47,887 research outputs found
The generalized second law of thermodynamics in generalized gravity theories
We investigate the generalized second law of thermodynamics (GSL) in
generalized theories of gravity. We examine the total entropy evolution with
time including the horizon entropy, the non-equilibrium entropy production, and
the entropy of all matter, field and energy components. We derive a universal
condition to protect the generalized second law and study its validity in
different gravity theories. In Einstein gravity, (even in the phantom-dominated
universe with a Schwarzschild black hole), Lovelock gravity, and braneworld
gravity, we show that the condition to keep the GSL can always be satisfied. In
gravity and scalar-tensor gravity, the condition to protect the GSL can
also hold because the gravity is always attractive and the effective Newton
constant should be approximate constant satisfying the experimental bounds.Comment: 19 pages, no figure, mistakes corrected, references added, to appear
in Class. Quantum Gra
Entropy and quantum gravity
We give a review, in the style of an essay, of the author's 1998
matter-gravity entanglement hypothesis which, unlike the standard approach to
entropy based on coarse-graining, offers a definition for the entropy of a
closed system as a real and objective quantity. We explain how this approach
offers an explanation for the Second Law of Thermodynamics in general and a
non-paradoxical understanding of information loss during black hole formation
and evaporation in particular. It also involves a radically different from
usual description of black hole equilibrium states in which the total state of
a black hole in a box together with its atmosphere is a pure state -- entangled
in just such a way that the reduced state of the black hole and of its
atmosphere are each separately approximately thermal. We also briefly recall
some recent work of the author which involves a reworking of the string-theory
understanding of black hole entropy consistent with this alternative
description of black hole equilibrium states and point out that this is free
from some unsatisfactory features of the usual string theory understanding. We
also recall the author's recent arguments based on this alternative description
which suggest that the AdS/CFT correspondence is a bijection between the
boundary CFT and just the matter degrees of freedom of the bulk theory.Comment: 15 pages. Considerably enlarged. 3 figures and many references added.
Also published in the recent special issue "Entropy in Quantum Gravity and
Quantum Cosmology" (ed. Remo Garattini) of the online journal "Entropy".
(Note: that journal version will soon be replaced with an updated version
where some typesetting errors are corrected. This arXiv version is also free
from those errors.
Holography and Entropy Bounds in Gauss-Bonnet Gravity
We discuss the holography and entropy bounds in Gauss-Bonnet gravity theory.
By applying a Geroch process to an arbitrary spherically symmetric black hole,
we show that the Bekenstein entropy bound always keeps its form as , independent of gravity theories. As a result, the
Bekenstein-Verlinde bound also remains unchanged. Along the Verlinde's
approach, we obtain the Bekenstein-Hawking bound and Hubble bound, which are
different from those in Einstein gravity. Furthermore, we note that when
, the three cosmological entropy bounds become identical as in the case
of Einstein gravity. But, the Friedmann equation in Gauss-Bonnet gravity can no
longer be cast to the form of cosmological Cardy formula.Comment: 8 pages, Late
On entanglement entropy functionals in higher derivative gravity theories
In arXiv:1310.5713 and arXiv:1310.6659 a formula was proposed as the
entanglement entropy functional for a general higher-derivative theory of
gravity, whose lagrangian consists of terms containing contractions of the
Riemann tensor. In this paper, we carry out some tests of this proposal. First,
we find the surface equation of motion for general four-derivative gravity
theory by minimizing the holographic entanglement entropy functional resulting
from this proposed formula. Then we calculate the surface equation for the same
theory using the generalized gravitational entropy method of arXiv:1304.4926.
We find that the two do not match in their entirety. We also construct the
holographic entropy functional for quasi-topological gravity, which is a
six-derivative gravity theory. We find that this functional gives the correct
universal terms. However, as in the four-derivative case, the generalized
gravitational entropy method applied to this theory does not give exactly the
surface equation of motion coming from minimizing the entropy functional.Comment: 34 pages; v3: Details added, typos fixed, references updated; version
to appear in JHE
- …