Covariant formulation of non-equilibrium thermodynamics in General Relativity

We construct a generally-covariant formulation of non-equilibrium thermodynamics in General Relativity. We find covariant entropic forces arising from gradients of the entropy density, and a corresponding non-conservation of the energy momentum tensor in terms of these forces. We also provide a Hamiltonian formulation of General Relativity in the context of non-equilibrium phenomena and write the Raychaudhuri equations for a congruence of geodesics. We find that a fluid satisfying the strong energy condition could avoid collapse for a positive and sufficiently large entropic-force contribution. We then study the forces arising from the internal production of "bulk" entropy of hydrodynamical matter, as well as from the entropy gradients in the boundary terms of the action, like those associated with black hole horizons. Finally, we apply the covariant formulation of non equilibrium thermodynamics to the expanding universe and obtain the modified Friedmann equations, with an extra term corresponding to an entropic force satisfying the second law of thermodynamics. (C) 2021 The Author(s). Published by Elsevier B.V.