Black Shell Thermodynamics

TL;DR

This paper explores the thermodynamic behavior of black shells, horizonless objects that mimic black holes, revealing complex phase structures influenced by a negative cosmological constant and electric charge.

Contribution

It introduces a detailed thermodynamic analysis of black shells, highlighting their phase transitions and potential implications for gauge/gravity duality.

Findings

Black shells exhibit a rich phase structure with a split Hawking-Page transition.

A black shell phase exists between thermal AdS and large AdS black holes.

No phase transitions occur above a critical electric charge.

Abstract

Black shells have been proposed as black hole mimickers, i.e. horizonless ultra-compact objects that replace black holes. In this paper, we assume the existence of black shells and consider their thermodynamic properties, but remain agnostic about their wider role in gravitational physics. An ambient negative cosmological constant is introduced in order to have a well-defined canonical ensemble, leading to a rich phase structure. In particular, the Hawking-Page transition between thermal AdS vacuum and large AdS black holes is split in two, with an intermediate black shell phase, which may play a role in gauge/gravity duality at finite volume. Similarly, for non-vanishing electric charge below a critical value, a black shell phase separates two black hole phases at low and high temperatures. Above the critical charge, there are no phase transitions and large AdS black holes always have the lowest free energy.