Black Hole Entropy Beyond the Wald Term in Nonminimally Coupled Gravity: A Covariant Phase Space Decomposition

Abstract

We study the entropy of static, spherically symmetric black holes in diffeomorphism-invariant theories with nonminimal matter--curvature couplings, using the covariant phase space formalism. For regular bifurcate Killing horizons, the Iyer--Wald construction gives the standard Wald entropy. If a matter field cannot be smoothly extended to the regular bifurcation surface, however, the entropy-sector horizon surface charge variation can contain finite contributions that are not included in the Wald entropy density. In the representative obtained by directly varying the action, and after ordinary non-gravitational boundary terms have been separated into the work sector, we decompose the entropy entering the first law of black hole thermodynamics as \(\SH=\SW+\Sone+\DeltaS\). Here \(\SW\) is the Wald entropy, \(\Sone\) is the non-Wald part of the entropy-sector Noether charge, and \(\DeltaS\) is the remaining integrable part of the entropy-sector horizon surface charge variation. Applying this criterion to Kalb--Ramond, bumblebee, and extended Gauss--Bonnet black holes, we find that the regular Kalb--Ramond branch has \(\SH=\SW\), the bumblebee branches yield either \(\Sone=0\) with \(\DeltaS\neq0\) or a cancellation between \(\Sone\) and \(\DeltaS\), and the Weyl-vector extended Gauss--Bonnet examples require both corrections. This provides a direct test of whether the Wald density is sufficient or whether the full horizon surface charge variation is required.