Entropy of the holographic dark energy and generalized second law

TL;DR

This paper investigates the thermodynamics of holographic dark energy in a flat universe, confirming the generalized second law at the apparent horizon under various conditions and analyzing its partial validity at the event horizon.

Contribution

It demonstrates that the generalized second law holds at the apparent horizon regardless of the universe's acceleration, and explores thermodynamic behavior under equilibrium and non-equilibrium states.

Findings

GSL is valid at the apparent horizon for any dark energy form.

GSL is only partially satisfied at the event horizon.

Non-equilibrium analysis shows GSL holds if dark energy temperature exceeds dark matter temperature.

Abstract

In this paper we have considered the holographic dark energy and studied it's cosmology and thermodynamics. We have analysed the generalized second law (GSL) of thermodynamics in a flat universe consists of interacting dark energy and dark matter. We did the analysis both under thermal equilibrium and non-equilibrium conditions. If the apparent horizon is taken as the boundary of the universe, we have shown that the rate of change of the total entropy of the universe is proportional to $(1+q)^2,$ which in fact shows that the GSL is valid at the apparent horizon irrespective of the sign of $q,$ the deceleration parameter. Hence for any form of dark energy the apparent horizon can be considered as a perfect thermodynamic boundary of the universe. We made a confirmation of this conclusion by using the holographic dark energy. When event horizon is taken as the boundary, we found that the GSL is only partially satisfied. The analysis under non-equilibrium conditions revealed that the GSL is satisfied if the dark energy temperature is greater than the temperature of dark matter.