Euclidean actions and static black hole entropy in teleparallel theories

TL;DR

This paper demonstrates that black hole thermodynamics results, traditionally derived in Einstein-Hilbert gravity, can also be obtained in teleparallel theories using quasilocal relations, with some differences in nonmetricity cases.

Contribution

It shows how to derive black hole entropy in teleparallel theories through boundary terms, extending the Euclidean path integral approach to torsion and nonmetricity frameworks.

Findings

Boundary terms reproduce Bekenstein-Gibbons-Hawking results

Bulk integrals vanish in torsion theories but not in nonmetricity theories

Regularization issues in nonmetricity case lead to zero partition function

Abstract

It is well-known that the results by Bekenstein, Gibbons and Hawking on the thermodynamics of black holes can be reproduced quite simply in the Euclidean path integral approach to Quantum Gravity. The corresponding partition function is obtained semiclassically, ultimately requiring only the on-shell Einstein-Hilbert action with opportune asymptotic subtractions. We elaborate on the fact that the same expressions for the thermodynamical quantities can be obtained within teleparallel equivalent theories, based on either torsion or nonmetricity, by employing quasilocal relations. Notably, the bulk integrals of these theories do not vanish on-shell but rather result in boundary terms themselves. Asymptotic subtractions of the latter are able to cancel out the divergences, ultimately leading to Bekenstein-Gibbons-Hawking's results. As a non-trivial cross-check, we compute the bulk integrals directly without reference to the boundary terms. While the result agrees with the previous method for the torsion-based teleparallel theory, it differs for the nonmetricity theory. Specifically, upon regularizing the bulk integral using a fiducial reference frame, we find that the semiclassical partition function vanishes. To address this problem, we propose a simple prescription for Schwarzschild black holes, which involves keeping the nonmetric connection arbitrary and imposing thermal equilibrium. Generalizations of the results to more general modified gravity theories with antisymmetric degrees of freedom are also discussed.