Different aspects of entropic cosmology

TL;DR

This paper reviews entropic cosmology based on thermodynamic laws at the apparent horizon, exploring how entropy relates to gravity theories and discussing implications for cosmic inflation, bounce, and primordial gravitational waves.

Contribution

It provides a comprehensive review of entropic cosmology, linking thermodynamic laws to gravity theories and discussing implications for early universe phenomena.

Findings

First law relates horizon entropy to gravity theories.

Second law constrains entropic parameters independently of models.

Discussion on entropy's role in inflation and primordial gravitational waves.

Abstract

We give a short review of the recent developments of entropic cosmology based on two thermodynamic laws of the apparent horizon, namely the first and the second laws of thermodynamics. The first law essentially provides the change of the entropy of the apparent horizon during the cosmic evolution of the universe, in particular, it is given by: $TdS = -d(\rho V) + W dV$ (where $W$ is the work density and other quantities have their usual meaning). In this way, the first law actually links various theories of gravity with the entropy of the apparent horizon. This leads to a natural question -- ``what will be the form of the horizon entropy corresponding to a general modified theory of gravity?'' The second law of horizon thermodynamics states that the change of total entropy (the sum of horizon entropy $+$ matter fields' entropy) with respect to cosmic time must be positive, where the matter fields behave like an open system characterised by a non-zero chemical potential. The second law of horizon thermodynamics importantly provides model-independent constraints on entropic parameters. Finally, we discuss the standpoint of entropic cosmology on inflation (or bounce), reheating and primordial gravitational waves from the perspective of generalised entropy function.