Black hole free energy landscapes from constrained instantons

Abstract

We apply gravitational path integral factorization to provide a semiclassical interpretation of generalized black hole free energy landscapes at fixed external temperature and angular velocities. Boundary integral transforms express the formal partition function as an integral over constrained sectors labeled by complete local partial-Neumann data on an auxiliary hypersurface. Independent variation establishes paired regular subregion solutions as constrained instantons, with Lagrange multipliers supporting lapse and tangential shift jumps that accommodate thermal and rotational mismatches. For selected nonextremal stationary families with an asymptotic boundary, the constrained saddle action takes the form $I_{\rm constr}=β\mathcal F_{\rm gen}=β(E_{\mathcal B}-Ω_{\mathcal B}^{I}J_{\mathcal B,I})-S$, with $β=T^{-1}$. Here $E_{\mathcal B}$ and $J_{\mathcal B,I}$ are renormalized Hamiltonian charges and $S$ is the black hole entropy, whose contribution to the action comes from the regular horizon cap. Interior stationary points of the landscape correspond to smooth equilibrium saddles when the retained variations independently span the entropy and angular momentum directions. We extend the construction to local, parity-even $f(\mathrm{Riemann})$ theories without curvature derivatives by retaining the complete auxiliary interface data. The same thermodynamic form follows with Wald entropy, without a small-deficit expansion. Schwarzschild and Kerr black holes illustrate the Einstein construction, while five-dimensional static AdS black holes in Einstein--Gauss--Bonnet gravity and the rotating BTZ branch of new massive gravity provide explicit checks of the higher-curvature extension.