Abstrakt
We study the thermodynamics of spherically symmetric, neutral and non-rotating black holes in conformal (Weyl) gravity. To this end, we apply different methods: (i) the evaluation of the specific heat; (ii) the study of the entropy concavity; (iii) the geometrical approach to thermodynamics known as thermodynamic geometry; (iv) the Poincare method that relates equilibrium and out-of-equilibrium thermodynamics.
We show that the thermodynamic geometry approach can be applied to conformal gravity too, because all the key thermodynamic variables are insensitive to Weyl scaling. The first two methods, (i) and (ii), indicate that the entropy of a de Sitter black hole is always in the interval 2/3 4/3 they are always unstable, hence there is no definite answer on whether such transition occurs.
Since thermodynamics geometry takes the view that the entropy is an extensive quantity, while the Poincare method does not require extensiveness, it is valuable to present here the analysis based on both approaches, and so we do.