Horizon thermodynamics in holographic cosmological models with a power-law term

Abstract

Thermodynamics on the horizon of a flat universe at late times is studied in holographic cosmological models that assume an associated entropy on the horizon. In such models, a $\Lambda (t)$ model similar to a time-varying $\Lambda (t)$ cosmology is favored because of the consistency of energy flows across the horizon. Based on this consistency, a $\Lambda (t)$ model with a power-law term proportional to $H^{\alpha }$ is formulated to systematically examine the evolution of the Bekenstein--Hawking entropy. Here, $H$ is the Hubble parameter and $\alpha$ is a free parameter whose value is a real number. The present model always satisfies the second law of thermodynamics on the horizon. In particular, the universe for $\alpha <2$ tends to approach thermodynamic equilibrium-like states. Consequently, when $\alpha <2$, the maximization of the entropy should be satisfied as well, at least in the last stage of the evolution of an expanding universe. A relaxation-like process before the last stage is also examined from a thermodynamics viewpoint.

Comments: Final version accepted for publication in PRD. Several paragraphs and references are added and revised. [15pages, 6 figures]