Mannheim--Kazanas Black Holes: Horizons, Temperatures and Thermodynamics

Abstract

Static black holes in conformal Weyl gravity differ from their Einstein counterparts in a simple but important way: the Mannheim--Kazanas geometry is encoded in a Bach-flat lapse function whose radial dependence is richer than that of Schwarzschild--de Sitter. We study the de Sitter branch of this solution in the Schwarzschild gauge, keeping the characteristic linear term together with the quadratic de Sitter term. In the branch continuously connected to the positive-mass Schwarzschild--de Sitter spacetime, a regular static region between a black-hole horizon and a cosmological horizon exists precisely when \[ -\frac{1}{3}<βγ<\frac{2}{3}, \qquad 0<κ<\frac{1+3βγ}{27β^{2}}. \] In this window the singularity at the origin is hidden, the two positive horizons obey $0<r_b<3β<r_c$, and the limiting endpoint is the Nariai geometry. The thermodynamic interpretation is subtler than the root structure: the two horizons generally have different temperatures, and the static patch has no asymptotic region that would select a unique normalization of time. We therefore compare the Killing, Bousso--Hawking normalized, Tolman local, and effective two-horizon temperature conventions, and we compute the corresponding Wald entropy for the pure Weyl-squared action. The result is a local horizon thermodynamic description that keeps the geometric facts separate from the ensemble-dependent choices needed for any global first law.