Black hole interiors in the thermal view of scalar-tensor gravity

The thermal view of scalar-tensor gravity is an analogy with a dissipative fluid. The scalar degree of freedom excites gravity to a positive ``temperature'', while Einstein gravity is the ``zero-temperature'' equilibrium state. We extend this thermal analogy to the interior region near the singularity of spherical, vacuum, uncharged black holes by using the universality of the Kasner behaviour near spacelike singularities. The singularity is ``hot'', meaning that gravity diverges from general relativity. The discussion is then extended to black holes in the presence of perfect or imperfect fluids with constant equation of state -- the latter determines whether Einstein gravity is approached or not.