Pressure and volume in the first law of black hole thermodynamics

TL;DR

This paper explores the thermodynamic properties of black holes, interpreting mass as enthalpy with pressure from the cosmological constant, and analyzes how this affects black hole efficiency and phase transitions.

Contribution

It introduces a thermodynamic framework with pressure and volume for black holes, including a virial expansion and critical point analysis, extending the first law of black hole thermodynamics.

Findings

Derived a relation between black hole volume and pressure.

Identified a van der Waals-like critical point.

Showed increased efficiency of black holes with negative cosmological constant.

Abstract

The mass of a black hole is interpreted, in terms of thermodynamic potentials, as being the enthalpy, with the pressure given by the cosmological constant. The volume is then defined as being the Legendre transform of the pressure and the resulting relation between volume and pressure is explored in the case of positive pressure. A virial expansion is developed and a van der Waals like critical point determined. The first law of black hole thermodynamics includes a PdV term which modifies the maximal efficiency of a Penrose process. It is shown that, in four dimensional space-time with a negative cosmological constant an extremal charged rotating black hole can have an efficiency of up to 75%, while for an electrically neutral rotating back hole this figure is reduced to 52%, compared to the corresponding values of 50% and 29% respectively when the cosmological constant is zero.