The purpose of this talk is two-fold: (1) in the first part, I will discuss why thermodynamics is important in cosmology, and then, (2) I will show that the laws of thermodynamics (including the second law) become naturally satisfied during the entire cosmic evolution of the universe, from inflation to the late dark energy era, without imposing any exotic condition. The first part is based on how the matter fields inside the horizon exhibit a flux during the cosmic expansion of the universe. Regarding the thermodynamic laws in cosmology, they are individually shown for two different class of gravitational theories: (a) having one thermodynamic degree of freedom, and (b) having more than one thermodynamics degrees of freedom. These make the inter-connection between cosmology and thermodynamics more concrete. Consequently, it also shows 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 ω = −1/3, leading to the transitions of the universe from an accelerating to a decelerating era and vice versa. The effect of viscosity on thermodynamic laws will be discussed, as well. We will explicitly show that viscosity makes universe expansion more irreversible compared to the case in the absence of viscosity.

The following papers may be treated as reference for this talk.

REF-1: Phys.Rev.D 111 (2025) 4, 043544

REF-2: Phys.Rev.D 111 (2025) 8, 083540