Natural validation of the second law of thermodynamics in cosmology

Abstract

The present work shows that the second law of thermodynamics gets naturally satisfied during the entire cosmic evolution of the universe starting from inflation to the late dark energy era, without imposing any exotic condition. This makes the inter-connection between cosmology and thermodynamics more concrete. Consequently, it also depicts that why the matter fields are not in thermal equilibrium with the apparent horizon during most of the cosmic era of the universe, except for the fluids with $\omega = -1/3$ leading to the transitions of the universe from an accelerating to a decelerating era and vice-versa.