29 research outputs found
Quantum buoyancy, generalized second law, and higher-dimensional entropy bounds
Bekenstein has presented evidence for the existence of a universal upper
bound of magnitude to the entropy-to-energy ratio of an
arbitrary {\it three} dimensional system of proper radius and negligible
self-gravity. In this paper we derive a generalized upper bound on the
entropy-to-energy ratio of a -dimensional system. We consider a box full
of entropy lowered towards and then dropped into a -dimensional black
hole in equilibrium with thermal radiation. In the canonical case of three
spatial dimensions, it was previously established that due to quantum buoyancy
effects the box floats at some neutral point very close to the horizon. We find
here that the significance of quantum buoyancy increases dramatically with the
number of spatial dimensions. In particular, we find that the neutral
(floating) point of the box lies near the horizon only if its length is
large enough such that , where is the Compton length of the
body and for . A consequence is that quantum
buoyancy severely restricts our ability to deduce the universal entropy bound
from the generalized second law of thermodynamics in higher-dimensional
spacetimes with . Nevertheless, we find that the universal entropy bound
is always a sufficient condition for operation of the generalized second law in
this type of gedanken experiments.Comment: 6 page
New AdS solitons and brane worlds with compact extra-dimensions
We construct new static, asymptotically AdS solutions where the conformal
infinity is the product of Minkowski spacetime and a sphere . Both
globally regular, soliton-type solutions and black hole solutions are
considered. The black holes can be viewed as natural AdS generalizations of the
Schwarzschild black branes in Kaluza-Klein theory. The solitons provide new
brane-world models with compact extra-dimensions. Different from the
Randall-Sundrum single-brane scenario, a Schwarzschild black hole on the Ricci
flat part of these branes does not lead to a naked singularity in the bulk.Comment: 28 pages, 4 figure
Radiation from a D-dimensional collision of shock waves: first order perturbation theory
We study the spacetime obtained by superimposing two equal Aichelburg-Sexl
shock waves in D dimensions traveling, head-on, in opposite directions.
Considering the collision in a boosted frame, one shock becomes stronger than
the other, and a perturbative framework to compute the metric in the future of
the collision is setup. The geometry is given, in first order perturbation
theory, as an integral solution, in terms of initial data on the null surface
where the strong shock has support. We then extract the radiation emitted in
the collision by using a D-dimensional generalisation of the Landau-Lifschitz
pseudo-tensor and compute the percentage of the initial centre of mass energy
epsilon emitted as gravitational waves. In D=4 we find epsilon=25.0%, in
agreement with the result of D'Eath and Payne. As D increases, this percentage
increases monotonically, reaching 40.0% in D=10. Our result is always within
the bound obtained from apparent horizons by Penrose, in D=4, yielding 29.3%,
and Eardley and Giddings, in D> 4, which also increases monotonically with
dimension, reaching 41.2% in D=10. We also present the wave forms and provide a
physical interpretation for the observed peaks, in terms of the null generators
of the shocks.Comment: 27 pages, 11 figures; v2 some corrections, including D dependent
factor in epsilon; matches version accepted in JHE
Quantum gravity effects in Myers-Perry space-times
We study quantum gravity effects for Myers-Perry black holes assuming that the leading contributions arise from the renormalization group evolution of Newton's coupling. Provided that gravity weakens following the asymptotic safety conjecture, we find that quantum effects lift a degeneracy of higher-dimensional black holes, and dominate over kinematical ones induced by rotation, particularly for small black hole mass, large angular momentum, and higher space-time dimensionality. Quantum-corrected space-times display inner and outer horizons, and show the existence of a black hole of smallest mass in any dimension. Ultra-spinning solutions no longer persist. Thermodynamic properties including temperature, specific heat, the Komar integrals, and aspects of black hole mechanics are studied as well. Observing a softening of the ring singularity, we also discuss the validity of classical energy conditions
f(R) theories
Over the past decade, f(R) theories have been extensively studied as one of
the simplest modifications to General Relativity. In this article we review
various applications of f(R) theories to cosmology and gravity - such as
inflation, dark energy, local gravity constraints, cosmological perturbations,
and spherically symmetric solutions in weak and strong gravitational
backgrounds. We present a number of ways to distinguish those theories from
General Relativity observationally and experimentally. We also discuss the
extension to other modified gravity theories such as Brans-Dicke theory and
Gauss-Bonnet gravity, and address models that can satisfy both cosmological and
local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in
Relativity, Published version, Comments are welcom
Black ringoids: spinning balanced black objects in d >= 5 dimensions - the codimension-two case
We propose a general framework for the study of asymptotically flat black objects with k+1 equal magnitude angular momenta in d >= 5 spacetime dimensions (with 0 0 are dubbed black ringoids. Based on the nonperturbative numerical results found for several values of (n, k), we propose a general picture for the properties and the phase diagram of these solutions and the associated black holes with spherical horizon topology: n = 1 black ringoids repeat the k = 0 pattern of black rings and Myers-Perry black holes in 5 dimensions, whereas n > 1 black ringoids follow the pattern of higher dimensional black rings associated with 'pinched' black holes and Myers-Perry black holes
Black Holes in Higher Dimensions
We review black hole solutions of higher-dimensional vacuum gravity, and of
higher-dimensional supergravity theories. The discussion of vacuum gravity is
pedagogical, with detailed reviews of Myers-Perry solutions, black rings, and
solution-generating techniques. We discuss black hole solutions of maximal
supergravity theories, including black holes in anti-de Sitter space. General
results and open problems are discussed throughout.Comment: 76 pages, 14 figures; review article for Living Reviews in
Relativity. v2: some improvements and refs adde
Hawking Radiation from Higher-Dimensional Black Holes
We review the quantum field theory description of Hawking radiation from evaporating black holes and summarize what is known about Hawking radiation from black holes in more than four space-time dimensions. In the context of the Large Extra Dimensions scenario, we present the theoretical formalism for all types of emitted fields and a selection of results on the radiation spectra. A detailed analysis of the Hawking fluxes in this case is essential for modelling the evaporation of higher-dimensional black holes at the LHC, whose creation is predicted by low-energy models of quantum gravity. We discuss the status of the quest for black-hole solutions in the context of the Randall-Sundrum brane-world model and, in the absence of an exact metric, we review what is known about Hawking radiation from such black holes
Exploring new physics frontiers through numerical relativity
The demand to obtain answers to highly complex problems within strong-field gravity has been met with significant progress in the numerical solution of Einstein's equations - along with some spectacular results - in various setups. We review techniques for solving Einstein's equations in generic spacetimes, focusing on fully nonlinear evolutions but also on how to benchmark those results with perturbative approaches. The results address problems in high-energy physics, holography, mathematical physics, fundamental physics, astrophysics and cosmology