Non-adiabatic-like accelerated expansion of the late universe in entropic cosmology

TL;DR

This paper explores a non-adiabatic-like accelerated expansion in entropic cosmology, proposing a simple model that aligns with observations and offers a different perspective on the universe's entropy evolution compared to the standard ΛCDM model.

Contribution

It introduces a novel entropic cosmology model with non-adiabatic expansion, matching observational data and providing new insights into entropy evolution in the universe.

Findings

The simple model agrees with observed accelerated expansion.

Entropy increase is likely uniform in the simple model.

The model suggests the present time is not a special epoch.

Abstract

In `entropic cosmology', instead of a cosmological constant $\Lambda$, an extra driving term is added to the Friedmann equation and the acceleration equation, taking into account the entropy and the temperature on the horizon of the universe. By means of the modified Friedmann and acceleration equations, we examine a non-adiabatic-like accelerated expansion of the universe in entropic cosmology. In this study, we consider a homogeneous, isotropic, and spatially flat universe, focusing on the single-fluid (single-component) dominated universe at late-times. To examine the properties of the late universe, we solve the modified Friedmann and acceleration equations, neglecting high-order corrections for the early universe. We derive the continuity (conservation) equation from the first law of thermodynamics, assuming non-adiabatic expansion caused by the entropy and temperature on the horizon. Using the continuity equation, we formulate the generalized Friedmann and acceleration equations, and propose a simple model. Through the luminosity distance, it is demonstrated that the simple model agrees well with both the observed accelerated expansion of the universe and a fine-tuned standard $\Lambda$CDM (lambda cold dark matter) model. However, we find that the increase of the entropy for the simple model is likely uniform, while the increase of the entropy for the standard $\Lambda$CDM model tends to be gradually slow especially after the present time. In other words, the simple model predicts that the present time is not a special time, unlike for the prediction of the standard $\Lambda$CDM model.