Black Hole Thermodynamic Ensembles, Euclidean Action and Legendre Transformation

TL;DR

This paper explores how Legendre transformations of the black hole on-shell action correspond to different boundary conditions and ensembles, extending thermodynamic concepts to gravitational theories with gauge fields and higher dimensions.

Contribution

It demonstrates the equivalence between Legendre transformations and boundary condition choices in black hole thermodynamics, including applications to five-dimensional supergravity with Chern-Simons terms.

Findings

Legendre transformation of the on-shell action relates to boundary conditions.

In four-dimensional dyonic black holes, the action depends on specific charge-potential pairs.

Extension to five-dimensional supergravity shows consistent thermodynamic formulation.

Abstract

In thermodynamics, a Legendre transformation of the free energy provides a mapping between different statistical ensembles. In this work, we demonstrate that performing a Legendre transformation of the black hole on-shell action is equivalent to imposing different boundary conditions on the fields. Consequently, the choice of ensemble must be consistent with, and cannot contradict, the imposed boundary conditions. From this perspective, it follows that for four-dimensional dyonic black holes, the on-shell action can only be expressed either as a function of the electric charge and the magnetic potential, or alternatively as a function of the magnetic charge and the electric potential. Inspired by the Legendre transformation of the Maxwell field, we argue that for purely gravitational theories whose metric geometries admit a \(U(1)\) fiber bundle structure, i.e.\ rotating, boosted, or Kaluza-Klein monopole configurations, one can similarly introduce appropriate Legendre terms, in the sense of dimensional reduction, to modify the thermodynamic ensemble of the black hole. Within the dimensional reduction framework, we study the on-shell action of black holes in five-dimensional minimal supergravity with a Chern-Simons term, analyze the corresponding Legendre transformation procedure, and show how the resulting formulation remains consistent with the Wald formalism.