Dissipation in Non-equilibrium Spacetime Thermodynamics

It has been recently realized [1, 2] that the thermodynamical derivation of the Einstein equation introduced by Jacobson [3] needs a generalization to a non-equilibrium thermodynamical setting. Here we show that the non-equilibrium character of spacetime thermodynamics - both in GR as well as in f(R) - gravity is generally related to non-local heat fluxes associated with the purely gravitational/internal degrees of freedom of the theory. In particular, we show that the allowed gravitational degrees of freedom can be fixed by the kinematics of the local spacetime causal structure, through the specific Equivalence Principle formulation. In this sense, the thermodynamical description seems to go beyond Einstein's theory as an intrinsic property of gravitation.