arXiv subjects

Ram Brustein

Publications and source records attributed to Ram Brustein.

At least 91 records · Page 5Linked to original sources

arXiv2010

Constraining the expansion history of the universe from the red shift evolution of cosmic shear

We present a quantitative analysis of the constraints on the total equation of state parameter that can be obtained from measuring the red shift evolution of the cosmic shear. We compare the constraints that can be obtained from measurements of the spin two angular multipole moments of the cosmic shear to those resulting from the two dimensional and three dimensional power spectra of the cosmic shear. We find that if the multipole moments of the cosmic shear are measured accurately enough for a few red shifts the constraints on the dark energy equation of state parameter improve significantly compared to those that can be obtained from other measurements.

astro-ph.CO↗

arXiv2010

Bounds on Black Hole Entropy in Unitary Theories of Gravity

We consider unitary and weakly coupled theories of gravity that extend Einstein gravity and reduce to it asymptotically at large distances. Our discussion is restricted to such theories that, similarly to Einstein gravity, contain black holes as semiclassical states in a range of scales. We show that, at a given scale, the entropy of these black holes has to be larger than the number of elementary light species in the theory. Our bound follows from the observation that the black hole entropy has to be larger than the product of its mass and horizon radius (in units of Planck's constant divided by the speed of light) and the fact that, for any semiclassical black hole, this product has to be larger than the number of light species. For theories that obey our assumptions, the bound resolves the "species problem": the tension between the geometric, species-independent nature of black hole entropy and the proportionality of ordinary thermodynamic entropy to the number of species. We then show that, when black holes in Einstein's theory are compared to those in the extended theories at a fixed value of mass, the entropy of the Einstein black holes will always be minimal. Similar considerations are also applied to the entropy density of black branes in anti-de Sitter space.

hep-th↗

arXiv2009

Proof of a universal lower bound on the shear viscosity to entropy density ratio

It has been conjectured, on the basis of the gauge-gravity duality, that the ratio of the shear viscosity to the entropy density should be universally bounded from below by 1/ 4 pi in units of the Planck constant divided by the Boltzmann constant. Here, we prove the bound for any ghost-free extension of Einstein gravity and the field-theory dual thereof. Our proof is based on the fact that, for such an extension, any gravitational coupling can only increase from its Einstein value. Therefore, since the shear viscosity is a particular gravitational coupling, it is minimal for Einstein gravity. Meanwhile, we show that the entropy density can always be calibrated to its Einstein value. Our general principles are demonstrated for a pair of specific models, one with ghosts and one without.

hep-th↗

arXiv2009

Expressing the equation of state parameter in terms of the three dimensional cosmic shear

We study the functional dependence of the spin-weighted angular moments of the two-point correlation function of the three dimensional cosmic shear on the expansion history of the universe. We first express the redshift dependent total equation of state parameter in terms of the growing mode of the gauge invariant metric perturbation in the conformal-Newtonian gauge for the case of adiabatic perturbations. We then express the redshift dependent angular moments of the shear two-point correlation function as an integral in terms of the metric perturbation. We present the final explicit expression for the case of a Harrison-Zeldovich spectrum of primordial perturbations. Our analysis is restricted to the linear regime. We use our results to make a preliminary study of the required sensitivity that will allow cosmic shear observations to add significant information about the expansion history of the universe.

astro-ph↗

arXiv2009

Wald's entropy is equal to a quarter of the horizon area in units of the effective gravitational coupling

The Bekenstein-Hawking entropy of black holes in Einstein's theory of gravity is equal to a quarter of the horizon area in units of Newton's constant. Wald has proposed that in general theories of gravity the entropy of stationary black holes with bifurcate Killing horizons is a Noether charge which is in general different from the Bekenstein-Hawking entropy. We show that the Noether charge entropy is equal to a quarter of the horizon area in units of the effective gravitational coupling on the horizon defined by the coefficient of the kinetic term of specific graviton polarizations on the horizon. We present several explicit examples of static spherically symmetric black holes.

hep-th↗

arXiv2009

The sound damping constant for generalized theories of gravity

The near-horizon metric for a black brane in Anti-de Sitter (AdS) space and the metric near the AdS boundary both exhibit hydrodynamic behavior. We demonstrate the equivalence of this pair of hydrodynamic systems for the sound mode of a conformal theory. This is first established for Einstein's gravity, but we then show how the sound damping constant will be modified, from its Einstein form, for a generalized theory. The modified damping constant is expressible as the ratio of a pair of gravitational couplings that are indicative of the sound-channel class of gravitons. This ratio of couplings differs from both that of the shear diffusion coefficient and the shear viscosity to entropy ratio. Our analysis is mostly limited to conformal theories but suggestions are made as to how this restriction might eventually be lifted.

hep-th↗

arXiv2008

The shear diffusion coefficient for generalized theories of gravity

Near the horizon of a black brane in Anti-de Sitter (AdS) space and near the AdS boundary, the long-wavelength fluctuations of the metric exhibit hydrodynamic behaviour. The gauge-gravity duality then relates the boundary hydrodynamics for generalized gravity to that of gauge theories with large finite values of 't Hooft coupling. We discuss, for this framework, the hydrodynamics of the shear mode in generalized theories of gravity in d+1 dimensions. It is shown that the shear diffusion coefficients of the near-horizon and boundary hydrodynamics are equal and can be expressed in a form that is purely local to the horizon. We find that the Einstein-theory relation between the shear diffusion coefficient and the shear viscosity to entropy ratio is modified for generalized gravity theories: Both can be explicitly written as the ratio of a pair of polarization-specific gravitational couplings but implicate differently polarized gravitons. Our analysis is restricted to the shear-mode fluctuations for simplicity and clarity; however, our methods can be applied to the hydrodynamics of all gravitational and matter fluctuation modes.

hep-th↗

arXiv2008

Entanglement Induced Fluctuations of Cold Bosons

We show that due to entanglement, quantum fluctuations may differ significantly from statistical fluctuations. We calculate quantum fluctuations of the particle number and of the energy in a sub-volume of a system of bosons in a pure state, and briefly discuss the possibility of measuring them. We find that energy fluctuations have a non-extensive nature.

hep-th↗

arXiv2008

The ratio of shear viscosity to entropy density in generalized theories of gravity

Near the horizon of a black brane solution in Anti-de Sitter space, the long-wavelength fluctuations of the metric exhibit hydrodynamic behaviour. For Einstein's theory, the ratio of the shear viscosity of near-horizon metric fluctuations eta to the entropy per unit of transverse volume s is eta/s=1/4 pi. We propose that, in generalized theories of gravity, this ratio is given by the ratio of two effective gravitational couplings and can be different than 1/4 pi. Our proposal implies that eta/s is equal for any pair of gravity theories that can be transformed into each other by a field redefinition. In particular, the ratio is 1/4 pi for any theory that can be transformed into Einstein's theory; such as F(R) gravity. Our proposal also implies that matter interactions -- except those including explicit or implicit factors of the Riemann tensor -- will not modify eta/s. The proposed formula reproduces, in a very simple manner, some recently found results for Gauss-Bonnet gravity. We also make a prediction for eta/s in Lovelock theories of any order or dimensionality.

hep-th↗

arXiv2008

Models of Modular Inflation and Their Phenomenological Consequences

We study models of modular inflation of the form expected to arise from low energy effective actions of superstring theories. We argue on general grounds that the most likely models of modular slow-roll inflation are small field models in which the inflaton moves about a Planck distance from an extremum of the potential. We then focus on models in which the inflaton is the bosonic component of a single (complex) chiral superfield and explain the generic difficulties in designing small field models of modular inflation. We then show that if the Kaehler potential (KP) of the inflaton is logarithmic as in perturbative string theories, then it is not possible to satisfy the slow-roll conditions for any superpotential. We find that if the corrections to the KP are large enough so it can be approximated by a canonical KP in the vicinity of the extremum, then viable slow-roll inflation is possible. In this case, several parameters have to be tuned to a fraction of a percent. We give a prescription for designing successful small field supergravity models of inflation when the KP is canonical and calculate the slow-roll parameters from the superpotential parameters. Our results strengthen the case for models in which the moduli slowly roll about a Planck distance from a relatively high scale extremum that is located in the vicinity of the central region of moduli space units. Generic models of this class predict a red spectrum of scalar perturbations and negligible spectral index running. They also predict a characteristic suppression of tensor perturbations despite the high scale of inflation. Consequently, a detection of primordial tensor anisotropies or spectral index running in cosmic microwave background observations in the foreseeable future will rule out this entire class of modular inflation models.

hep-th↗

arXiv2007

Entanglement and Nonunitary Evolution

We consider a collapsing relativistic spherical shell for a free quantum field. Once the center of the wavefunction of the shell passes a certain radius R, the degrees of freedom inside R are traced over. We show that an observer outside this region will determine that the evolution of the system is nonunitary. We argue that this phenomenon is generic to entangled systems, and discuss a possible relation to black hole physics.

hep-th↗

arXiv2007

Cosmological Entropy Bounds

I review some basic facts about entropy bounds in general and about cosmological entropy bounds. Then I review the Causal Entropy Bound, the conditions for its validity and its application to the study of cosmological singularities. This article is based on joint work with Gabriele Veneziano and subsequent related research.

hep-th↗

arXiv2005

Entanglement Interpretation of Black Hole Entropy in String Theory

We show that the entropy resulting from the counting of microstates of non extremal black holes using field theory duals of string theories can be interpreted as arising from entanglement. The conditions for making such an interpretation consistent are discussed. First, we interpret the entropy (and thermodynamics) of spacetimes with non degenerate, bifurcating Killing horizons as arising from entanglement. We use a path integral method to define the Hartle-Hawking vacuum state in such spacetimes and discuss explicitly its entangled nature and its relation to the geometry. If string theory on such spacetimes has a field theory dual, then, in the low-energy, weak coupling limit, the field theory state that is dual to the Hartle-Hawking state is a thermofield double state. This allows the comparison of the entanglement entropy with the entropy of the field theory dual, and thus, with the Bekenstein-Hawking entropy of the black hole. As an example, we discuss in detail the case of the five dimensional anti-de Sitter, black hole spacetime.

hep-th↗

arXiv2005

Dimensional reduction from entanglement in Minkowski space

Using a quantum field theoretic setting, we present evidence for dimensional reduction of any sub-volume of Minkowksi space. First, we show that correlation functions of a class of operators restricted to a sub-volume of D-dimensional Minkowski space scale as its surface area. A simple example of such area scaling is provided by the energy fluctuations of a free massless quantum field in its vacuum state. This is reminiscent of area scaling of entanglement entropy but applies to quantum expectation values in a pure state, rather than to statistical averages over a mixed state. We then show, in a specific case, that fluctuations in the bulk have a lower-dimensional representation in terms of a boundary theory at high temperature.

hep-th↗

arXiv2004

A braneworld puzzle about entropy bounds and a maximal temperature

Entropy bounds applied to a system of N species of light quantum fields in thermal equilibrium at temperature T are saturated in four dimensions at a maximal temperature T_max=M_Planck/N^1/2. We show that the correct setup for understanding the reason for the saturation is a cosmological setup, and that a possible explanation is the copious production of black holes at this maximal temperature. The proposed explanation implies, if correct, that N light fields cannot be in thermal equilibrium at temperatures T above T_max. However, we have been unable to identify a concrete mechanism that is efficient and quick enough to prevent the universe from exceeding this limiting temperature. The same issues can be studied in the framework of AdS/CFT by using a brane moving in a five dimensional AdS-Schwarzschild space to model a radiation dominated universe. In this case we show that T_max is the temperature at which the brane just reaches the horizon of the black hole, and that entropy bounds and the generalized second law of thermodynamics seem to be violated when the brane continues to fall into the black hole. We find, again, that the known physical mechanisms, including black hole production, are not efficient enough to prevent the brane from falling into the black hole. We propose several possible explanations for the apparent violation of entropy bounds, but none is a conclusive one.

hep-th↗

arXiv2003

Thermodynamics and area in Minkowski space: Heat capacity of entanglement

Tracing over the degrees of freedom inside (or outside) a sub-volume V of Minkowski space in a given quantum state |psi>, results in a statistical ensemble described by a density matrix rho. This enables one to relate quantum fluctuations in V when in the state |psi>, to statistical fluctuations in the ensemble described by rho. These fluctuations scale linearly with the surface area of V. If V is half of space, then rho is the density matrix of a canonical ensemble in Rindler space. This enables us to `derive' area scaling of thermodynamic quantities in Rindler space from area scaling of quantum fluctuations in half of Minkowski space. When considering shapes other than half of Minkowski space, even though area scaling persists, rho does not have an interpretation as a density matrix of a canonical ensemble in a curved, or geometrically non-trivial, background.

hep-th↗

arXiv2003

Implications of area scaling of quantum fluctuations

Quantum fluctuations of a certain class of bulk operators defined in spatial sub-volumes of Minkowski space-time, have an unexpected area scaling property. We wish to present evidence that such area scaling may be ascribed to a boundary theory. We first highlight the implications of area scaling with two examples in which the boundary area of the spatial regions is not monotonous with their volume. Next, we prove that the covariance of two operators that are restricted to two different regions in Minkowski space scales linearly with their mutual boundary area. Finally, we present an example which demonstrates why this implies an underlying boundary theory.

hep-th↗

arXiv2002

Classical dynamics of quantum fluctuations

It is shown that the vacuum state of weakly interacting quantum field theories can be described, in the Heisenberg picture, as a linear combination of randomly distributed incoherent paths that obey classical equations of motion with constrained initial conditions. We call such paths "pseudoclassical" paths and use them to define the dynamics of quantum fluctuations. Every physical observable is assigned a time-dependent value on each path in a way that respects the uncertainty principle, but in consequence, some of the standard algebraic relations between quantum observables are not necessarily fulfilled by their time-dependent values on paths. We define "collective observables" which depend on a large number of independent degrees of freedom, and show that the dynamics of their quantum fluctuations can be described in terms of unconstrained classical stochastic processes without reference to any additional external system or to an environment. Our analysis can be generalized to states other than the vacuum. Finally, we compare our formalism to the formalism of coherent states, and highlight their differences.

hep-th↗