvös experiments can probe unconstrained regions of parameter space.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/qnddx-grs67Lorentz violation in Goldstone gravity
https://resolver.caltech.edu/CaltechAUTHORS:20090817-144813345
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Tam-H', 'name': {'family': 'Tam', 'given': 'Heywood'}}, {'id': 'Wehus-I-K', 'name': {'family': 'Wehus', 'given': 'Ingunn Kathrine'}}]}
Year: 2009
DOI: 10.1103/PhysRevD.80.025020
We consider a theory of gravity in which a symmetric two-index tensor in Minkowski spacetime acquires a vacuum expectation value (vev) via a potential, thereby breaking Lorentz invariance spontaneously. When the vev breaks all the generators of the Lorentz group, six Goldstone modes emerge, two linear combinations of which have properties that are identical to those of the graviton in general relativity. Integrating out massive modes yields an infinite number of Lorentz-violating radiative-correction terms in the low-energy effective Lagrangian. We examine a representative subset of these terms and show that they modify the dispersion relation of the two propagating graviton modes such that their phase velocity is direction dependent. If the phase velocity of the Goldstone gravitons is subluminal, cosmic rays can emit gravi-Cherenkov radiation, and the detection of high-energy cosmic rays can be used to constrain these radiative-correction terms. Test particles in the vicinity of the Goldstone gravitons undergo longitudinal oscillations in addition to the usual transverse oscillations as predicted by general relativity. Finally, we discuss the possibility of having vevs that do not break all six generators and examine in detail one such theory.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/r8v5c-37c12Extremal limits and black hole entropy
https://resolver.caltech.edu/CaltechAUTHORS:20100120-093837694
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Johnson-M-C', 'name': {'family': 'Johnson', 'given': 'Matthew C.'}, 'orcid': '0000-0002-5099-8185'}, {'id': 'Randall-L', 'name': {'family': 'Randall', 'given': 'Lisa'}}]}
Year: 2009
DOI: 10.1088/1126-6708/2009/11/109
Taking the extremal limit of a non-extremal Reissner-Nordström black hole (by externally varying the mass or charge), the region between the inner and outer event horizons experiences an interesting fate — while this region is absent in the extremal case, it does not disappear in the extremal limit but rather approaches a patch of AdS_2 × S^2. In other words, the approach to extremality is not continuous, as the non-extremal Reissner-Nordström solution splits into two spacetimes at extremality: an extremal black hole and a disconnected AdS space. We suggest that the unusual nature of this limit may help in understanding the entropy of extremal black holes.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/j8ff5-dzd10Dynamical compactification from de Sitter space
https://resolver.caltech.edu/CaltechAUTHORS:20100120-093214799
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Johnson-M-C', 'name': {'family': 'Johnson', 'given': 'Matthew C.'}, 'orcid': '0000-0002-5099-8185'}, {'id': 'Randall-L', 'name': {'family': 'Randall', 'given': 'Lisa'}}]}
Year: 2009
DOI: 10.1088/1126-6708/2009/11/094
We show that D-dimensional de Sitter space is unstable to the nucleation of non-singular geometries containing spacetime regions with different numbers of macroscopic dimensions, leading to a dynamical mechanism of compactification. These and other solutions to Einstein gravity with flux and a cosmological constant are constructed by performing a dimensional reduction under the assumption of q-dimensional spherical symmetry in the full D-dimensional geometry. In addition to the familiar black holes, black branes, and compactification solutions we identify a number of new geometries, some of which are completely non-singular. The dynamical compactification mechanism populates lower-dimensional vacua very differently from false vacuum eternal inflation, which occurs entirely within the context of four-dimensions. We outline the phenomenology of the nucleation rates, finding that the dimensionality of the vacuum plays a key role and that among vacua of the same dimensionality, the rate is highest for smaller values of the cosmological constant. We consider the cosmological constant problem and propose a novel model of slow-roll inflation that is triggered by the compactification process.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/2j1he-16137Dark matter and dark radiation
https://resolver.caltech.edu/CaltechAUTHORS:20100715-111557235
Authors: {'items': [{'id': 'Ackerman-L', 'name': {'family': 'Ackerman', 'given': 'Lotty'}}, {'id': 'Buckley-M-R', 'name': {'family': 'Buckley', 'given': 'Matthew R.'}}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Kamionkowski-M', 'name': {'family': 'Kamionkowski', 'given': 'Marc'}, 'orcid': '0000-0001-7018-2055'}]}
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://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/qvvh9-2y366The origin of the universe and the arrow of time
https://resolver.caltech.edu/CaltechAUTHORS:20170316-132438668
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2010
DOI: 10.1109/AERO.2010.5447043
One of the most obvious facts about the universe is that the past is different from the future. The world around us is full of irreversible processes: we can turn an egg into an omelet, but can't turn an omelet into an egg. Physicists have codified this difference into the Second Law of Thermodynamics: the entropy of a closed system always increases with time. But why? The ultimate explanation is to be found in cosmology: special conditions in the early universe are responsible for the arrow of time. I will talk about the nature of time, the origin of entropy, and how what happened before the Big Bang may be responsible for the arrow of time we observe today.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/9zzjd-rmx46Implications of a scalar dark force for terrestrial experiments
https://resolver.caltech.edu/CaltechAUTHORS:20100520-140754035
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Mantry-S', 'name': {'family': 'Mantry', 'given': 'Sonny'}}, {'id': 'Ramsey-Musolf-M-J', 'name': {'family': 'Ramsey-Musolf', 'given': 'Michael J.'}, 'orcid': '0000-0001-8110-2479'}]}
Year: 2010
DOI: 10.1103/PhysRevD.81.063507
A long-range intergalactic force between dark matter (DM) particles, mediated by an ultralight scalar, is tightly constrained by galactic dynamics and large scale structure formation. We examine the implications of such a ''dark force" for several terrestrial experiments, including Eötvös tests of the Weak Equivalence Principle (WEP), direct-detection DM searches, and collider studies. The presence of a dark force implies a nonvanishing effect in Eötvös tests that could be probed by current and future experiments depending on the DM model. For scalar DM that is a singlet under the standard model gauge groups, a dark force of astrophysically relevant magnitude is ruled out in large regions of parameter space by the DM relic density and WEP constraints. WEP tests also imply constraints on the Higgs-exchange contributions to the spin-independent (SI) DM-nucleus direct-detection cross section. For WIMP scenarios, these considerations constrain Higgs-exchange contributions to the SI cross section to be subleading compared to gauge-boson mediated contributions. In multicomponent DM scenarios, a dark force would preclude large shifts in the rate for Higgs decay to two photons associated with DM-multiplet loops that might otherwise lead to measurable deviations at the LHC or a future linear collider. The combination of observations from galactic dynamics, large scale structure formation, Eötvös experiments, DM-direct-detection experiments, and colliders can further constrain the size of new long-range forces in the dark sector.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/5qsp1-v8315Translational invariance and the anisotropy of the cosmic microwave background
https://resolver.caltech.edu/CaltechAUTHORS:20100524-141735641
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Tseng-C-Y', 'name': {'family': 'Tseng', 'given': 'Chien-Yao'}}, {'id': 'Wise-M-B', 'name': {'family': 'Wise', 'given': 'Mark B.'}, 'orcid': '0000-0002-9125-801X'}]}
Year: 2010
DOI: 10.1103/PhysRevD.81.083501
Primordial quantum fluctuations produced by inflation are conventionally assumed to be statistically
homogeneous, a consequence of translational invariance. In this paper we quantify the potentially
observable effects of a small violation of translational invariance during inflation, as characterized by
the presence of a preferred point, line, or plane.We explore the imprint such a violation would leave on the
cosmic microwave background anisotropy, and provide explicit formulas for the expected amplitudes <а_(lm)а^*_(l'm')> of the spherical-harmonic coefficients.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/evvxm-x6f78Out of equilibrium: understanding cosmological evolution to lower-entropy states
https://resolver.caltech.edu/CaltechAUTHORS:20120516-092014134
Authors: {'items': [{'id': 'Aguirre-A', 'name': {'family': 'Aguirre', 'given': 'Anthony'}}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Johnson-M-C', 'name': {'family': 'Johnson', 'given': 'Matthew C.'}, 'orcid': '0000-0002-5099-8185'}]}
Year: 2012
DOI: 10.1088/1475-7516/2012/02/024
Despite the importance of the Second Law of Thermodynamics, it is not absolute. Statistical mechanics implies that, given sufficient time, systems near equilibrium will spontaneously fluctuate into lower-entropy states, locally reversing the thermodynamic arrow of time. We study the time development of such fluctuations, especially the very large fluctuations relevant to cosmology. Under fairly general assumptions, the most likely history of a fluctuation out of equilibrium is simply the CPT conjugate of the most likely way a system relaxes back to equilibrium. We use this idea to elucidate the spacetime structure of various fluctuations in (stable and metastable) de Sitter space and thermal anti-de Sitter space.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/g6y34-84634Dark matter with density-dependent interactions
https://resolver.caltech.edu/CaltechAUTHORS:20130131-161047251
Authors: {'items': [{'id': 'Boddy-K-K', 'name': {'family': 'Boddy', 'given': 'Kimberly K.'}, 'orcid': '0000-0003-1928-4667'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Trodden-M', 'name': {'family': 'Trodden', 'given': 'Mark'}}]}
Year: 2012
DOI: 10.1103/PhysRevD.86.123529
The decay and annihilation cross sections of dark matter particles may depend on the value of a chameleonic scalar field that both evolves cosmologically and takes different values depending on the local matter density. This possibility introduces a separation between the physics relevant for freeze-out and that responsible for dynamics and detection in the late universe. We investigate how such dark sector interactions might be implemented in a particle physics Lagrangian and consider how current and upcoming observations and experiments bound such dark matter candidates. A specific simple model allows for an increase in the annihilation cross section by a factor of 10^6 between freeze-out and today, while more complicated models should also allow for scattering cross sections near the astrophysical bounds.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/xbw9f-11d97Fiction: Silicon and surveillance [Book Review]
https://resolver.caltech.edu/CaltechAUTHORS:20131017-111952775
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2013
DOI: 10.1038/501312a
Sean Carroll finds Thomas Pynchon on compelling form in a tale of big data and bigger conspiracies.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/hmg5d-cfs02Can the Higgs Boson Save Us From the Menace of the Boltzmann Brains?
https://resolver.caltech.edu/CaltechAUTHORS:20130925-113239870
Authors: {'items': [{'id': 'Boddy-K-K', 'name': {'family': 'Boddy', 'given': 'Kimberly K.'}, 'orcid': '0000-0003-1928-4667'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2013
DOI: 10.48550/arXiv.1308.4686
The standard ΛCDM model provides an excellent fit to current cosmological observations but suffers from a potentially serious Boltzmann Brain problem. If the universe enters a de Sitter vacuum phase that is truly eternal, there will be a finite temperature in empty space and corresponding thermal fluctuations. Among these fluctuations will be intelligent observers, as well as configurations that reproduce any local region of the current universe to arbitrary precision. We discuss the possibility that the escape from this unacceptable situation may be found in known physics: vacuum instability induced by the Higgs field. Avoiding Boltzmann Brains in a measure-independent way requires a decay timescale of order the current age of the universe, which can be achieved if the top quark pole mass is approximately 178 GeV. Otherwise we must invoke new physics or a particular cosmological measure before we can consider ΛCDM to be an empirical success.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/58bg0-6c617Attractor solutions in scalar-field cosmology
https://resolver.caltech.edu/CaltechAUTHORS:20131125-140730326
Authors: {'items': [{'id': 'Remmen-G-N', 'name': {'family': 'Remmen', 'given': 'Grant N.'}, 'orcid': '0000-0001-6569-8866'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2013
DOI: 10.1103/PhysRevD.88.083518
Models of cosmological scalar fields often feature "attractor solutions" to which the system evolves for a wide range of initial conditions. There is some tension between this well-known fact and another well-known fact: Liouville's theorem forbids true attractor behavior in a Hamiltonian system. In universes with vanishing spatial curvature, the field variables ϕ and ϕ˙ specify the system completely, defining an effective phase space. We investigate whether one can define a unique conserved measure on this effective phase space, showing that it exists for m^2ϕ^2 potentials and deriving conditions for its existence in more general theories. We show that apparent attractors are places where this conserved measure diverges in the ϕ-ϕ˙ variables and suggest a physical understanding of attractor behavior that is compatible with Liouville's theorem.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/kthdh-6we85Many Worlds, the Born Rule, and Self-Locating Uncertainty
https://resolver.caltech.edu/CaltechAUTHORS:20141216-203110170
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2014
DOI: 10.1007/978-88-470-5217-8_10
We provide a derivation of the Born Rule in the context of the Everett (Many-Worlds) approach to quantum mechanics. Our argument is based on the idea of self-locating uncertainty: in the period between the wave function branching via decoherence and an observer registering the outcome of the measurement, that observer can know the state of the universe precisely without knowing which branch they are on. We show that there is a uniquely rational way to apportion credence in such cases, which leads directly to the Born Rule. [Editors note: for a video of the talk given by Prof. Carroll at the Aharonov-80 conference in 2012 at Chapman University, see quantum.chapman.edu/talk-14.]https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/9cdj7-axw89Consistent effective theory of long-wavelength cosmological perturbations
https://resolver.caltech.edu/CaltechAUTHORS:20141120-145800084
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Leichenauer-S', 'name': {'family': 'Leichenauer', 'given': 'Stefan'}}, {'id': 'Pollack-J-A', 'name': {'family': 'Pollack', 'given': 'Jason'}, 'orcid': '0000-0003-4754-4905'}]}
Year: 2014
DOI: 10.1103/PhysRevD.90.023518
Effective field theory provides a perturbative framework to study the evolution of cosmological large-scale structure. We investigate the underpinnings of this approach, and suggest new ways to compute correlation functions of cosmological observables. We find that, in contrast with quantum field theory, the appropriate effective theory of classical cosmological perturbations involves interactions that are nonlocal in time. We describe an alternative to the usual approach of smoothing the perturbations, based on a path-integral formulation of the renormalization group equations. This technique allows for improved handling of short-distance modes that are perturbatively generated by long-distance interactions.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/5yqdv-9vk77In What Sense Is the Early Universe Fine-Tuned?
https://resolver.caltech.edu/CaltechAUTHORS:20140716-105154654
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2014
DOI: 10.48550/arXiv.1406.3057v1
It is commonplace in discussions of modern cosmology to assert that the early
universe began in a special state. Conventionally, cosmologists characterize
this fine-tuning in terms of the horizon and flatness problems. I argue that
the fine-tuning is real, but these problems aren't the best way to think about
it: causal disconnection of separated regions isn't the real problem, and
flatness isn't a problem at all. Fine-tuning is better understood in terms of a
measure on the space of trajectories: given reasonable conditions in the late
universe, the fraction of cosmological histories that were smooth at early
times is incredibly tiny. This discussion helps clarify what is required by a
complete theory of cosmological initial conditions.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/3epzb-qf215How Many e-Folds Should We Expect from High-Scale Inflation?
https://resolver.caltech.edu/CaltechAUTHORS:20140529-121305448
Authors: {'items': [{'id': 'Remmen-G-N', 'name': {'family': 'Remmen', 'given': 'Grant N.'}, 'orcid': '0000-0001-6569-8866'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2014
DOI: 10.1103/PhysRevD.90.063517
We address the issue of how many e-folds we would naturally expect if inflation occurred at an energy scale of order 10^(16) GeV. We use the canonical measure on trajectories in classical phase space, specialized to the case of flat universes with a single scalar field. While there is no exact analytic expression for the measure, we are able to derive conditions that determine its behavior. For a quadratic potential V(ϕ)=m^2ϕ^2/2 with m=2×10^(13) GeV and cutoff at M_(Pl)=2.4×10^(18) GeV, we find an expectation value of 2×1010 e-folds on the set of FRW trajectories. For cosine inflation V(ϕ)=Λ^4[1−cos(ϕ/f)] with f=1.5×10^(19) GeV, we find that the expected total number of e-folds is 50, which would just satisfy the observed requirements of our own Universe; if f is larger, more than 50 e-folds are generically attained. We conclude that one should expect a large amount of inflation in large-field models and more limited inflation in small-field (hilltop) scenarios.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/32jdd-1ys77Quantifying the Rise and Fall of Complexity in Closed Systems: The Coffee Automaton
https://resolver.caltech.edu/CaltechAUTHORS:20150316-131020808
Authors: {'items': [{'id': 'Aaronson-S', 'name': {'family': 'Aaronson', 'given': 'Scott'}}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Oullette-L', 'name': {'family': 'Ouellette', 'given': 'Lauren'}}]}
Year: 2015
DOI: 10.48550/arXiv.1405.6903
In contrast to entropy, which increases monotonically, the "complexity" or
"interestingness" of closed systems seems intuitively to increase at first and
then decrease as equilibrium is approached. For example, our universe lacked
complex structures at the Big Bang and will also lack them after black holes
evaporate and particles are dispersed. This paper makes an initial attempt to
quantify this pattern. As a model system, we use a simple, two-dimensional
cellular automaton that simulates the mixing of two liquids ("coffee" and
"cream"). A plausible complexity measure is then the Kolmogorov complexity of a
coarse-grained approximation of the automaton's state, which we dub the
"apparent complexity." We study this complexity measure, and show analytically
that it never becomes large when the liquid particles are non-interacting. By
contrast, when the particles do interact, we give numerical evidence that the
complexity reaches a maximum comparable to the "coffee cup's" horizontal
dimension. We raise the problem of proving this behavior analytically.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/9x9cs-z5291Consistency conditions for an AdS multiscale entanglement renormalization ansatz correspondence
https://resolver.caltech.edu/CaltechAUTHORS:20150601-131525807
Authors: {'items': [{'id': 'Bao-Ning', 'name': {'family': 'Bao', 'given': 'Ning'}, 'orcid': '0000-0002-3296-1039'}, {'id': 'Cao-ChunJun', 'name': {'family': 'Cao', 'given': 'ChunJun'}, 'orcid': '0000-0002-5761-5474'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Chatwin-Davies-A', 'name': {'family': 'Chatwin-Davies', 'given': 'Aidan'}, 'orcid': '0000-0003-1406-9271'}, {'id': 'Hunter-Jones-N', 'name': {'family': 'Hunter-Jones', 'given': 'Nicholas'}}, {'id': 'Pollack-J-A', 'name': {'family': 'Pollack', 'given': 'Jason'}, 'orcid': '0000-0003-4754-4905'}, {'id': 'Remmen-G-N', 'name': {'family': 'Remmen', 'given': 'Grant N.'}, 'orcid': '0000-0001-6569-8866'}]}
Year: 2015
DOI: 10.1103/PhysRevD.91.125036
The multiscale entanglement renormalization ansatz (MERA) is a tensor network that provides an efficient way of variationally estimating the ground state of a critical quantum system. The network geometry resembles a discretization of spatial slices of an anti–de Sitter (AdS) spacetime and "geodesics" in the MERA reproduce the Ryu-Takayanagi formula for the entanglement entropy of a boundary region in terms of bulk properties. It has therefore been suggested that there could be an AdS/MERA correspondence, relating states in the Hilbert space of the boundary quantum system to ones defined on the bulk lattice. Here we investigate this proposal and derive necessary conditions for it to apply, using geometric features and entropy inequalities that we expect to hold in the bulk. We show that, perhaps unsurprisingly, the MERA lattice can only describe physics on length scales larger than the AdS radius. Further, using the covariant entropy bound in the bulk, we show that there are no conventional MERA parameters that completely reproduce bulk physics even on super-AdS scales. We suggest modifications or generalizations of this kind of tensor network that may be able to provide a more robust correspondence.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/c0qbf-wrx40Bayesian second law of thermodynamics
https://resolver.caltech.edu/CaltechAUTHORS:20150812-142230808
Authors: {'items': [{'id': 'Bartolotta-A', 'name': {'family': 'Bartolotta', 'given': 'Anthony'}}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Leichenauer-S', 'name': {'family': 'Leichenauer', 'given': 'Stefan'}}, {'id': 'Pollack-J-A', 'name': {'family': 'Pollack', 'given': 'Jason'}, 'orcid': '0000-0003-4754-4905'}]}
Year: 2015
DOI: 10.1103/PhysRevE.94.022102
We derive a generalization of the second law of thermodynamics that uses Bayesian updates to explicitly incorporate the effects of a measurement of a system at some point in its evolution. By allowing an experimenter's knowledge to be updated by the measurement process, this formulation resolves a tension between the fact that the entropy of a statistical system can sometimes fluctuate downward and the information-theoretic idea that knowledge of a stochastically evolving system degrades over time. The Bayesian second law can be written as ΔH(ρ_m,ρ)+(2)F|M ⩾ 0, where ΔH(ρ_m,ρ) is the change in the cross entropy between the original phase-space
probability distribution ρ and the measurement-updated distribution ρ_m and (2)F|m is the expectation value of a
generalized heat flow out of the system. We also derive refined versions of the second law that bound the entropy
increase from below by a non-negative number, as well as Bayesian versions of integral fluctuation theorems.
We demonstrate the formalism using simple analytical and numerical examples.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/vydfg-wvm39Why Boltzmann Brains Don't Fluctuate Into Existence From the De Sitter Vacuum
https://resolver.caltech.edu/CaltechAUTHORS:20150814-091340697
Authors: {'items': [{'id': 'Boddy-K-K', 'name': {'family': 'Boddy', 'given': 'Kimberly K.'}, 'orcid': '0000-0003-1928-4667'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Pollack-J-A', 'name': {'family': 'Pollack', 'given': 'Jason'}, 'orcid': '0000-0003-4754-4905'}]}
Year: 2015
DOI: 10.48550/arXiv.1505.02780
Many modern cosmological scenarios feature large volumes of spacetime in a de
Sitter vacuum phase. Such models are said to be faced with a "Boltzmann Brain
problem" - the overwhelming majority of observers with fixed local conditions
are random fluctuations in the de Sitter vacuum, rather than arising via
thermodynamically sensible evolution from a low-entropy past. We argue that
this worry can be straightforwardly avoided in the Many-Worlds (Everett)
approach to quantum mechanics, as long as the underlying Hilbert space is
infinite-dimensional. In that case, de Sitter settles into a truly stationary
quantum vacuum state. While there would be a nonzero probability for observing
Boltzmann-Brain-like fluctuations in such a state, "observation" refers to a
specific kind of dynamical process that does not occur in the vacuum (which is,
after all, time-independent). Observers are necessarily out-of-equilibrium
physical systems, which are absent in the vacuum. Hence, the fact that
projection operators corresponding to states with observers in them do not
annihilate the vacuum does not imply that such observers actually come into
existence. The Boltzmann Brain problem is therefore much less generic than has
been supposed.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/dnpg4-wnf59How to Recover a Qubit That Has Fallen Into a Black Hole
https://resolver.caltech.edu/CaltechAUTHORS:20150731-184935946
Authors: {'items': [{'id': 'Chatwin-Davies-A', 'name': {'family': 'Chatwin-Davies', 'given': 'Aidan'}, 'orcid': '0000-0003-1406-9271'}, {'id': 'Jermyn-A-S', 'name': {'family': 'Jermyn', 'given': 'Adam S.'}, 'orcid': '0000-0001-5048-9973'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2015
DOI: 10.1103/PhysRevLett.115.261302
We demonstrate an algorithm for the retrieval of a qubit, encoded in spin angular momentum, that has been dropped into a no-firewall black hole. Retrieval is achieved analogously to quantum teleportation by collecting Hawking radiation and performing measurements on the black hole. Importantly, these methods require only the ability to perform measurements from outside the event horizon.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/d3trc-3h036De Sitter Space Without Dynamical Quantum Fluctuations
https://resolver.caltech.edu/CaltechAUTHORS:20150316-131744979
Authors: {'items': [{'id': 'Boddy-K-K', 'name': {'family': 'Boddy', 'given': 'Kimberly K.'}, 'orcid': '0000-0003-1928-4667'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Pollack-J-A', 'name': {'family': 'Pollack', 'given': 'Jason'}, 'orcid': '0000-0003-4754-4905'}]}
Year: 2016
DOI: 10.1007/s10701-016-9996-8
We argue that, under certain plausible assumptions, de Sitter space settles into a quiescent vacuum in which there are no dynamical quantum fluctuations. Such fluctuations require either an evolving microstate, or time-dependent histories of out-of-equilibrium recording devices, which we argue are absent in stationary states. For a massive scalar field in a fixed de Sitter background, the cosmic no-hair theorem implies that the state of the patch approaches the vacuum, where there are no fluctuations. We argue that an analogous conclusion holds whenever a patch of de Sitter is embedded in a larger theory with an infinite-dimensional Hilbert space, including semiclassical quantum gravity with false vacua or complementarity in theories with at least one Minkowski vacuum. This reasoning provides an escape from the Boltzmann brain problem in such theories. It also implies that vacuum states do not uptunnel to higher-energy vacua and that perturbations do not decohere while slow-roll inflation occurs, suggesting that eternal inflation is much less common than often supposed. On the other hand, if a de Sitter patch is a closed system with a finite-dimensional Hilbert space, there will be Poincaré recurrences and dynamical Boltzmann fluctuations into lower-entropy states. Our analysis does not alter the conventional understanding of the origin of density fluctuations from primordial inflation, since reheating naturally generates a high-entropy environment and leads to decoherence, nor does it affect the existence of non-dynamical vacuum fluctuations such as those that give rise to the Casimir effect.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/js57r-afc07What is the Entropy in Entropic Gravity?
https://resolver.caltech.edu/CaltechAUTHORS:20160210-142850363
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Remmen-G-N', 'name': {'family': 'Remmen', 'given': 'Grant N.'}, 'orcid': '0000-0001-6569-8866'}]}
Year: 2016
DOI: 10.1103/PhysRevD.93.124052
We investigate theories in which gravity arises as a consequence of entropy. We distinguish between two approaches to this idea: holographic gravity, in which Einstein's equation arises from keeping entropy stationary in equilibrium under variations of the geometry and quantum state of a small region, and thermodynamic gravity, in which Einstein's equation emerges as a local equation of state from constraints on the area of a dynamical light sheet in a fixed spacetime background. Examining holographic gravity, we argue that its underlying assumptions can be justified in part using recent results on the form of the modular energy in quantum field theory. For thermodynamic gravity, on the other hand, we find that it is difficult to formulate a self-consistent definition of the entropy, which represents an obstacle for this approach. This investigation points the way forward in understanding the connections between gravity and entanglement.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/p6zm2-gaz11Space from Hilbert Space: Recovering Geometry from Bulk Entanglement
https://resolver.caltech.edu/CaltechAUTHORS:20160704-200753575
Authors: {'items': [{'id': 'Cao-ChunJun', 'name': {'family': 'Cao', 'given': 'ChunJun'}, 'orcid': '0000-0002-5761-5474'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Michalakis-S', 'name': {'family': 'Michalakis', 'given': 'Spyridon'}, 'orcid': '0000-0003-4963-1156'}]}
Year: 2017
DOI: 10.1103/PhysRevD.95.024031
We examine how to construct a spatial manifold and its geometry from the entanglement structure of an abstract quantum state in Hilbert space. Given a decomposition of Hilbert space H into a tensor product of factors, we consider a class of "redundancy-constrained states" in H that generalize the area-law behavior for entanglement entropy usually found in condensed-matter systems with gapped local Hamiltonians. Using mutual information to define a distance measure on the graph, we employ classical multidimensional scaling to extract the best-fit spatial dimensionality of the emergent geometry. We then show that entanglement perturbations on such emergent geometries naturally give rise to local modifications of spatial curvature which obey a (spatial) analog of Einstein's equation. The Hilbert space corresponding to a region of flat space is finite-dimensional and scales as the volume, though the entropy (and the maximum change thereof) scales like the area of the boundary. A version of the ER=EPR conjecture is recovered, in that perturbations that entangle distant parts of the emergent geometry generate a configuration that may be considered as a highly quantum wormhole.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/957ax-76c67Why Boltzmann Brains Are Bad
https://resolver.caltech.edu/CaltechAUTHORS:20170209-151733308
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2017
DOI: 10.48550/arXiv.1702.00850
Some modern cosmological models predict the appearance of Boltzmann Brains:
observers who randomly fluctuate out of a thermal bath rather than naturally
evolving from a low-entropy Big Bang. A theory in which most observers are of
the Boltzmann Brain type is generally thought to be unacceptable, although
opinions differ. I argue that such theories are indeed unacceptable: the real
problem is with fluctuations into observers who are locally identical to
ordinary observers, and their existence cannot be swept under the rug by a
choice of probability distributions over observers. The issue is not that the
existence of such observers is ruled out by data, but that the theories that
predict them are cognitively unstable: they cannot simultaneously be true and
justifiably believed.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/5nj2j-mng35Quantum Circuit Cosmology: The Expansion of the Universe Since the First Qubit
https://resolver.caltech.edu/CaltechAUTHORS:20170228-191741490
Authors: {'items': [{'id': 'Bao-Ning', 'name': {'family': 'Bao', 'given': 'Ning'}, 'orcid': '0000-0002-3296-1039'}, {'id': 'Cao-ChunJun', 'name': {'family': 'Cao', 'given': 'ChunJun'}, 'orcid': '0000-0002-5761-5474'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'McAllister-L', 'name': {'family': 'McAllister', 'given': 'Liam'}}]}
Year: 2017
DOI: 10.48550/arXiv.1702.06959
We consider cosmological evolution from the perspective of quantum
information. We present a quantum circuit model for the expansion of a comoving
region of space, in which initially-unentangled ancilla qubits become entangled
as expansion proceeds. We apply this model to the comoving region that now
coincides with our Hubble volume, taking the number of entangled degrees of
freedom in this region to be proportional to the de Sitter entropy. The quantum
circuit model is applicable for at most 140 e-folds of inflationary and
post-inflationary expansion: we argue that no geometric description was
possible before the time t_1 when our comoving region was one Planck length
across, and contained one pair of entangled degrees of freedom. This approach
could provide a framework for modeling the initial state of inflationary
perturbations.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/9x3hm-03b32A Nonlocal Approach to the Cosmological Constant Problem
https://resolver.caltech.edu/CaltechAUTHORS:20170330-135637254
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Remmen-G-N', 'name': {'family': 'Remmen', 'given': 'Grant N.'}, 'orcid': '0000-0001-6569-8866'}]}
Year: 2017
DOI: 10.1103/PhysRevD.95.123504
We construct a model in which the cosmological constant is canceled from the gravitational equations of motion. Our model relies on two key ingredients: a nonlocal constraint on the action, which forces the spacetime average of the Lagrangian density to vanish, and a dynamical way for this condition to be satisfied classically with arbitrary matter content. We implement the former condition with a spatially constant Lagrange multiplier associated with the volume form and the latter by including a free four-form gauge field strength in the action. These two features are enough to remove the cosmological constant from the Einstein equation. The model is consistent with all cosmological and experimental bounds on modification of gravity and allows for both cosmic inflation and the present epoch of acceleration.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/dj8hr-mzm06POP goes the universe
https://resolver.caltech.edu/CaltechAUTHORS:20170814-133646653
Authors: {'items': [{'id': 'Guth-A-H', 'name': {'family': 'Guth', 'given': 'Alan H.'}}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2017
DOI: 10.1038/scientificamerican0717-5
The origins of space and time are among the most mysterious and contentious topics in science. Our February 2017 article "Pop Goes the Universe" argues against the dominant idea that the early cosmos underwent an extremely rapid expansion called inflation. Its authors instead advocate for another scenario—that our universe began not with a bang but with a bounce from a previously contracting cosmos. In the letter below, a group of 33 physicists who study inflationary cosmology respond to that article. It is followed by a reply from the authors.
In "Pop Goes the Universe," by Anna Ijjas, Paul J. Steinhardt and Abraham Loeb, the authors (hereafter "IS&L") make the case for a bouncing cosmology, as was proposed by Steinhardt and others in 2001. They close by making the extraordinary claim that inflationary cosmology "cannot be evaluated using the scientific method" and go on to assert that some scientists who accept inflation have proposed "discarding one of [science's] defining properties: empirical testability," thereby "promoting the idea of some kind of nonempirical science." We have no idea what scientists they are referring to. We disagree with a number of statements in their article, but in this letter, we will focus on our categorical disagreement with these statements about the testability of inflation.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/f8hgg-sav44How decoherence affects the probability of slow-roll eternal inflation
https://resolver.caltech.edu/CaltechAUTHORS:20170728-110444626
Authors: {'items': [{'id': 'Boddy-K-K', 'name': {'family': 'Boddy', 'given': 'Kimberly K.'}, 'orcid': '0000-0003-1928-4667'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Pollack-J-A', 'name': {'family': 'Pollack', 'given': 'Jason'}, 'orcid': '0000-0003-4754-4905'}]}
Year: 2017
DOI: 10.1103/PhysRevD.96.023539
Slow-roll inflation can become eternal if the quantum variance of the inflaton field around its slowly rolling classical trajectory is converted into a distribution of classical spacetimes inflating at different rates, and if the variance is large enough compared to the rate of classical rolling that the probability of an increased rate of expansion is sufficiently high. Both of these criteria depend sensitively on whether and how perturbation modes of the inflaton interact and decohere. Decoherence is inevitable as a result of gravitationally sourced interactions whose strength are proportional to the slow-roll parameters. However, the weakness of these interactions means that decoherence is typically delayed until several Hubble times after modes grow beyond the Hubble scale. We present perturbative evidence that decoherence of long-wavelength inflaton modes indeed leads to an ensemble of classical spacetimes with differing cosmological evolutions. We introduce the notion of per-branch observables—expectation values with respect to the different decohered branches of the wave function—and show that the evolution of modes on individual branches varies from branch to branch. Thus, single-field slow-roll inflation fulfills the quantum-mechanical criteria required for the validity of the standard picture of eternal inflation. For a given potential, the delayed decoherence can lead to slight quantitative adjustments to the regime in which the inflaton undergoes eternal inflation.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/d5c0n-vfh29The Hilbert Space of Quantum Gravity Is Locally Finite-Dimensional
https://resolver.caltech.edu/CaltechAUTHORS:20170427-143158126
Authors: {'items': [{'id': 'Bao-Ning', 'name': {'family': 'Bao', 'given': 'Ning'}, 'orcid': '0000-0002-3296-1039'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Singh-Ashmeet', 'name': {'family': 'Singh', 'given': 'Ashmeet'}, 'orcid': '0000-0002-4404-1416'}]}
Year: 2017
DOI: 10.1142/S0218271817430131
We argue in a model-independent way that the Hilbert space of quantum gravity is locally finite-dimensional. In other words, the density operator describing the state corresponding to a small region of space, when such a notion makes sense, is defined on a finite-dimensional factor of a larger Hilbert space. Because quantum gravity potentially describes superpositions of different geometries, it is crucial that we associate Hilbert-space factors with spatial regions only on individual decohered branches of the universal wave function. We discuss some implications of this claim, including the fact that quantum-field theory cannot be a fundamental description of nature.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/62sq6-kam37de Sitter space as a tensor network: Cosmic no-hair, complementarity, and complexity
https://resolver.caltech.edu/CaltechAUTHORS:20180105-090200316
Authors: {'items': [{'id': 'Bao-Ning', 'name': {'family': 'Bao', 'given': 'Ning'}, 'orcid': '0000-0002-3296-1039'}, {'id': 'Cao-ChunJun', 'name': {'family': 'Cao', 'given': 'ChunJun'}, 'orcid': '0000-0002-5761-5474'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Chatwin-Davies-A', 'name': {'family': 'Chatwin-Davies', 'given': 'Aidan'}, 'orcid': '0000-0003-1406-9271'}]}
Year: 2017
DOI: 10.1103/PhysRevD.96.123536
We investigate the proposed connection between de Sitter spacetime and the multiscale entanglement renormalization ansatz (MERA) tensor network, and ask what can be learned via such a construction. We show that the quantum state obeys a cosmic no-hair theorem: the reduced density operator describing a causal patch of the MERA asymptotes to a fixed point of a quantum channel, just as spacetimes with a positive cosmological constant asymptote to de Sitter space. The MERA is potentially compatible with a weak form of complementarity (local physics only describes single patches at a time, but the overall Hilbert space is infinite dimensional) or, with certain specific modifications to the tensor structure, a strong form (the entire theory describes only a single patch plus its horizon, in a finite-dimensional Hilbert space). We also suggest that de Sitter evolution has an interpretation in terms of circuit complexity, as has been conjectured for anti–de Sitter space.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/15h82-hak05Beyond Falsifiability: Normal Science in a Multiverse
https://resolver.caltech.edu/CaltechAUTHORS:20180117-155705369
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2018
DOI: 10.48550/arXiv.1801.05016
Cosmological models that invoke a multiverse - a collection of unobservable
regions of space where conditions are very different from the region around us
- are controversial, on the grounds that unobservable phenomena shouldn't play
a crucial role in legitimate scientific theories. I argue that the way we
evaluate multiverse models is precisely the same as the way we evaluate any
other models, on the basis of abduction, Bayesian inference, and empirical
success. There is no scientifically respectable way to do cosmology without
taking into account different possibilities for what the universe might be like
outside our horizon. Multiverse theories are utterly conventionally scientific,
even if evaluating them can be difficult in practice.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/gp3d4-9b590Cosmic Equilibration: A Holographic No-Hair Theorem from the Generalized Second Law
https://resolver.caltech.edu/CaltechAUTHORS:20170330-143452297
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Chatwin-Davies-A', 'name': {'family': 'Chatwin-Davies', 'given': 'Aidan'}, 'orcid': '0000-0003-1406-9271'}]}
Year: 2018
DOI: 10.1103/PhysRevD.97.046012
In a wide class of cosmological models, a positive cosmological constant drives cosmological evolution toward an asymptotically de Sitter phase. Here we connect this behavior to the increase of entropy over time, based on the idea that de Sitter spacetime is a maximum-entropy state. We prove a cosmic no-hair theorem for Robertson-Walker and Bianchi I spacetimes that admit a Q-screen ("quantum" holographic screen) with certain entropic properties: If generalized entropy, in the sense of the cosmological version of the generalized second law conjectured by Bousso and Engelhardt, increases up to a finite maximum value along the screen, then the spacetime is asymptotically de Sitter in the future. Moreover, the limiting value of generalized entropy coincides with the de Sitter horizon entropy. We do not use the Einstein field equations in our proof, nor do we assume the existence of a positive cosmological constant. As such, asymptotic relaxation to a de Sitter phase can, in a precise sense, be thought of as cosmological equilibration.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/kp4e1-p4730Why Is There Something, Rather Than Nothing?
https://resolver.caltech.edu/CaltechAUTHORS:20180221-105036200
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2018
DOI: 10.48550/arXiv.1802.02231
It seems natural to ask why the universe exists at all. Modern physics
suggests that the universe can exist all by itself as a self-contained system,
without anything external to create or sustain it. But there might not be an
absolute answer to why it exists. I argue that any attempt to account for the
existence of something rather than nothing must ultimately bottom out in a set
of brute facts; the universe simply is, without ultimate cause or explanation.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/xpehz-yrn86Quantum decimation in Hilbert space: Coarse graining without structure
https://resolver.caltech.edu/CaltechAUTHORS:20180327-074233712
Authors: {'items': [{'id': 'Singh-Ashmeet', 'name': {'family': 'Singh', 'given': 'Ashmeet'}, 'orcid': '0000-0002-4404-1416'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2018
DOI: 10.1103/PhysRevA.97.032111
We present a technique to coarse grain quantum states in a finite-dimensional Hilbert space. Our method is distinguished from other approaches by not relying on structures such as a preferred factorization of Hilbert space or a preferred set of operators (local or otherwise) in an associated algebra. Rather, we use the data corresponding to a given set of states, either specified independently or constructed from a single state evolving in time. Our technique is based on principle component analysis (PCA), and the resulting coarse-grained quantum states live in a lower-dimensional Hilbert space whose basis is defined using the underlying (isometric embedding) transformation of the set of fine-grained states we wish to coarse grain. Physically, the transformation can be interpreted to be an "entanglement coarse-graining" scheme that retains most of the global, useful entanglement structure of each state, while needing fewer degrees of freedom for its reconstruction. This scheme could be useful for efficiently describing collections of states whose number is much smaller than the dimension of Hilbert space, or a single state evolving over time.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/w27cy-rvt77Self-Locating Uncertainty and the Origin of Probability in Everettian Quantum Mechanics
https://resolver.caltech.edu/CaltechAUTHORS:20141216-201958532
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2018
DOI: 10.1093/bjps/axw004
A longstanding issue in attempts to understand the Everett (many-worlds) approach to quantum mechanics is the origin of the Born rule: why is the probability given by the square of the amplitude? Following Vaidman, we note that observers are in a position of self-locating uncertainty during the period between the branches of the wave function splitting via decoherence and the observer registering the outcome of the measurement. In this period, it is tempting to regard each branch as equiprobable, but we argue that the temptation should be resisted. Applying lessons from this analysis, we demonstrate (using methods similar to those of Zurek's envariance-based derivation) that the Born rule is the uniquely rational way of apportioning credence in Everettian quantum mechanics. In doing so, we rely on a single key principle: changes to the environment alone do not affect the probabilities one ought to assign to measurement outcomes in a local subsystem. We arrive at a method for assigning probabilities in cases that involve both classical and quantum self-locating uncertainty. This method provides unique answers to quantum Sleeping Beauty problems, as well as a well-defined procedure for calculating probabilities in quantum cosmological multiverses with multiple similar observers.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/a0vv6-h2t23Bulk Entanglement Gravity without a Boundary: Towards Finding Einstein's Equation in Hilbert Space
https://resolver.caltech.edu/CaltechAUTHORS:20171214-111114409
Authors: {'items': [{'id': 'Cao-ChunJun', 'name': {'family': 'Cao', 'given': 'ChunJun'}, 'orcid': '0000-0002-5761-5474'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2018
DOI: 10.1103/PhysRevD.97.086003
We consider the emergence from quantum entanglement of spacetime geometry in a bulk region. For certain classes of quantum states in an appropriately factorized Hilbert space, a spatial geometry can be defined by associating areas along codimension-one surfaces with the entanglement entropy between either side. We show how radon transforms can be used to convert these data into a spatial metric. Under a particular set of assumptions, the time evolution of such a state traces out a four-dimensional spacetime geometry, and we argue using a modified version of Jacobson's "entanglement equilibrium" that the geometry should obey Einstein's equation in the weak-field limit. We also discuss how entanglement equilibrium is related to a generalization of the Ryu-Takayanagi formula in more general settings, and how quantum error correction can help specify the emergence map between the full quantum-gravity Hilbert space and the semiclassical limit of quantum fields propagating on a classical spacetime.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/2m2vx-01829Branches of the Black Hole Wave Function Need Not Contain Firewalls
https://resolver.caltech.edu/CaltechAUTHORS:20180109-165521863
Authors: {'items': [{'id': 'Bao-Ning', 'name': {'family': 'Bao', 'given': 'Ning'}, 'orcid': '0000-0002-3296-1039'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Chatwin-Davies-A', 'name': {'family': 'Chatwin-Davies', 'given': 'Aidan'}, 'orcid': '0000-0003-1406-9271'}, {'id': 'Pollack-J-A', 'name': {'family': 'Pollack', 'given': 'Jason'}, 'orcid': '0000-0003-4754-4905'}, {'id': 'Remmen-G-N', 'name': {'family': 'Remmen', 'given': 'Grant N.'}, 'orcid': '0000-0001-6569-8866'}]}
Year: 2018
DOI: 10.1103/PhysRevD.97.126014
We discuss the branching structure of the quantum-gravitational wave function that describes the evaporation of a black hole. A global wave function which initially describes a classical Schwarzschild geometry is continually decohered into distinct semiclassical branches by the emission of Hawking radiation. The laws of quantum mechanics dictate that the wave function evolves unitarily, but this unitary evolution is only manifest when considering the global description of the wave function; it is not implemented by time evolution on a single semiclassical branch. Conversely, geometric notions like the position or smoothness of a horizon only make sense on the level of individual branches. We consider the implications of this picture for probes of black holes by classical observers in definite geometries, like those involved in the Almheiri-Marolf-Polchinski-Sully construction. We argue that individual branches can describe semiclassical geometries free of firewalls, even as the global wave function evolves unitarily. We show that the pointer states of infalling detectors that are robust under Hamiltonian evolution are distinct from, and incompatible with, those of exterior detectors stationary with respect to the black hole horizon, in the sense that the pointer bases are related to each other via nontrivial transformations that mix the system, apparatus, and environment. This result describes a Hilbert-space version of black hole complementarity.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/6ccwx-53c78Modeling Position and Momentum in Finite-Dimensional Hilbert Spaces via Generalized Clifford Algebra
https://resolver.caltech.edu/CaltechAUTHORS:20180710-134008015
Authors: {'items': [{'id': 'Singh-Ashmeet', 'name': {'family': 'Singh', 'given': 'Ashmeet'}, 'orcid': '0000-0002-4404-1416'}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2018
DOI: 10.48550/arXiv.1806.10134
The finite entropy of black holes suggests that local regions of spacetime
are described by finite-dimensional factors of Hilbert space, in contrast with
the infinite-dimensional Hilbert spaces of quantum field theory. With this in
mind, we explore how to cast finite-dimensional quantum mechanics in a form
that matches naturally onto the smooth case, especially the recovery of
conjugate position/momentum variables, in the limit of large Hilbert-space
dimension. A natural tool for this task is the generalized Clifford algebra
(GCA). Based on an exponential form of Heisenberg's canonical commutation
relation, the GCA offers a finite-dimensional generalization of conjugate
variables without relying on any a priori structure on Hilbert space. We
highlight some features of the GCA, its importance in studying concepts such as
locality of operators, and point out departures from infinite-dimensional
results (possibly with a cutoff) that might play a crucial role in our
understanding of quantum gravity. We introduce the concept of "Schwinger
locality," which characterizes how the action of an operator spreads a quantum
state along conjugate directions. We illustrate these concepts with a worked
example of a finite-dimensional harmonic oscillator, demonstrating how the
energy spectrum deviates from the familiar infinite-dimensional case.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/tr65p-81x78Mad-Dog Everettianism: Quantum Mechanics at Its Most Minimal
https://resolver.caltech.edu/CaltechAUTHORS:20180228-094539014
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Singh-Ashmeet', 'name': {'family': 'Singh', 'given': 'Ashmeet'}, 'orcid': '0000-0002-4404-1416'}]}
Year: 2019
DOI: 10.1007/978-3-030-11301-8_10
To the best of our current understanding, quantum mechanics is part of the most fundamental picture of the universe. It is natural to ask how pure and minimal this fundamental quantum description can be. The simplest quantum ontology is that of the Everett or Many-Worlds interpretation, based on a vector in Hilbert space and a Hamiltonian. Typically one also relies on some classical structure, such as space and local configuration variables within it, which then gets promoted to an algebra of preferred observables. We argue that even such an algebra is unnecessary, and the most basic description of the world is given by the spectrum of the Hamiltonian (a list of energy eigenvalues) and the components of some particular vector in Hilbert space. Everything else—including space and fields propagating on it—is emergent from these minimal elements.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/5f5w2-x7g75Woven from weirdness
https://resolver.caltech.edu/CaltechAUTHORS:20190917-142245143
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2019
DOI: 10.1016/s0262-4079(19)31728-2
The true origins of space-time, the backdrop to reality, are hidden in the quantum realm, writes physicist Sean Carroll.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/42n1d-bwm95Consciousness and the Laws of Physics
https://resolver.caltech.edu/CaltechAUTHORS:20211006-213336168
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'S.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2021
DOI: 10.53765/20512201.28.9.016
We have a much better understanding of the dynamics and ontology of physics than we do of consciousness. I consider ways in which intrinsically mental aspects of fundamental ontology might induce modifications of the known laws of physics, or whether they could be relevant to accounting for consciousness if no such modifications exist. I suggest that our current knowledge of physics should make us sceptical of hypothetical modifications of the known rules, and that without such modifications it's hard to imagine how intrinsically mental aspects could play a useful explanatory role.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/53amt-jfa79The Quantum Field Theory on Which the Everyday World Supervenes
https://resolver.caltech.edu/CaltechAUTHORS:20210122-075332117
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2021
DOI: 10.48550/arXiv.2101.07884
Effective Field Theory (EFT) is the successful paradigm underlying modern theoretical physics, including the "Core Theory" of the Standard Model of particle physics plus Einstein's general relativity. I will argue that EFT grants us a unique insight: each EFT model comes with a built-in specification of its domain of applicability. Hence, once a model is tested within some domain (of energies and interaction strengths), we can be confident that it will continue to be accurate within that domain. Currently, the Core Theory has been tested in regimes that include all of the energy scales relevant to the physics of everyday life (biology, chemistry, technology, etc.). Therefore, we have reason to be confident that the laws of physics underlying the phenomena of everyday life are completely known.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/xx0f8-p0s42Quantum mereology: Factorizing Hilbert space into subsystems with quasiclassical dynamics
https://resolver.caltech.edu/CaltechAUTHORS:20200528-092234945
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Singh-Ashmeet', 'name': {'family': 'Singh', 'given': 'Ashmeet'}, 'orcid': '0000-0002-4404-1416'}]}
Year: 2021
DOI: 10.1103/PhysRevA.103.022213
We study the question of how to decompose Hilbert space into a preferred tensor-product factorization without any preexisting structure other than a Hamiltonian operator, in particular the case of a bipartite decomposition into "system" and "environment." Such a decomposition can be defined by looking for subsystems that exhibit quasiclassical behavior. The correct decomposition is one in which pointer states of the system are relatively robust against environmental monitoring (their entanglement with the environment does not continually and dramatically increase) and remain localized around approximately classical trajectories. We present an in-principle algorithm for finding such a decomposition by minimizing a combination of entanglement growth and internal spreading of the system. Both of these properties are related to locality in different ways. This formalism is relevant to questions in the foundations of quantum mechanics and the emergence of spacetime from quantum entanglement.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/5cev0-g6840Reality as a Vector in Hilbert Space
https://resolver.caltech.edu/CaltechAUTHORS:20210323-141639318
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2021
DOI: 10.48550/arXiv.2103.09780
I defend the extremist position that the fundamental ontology of the world consists of a vector in Hilbert space evolving according to the Schrödinger equation. The laws of physics are determined solely by the energy eigenspectrum of the Hamiltonian. The structure of our observed world, including space and fields living within it, should arise as a higher-level emergent description. I sketch how this might come about, although much work remains to be done.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/x1dxg-01s41Energy Non-conservation in Quantum Mechanics
https://resolver.caltech.edu/CaltechAUTHORS:20210128-100731565
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Lodman-Jacqueline-J', 'name': {'family': 'Lodman', 'given': 'Jackie'}, 'orcid': '0000-0003-0923-9440'}]}
Year: 2021
DOI: 10.1007/s10701-021-00490-5
We study the conservation of energy, or lack thereof, when measurements are performed in quantum mechanics. The expectation value of the Hamiltonian of a system changes when wave functions collapse in accordance with the standard textbook (Copenhagen) treatment of quantum measurement, but one might imagine that the change in energy is compensated by the measuring apparatus or environment. We show that this is not true; the change in the energy of a state after measurement can be arbitrarily large, independent of the physical measurement process. In Everettian quantum theory, while the expectation value of the Hamiltonian is conserved for the wave function of the universe (including all the branches), it is not constant within individual worlds. It should therefore be possible to experimentally measure violations of conservation of energy, and we suggest an experimental protocol for doing so.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/9w3nd-thb42Mysteries of Modern Physics
https://resolver.caltech.edu/CaltechAUTHORS:20230420-574389200.2
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2022
DOI: 10.1017/9781009232517.003
This chapter guides the reader through three related enigmas of modern physics. The first is a mystery of quantum mechanics. Important aspects of quantum mechanics are still not truly understood, although competing theories have been proposed, including the Many-Worlds approach. The second enigma is the emergence of spacetime, and especially the way it interacts with gravity. Rather than following the traditional methodology of 'quantising' classical theories, the author proposes an alternative approach and instead seeks gravity within quantum mechanics. The chapter concludes with a discussion of the mystery of the arrow of time: what distinguishes the past from the future. Together, these three mysteries of modern physics serve as an important reminder of the endurance of enigmas in the very foundations of scholarly fields. Re-examining founding principles can provide a constructive alternative means of investigating mysteries, not only in modern science but also across other disciplines.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/p863p-e9c71