Thermodynamics in Modified Gravity Theories

Abstract

We demonstrate that there does exist an equilibrium description of thermodynamics on the apparent horizon in the expanding cosmological background for a wide class of modified gravity theories with the Lagrangian density $f(R,\varphi ,X)$, where $R$ is the Ricci scalar and $X$ is the kinetic energy of a scalar field $\varphi$. This comes from a suitable definition of an energy momentum tensor of the "dark" component obeying the local energy conservation law in the Jordan frame. It is shown that the equilibrium description in terms of the horizon entropy $S$ is convenient because it takes into account the contribution of the horizon entropy $\hat{S}$ in non-equilibrium thermodynamics as well as an entropy production term.

8 pages, no figure, revised version, to be published in the Proceedings of the "2nd International Workshop on Dark Matter, Dark Energy and Matter-Antimatter Asymmetry."

References in corpus (3)

• Friedmann Equations and Thermodynamics of Apparent Horizons
• The generalized second law of thermodynamics in generalized gravity theories
• F(R) gravity equation of state