Limits on the Statistical Description of Charged de Sitter Black Holes

TL;DR

This paper investigates the thermodynamics of charged de Sitter black holes, proposing a normalization based on a freely-falling observer to derive new thermodynamic laws and analyze heat capacity behavior.

Contribution

It introduces a normalization framework for de Sitter black hole thermodynamics that resolves existing ambiguities and examines the limits of statistical descriptions in extremal regimes.

Findings

Heat capacity remains finite near-extremal Nariai limit.

Heat capacity vanishes in the cold limit, indicating limitations.

Fundamental statistical description limitations persist in ultracold regimes.

Abstract

The thermodynamics of de Sitter black holes is complicated by the presence of two horizons and the absence of a globally defined timelike Killing vector. The standard choice of the Gibbons-Hawking Killing vector is at odds with the interpretation of the surface gravity as an acceleration measured by a physical observer at rest. Focusing on four-dimensional Reissner-Nordstr\"om de Sitter black holes we show that this issue can be resolved by adopting a normalization originally proposed by Bousso and Hawking, which defines thermodynamic quantities relative to the unique freely-falling observer at a fixed radial coordinate. Within this framework, we derive new first laws for the black hole and cosmological horizon and re-examine the black hole's heat capacity. We find that the heat capacity remains finite in the near-extremal Nariai limit, thus averting a breakdown of the semi-classical thermodynamic description. However, the heat capacity does vanish in the cold limit, as expected, and for Nariai black holes in the ultracold limit, indicating that fundamental limitations on the statistical description persist in these regimes. We discuss the implications of our results for log-$T$ corrections to near-extremal de Sitter black holes.