Black hole enthalpy and an entropy inequality for the thermodynamic volume
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Open Access
- 20 July 2011
- journal article
- research article
- Published by American Physical Society (APS) in Physical Review D
- Vol. 84 (2)
- https://doi.org/10.1103/physrevd.84.024037
Abstract
In a theory where the cosmological constant or the gauge coupling constant arises as the vacuum expectation value, its variation should be included in the first law of thermodynamics for black holes. This becomes , where is now the enthalpy of the spacetime, and , the thermodynamic conjugate of , is proportional to an effective volume “inside the event horizon.” Here we calculate and for a wide variety of -dimensional charged rotating asymptotically anti-de Sitter (AdS) black hole spacetimes, using the first law or the Smarr relation. We compare our expressions with those obtained by implementing a suggestion of Kastor, Ray, and Traschen, involving Komar integrals and Killing potentials, which we construct from conformal Killing-Yano tensors. We conjecture that the volume and the horizon area satisfy the inequality , where is the volume of the unit () sphere, and we show that this is obeyed for a wide variety of black holes, and saturated for Schwarzschild-AdS. Intriguingly, this inequality is the “inverse” of the isoperimetric inequality for a volume in Euclidean () space bounded by a surface of area , for which . Our conjectured reverse isoperimetric inequality can be interpreted as the statement that the entropy inside a horizon of a given ”volume” is maximized for Schwarzschild-AdS. The thermodynamic definition of requires a cosmological constant (or gauge coupling constant). However, except in seven dimensions, a smooth limit exists where or goes to zero, providing a definition of even for asymptotically flat black holes. DOI: http://dx.doi.org/10.1103/PhysRevD.84.024037 © 2011 American Physical Society
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Funding Information
- U.S. Department of Energy
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