Black holes and phase transitions in Conformal Gravity

Abstract

We explore the thermodynamic properties of static black holes in conformal gravities in arbitrary, even dimensions. In the spherically symmetric case, up to local Weyl rescalings, these spacetimes are static and unique, due to a Birkhoff's theorem that applies for all conformal gravities whose Lagrangians are complete contractions of $k$ Weyl tensors in dimension $D=2k$. The solutions can be chosen to be asymptotically locally (A)dS or flat, and the horizon can be extended to be a surface of arbitrary constant curvature, $γ$. We focus on the asymptotically AdS case, for which the cosmological constant at infinity is an integration constant, and compute the mass of the spacetime in terms of the diverse hair parameters which have a purely gravitational origin. We show that the first law of thermodynamics is satisfied, and study phase transitions in the canonical ensemble, showing the existence of zeroth-order black-hole/black-hole phase transitions for arbitrary dimension.