Horizon thermodynamics and spacetime mappings

When black holes are dynamical, event horizons are replaced by apparent and trapping horizons. Conformal and Kerr-Schild transformations are widely used in relation with dynamical black holes and we study the behaviour under such transformations of quantities related to the thermodynamics of these horizons, such as the Misner-Sharp-Hernandez mass (internal energy), the Kodama vector, surface gravity, and temperature. The transformation properties are not those expected on the basis of naive arguments.Comment: 12 page

Black hole boundaries

Classical black holes and event horizons are highly non-local objects, defined in relation to the causal past of future null infinity. Alternative, quasilocal characterizations of black holes are often used in mathematical, quantum, and numerical relativity. These include apparent, killing, trapping, isolated, dynamical, and slowly evolving horizons. All of these are closely associated with two-surfaces of zero outward null expansion. This paper reviews the traditional definition of black holes and provides an overview of some of the more recent work on alternative horizons.Comment: 27 pages, 8 figures, invited Einstein Centennial Review Article for CJP, final version to appear in journal - glossary of terms added, typos correcte

From Geometry to Numerics: interdisciplinary aspects in mathematical and numerical relativity

This article reviews some aspects in the current relationship between mathematical and numerical General Relativity. Focus is placed on the description of isolated systems, with a particular emphasis on recent developments in the study of black holes. Ideas concerning asymptotic flatness, the initial value problem, the constraint equations, evolution formalisms, geometric inequalities and quasi-local black hole horizons are discussed on the light of the interaction between numerical and mathematical relativists.Comment: Topical review commissioned by Classical and Quantum Gravity. Discussion inspired by the workshop "From Geometry to Numerics" (Paris, 20-24 November, 2006), part of the "General Relativity Trimester" at the Institut Henri Poincare (Fall 2006). Comments and references added. Typos corrected. Submitted to Classical and Quantum Gravit

Horizons in the near-equilibrium regime

Quasi-static systems are an important concept in thermodynamics: they are dynamic but close enough to equilibrium that many properties of equilibrium systems still hold. Slowly evolving horizons are the corresponding concept for quasilocally defined black holes: they are "nearly isolated" future outer trapping horizons. This article reviews the definition and properties of these objects including both their mechanics and the role that they play in the fluid-gravity correspondence. It also introduces a new property: there is an event horizon candidate in close proximity to any slowly evolving horizon.Comment: 19 pages, 2 figures, will appear as a chapter of "Black Holes: New Horizons" edited by S. Haywar

Two physical characteristics of numerical apparent horizons

This article translates some recent results on quasilocal horizons into the language of $(3+1)$ general relativity so as to make them more useful to numerical relativists. In particular quantities are described which characterize how quickly an apparent horizon is evolving and how close it is to either equilibrium or extremality.Comment: 6 pages, 2 figures, conference proceedings loosely based on talk given at Theory Canada III (Edmonton, Alberta, 2007). V2: Minor changes in response to referees comments to improve clarity and fix typos. One reference adde

Dynamics and Thermodynamics of (2+1)-Dimensional Evolving Lorentzian Wormhole

In this paper we study the relationship between the Einstein field equations for the (2+1)-dimensional evolving wormhole and the first law of thermodynamics. It has been shown that the Einstein field equations can be rewritten as a similar form of the first law of thermodynamics at the dynamical trapping horizon (as proposed by Hayward) for the dynamical spacetime which describes intrinsic thermal properties associated with the trapping horizon. For a particular choice of the shape and potential functions we are able to express field equations as a similar form of first law of thermodynamics $dE=-TdS+WdA$ at the trapping horizons. Here $E=\rho A$, $T=-\kappa /2\pi$, $S=4\pi \tilde{r}_{A}$, $W=(\rho -p)/2$%, and $A=\pi \tilde{r}_{A}^{2}$, are the total matter energy, horizon temperature, wormhole entropy, work density and volume of the evolving wormhole respectively.Comment: 20 pages, 4 figures, paper presented at the 3rd Algerian Workshop on Astronomy and Astrophysic

Black hole entropy in scalar-tensor and f(R) gravity: an overview

A short overview of black hole entropy in alternative gravitational theories is presented. Motivated by the recent attempts to explain the cosmic acceleration without dark energy, we focus on metric and Palatini f(R) gravity and on scalar-tensor theories.Comment: 24 pages, latex, to appear in "Entropy in Quantum Gravity", special issue of Entropy, R. Garattini editor

Black brane entropy and hydrodynamics: the boost-invariant case

The framework of slowly evolving horizons is generalized to the case of black branes in asymptotically anti-de Sitter spaces in arbitrary dimensions. The results are used to analyze the behavior of both event and apparent horizons in the gravity dual to boost-invariant flow. These considerations are motivated by the fact that at second order in the gradient expansion the hydrodynamic entropy current in the dual Yang-Mills theory appears to contain an ambiguity. This ambiguity, in the case of boost-invariant flow, is linked with a similar freedom on the gravity side. This leads to a phenomenological definition of the entropy of black branes. Some insights on fluid/gravity duality and the definition of entropy in a time-dependent setting are elucidated.Comment: RevTeX, 42 pages, 4 figure