Thermodynamics of the Gravity from Entropy theory

Abstract

The Gravity from Entropy (GfE) action posits that gravity that is fundamentally given by the information encoded in the interplay between matter and geometry. The GfE Lagrangian is given by the Geometric Quantum Relative Entropy (GQRE) between the physical metric and the metric induced by matter and curvature, leading to modified gravitational field equations with an emergent dynamical effective dark energy term, which reduce to Einstein's equations in the low energy, small curvature limit. Adopting a thermodynamic viewpoint, we identify the GfE energy density with this emergent effective dark energy term. For homogeneous and isotropic FRW spacetimes, we show that GfE universes admit a thermal description: locally, they are characterized by $k$-temperatures and $k$-pressures satisfying a first law of GfE thermodynamics. In the low energy, small curvature regime with perfect-fluid matter and radiation, GfE solutions are well approximated by Friedmann cosmologies. We show that, while the total GQRE per unit volume does not increase, consistent with its nature as a relative entropy, the total entropy of GfE universes is non-decreasing in time. These results provide a thermodynamic interpretation of GfE cosmologies and of general relativity (GR) itself, recovered in the low energy, small curvature limit of the theory, offering a framework to reconcile local order and complexity with the global increase of entropy in the universe.