# A thermodynamics revision of Rastall gravity

## TL;DR

This paper examines the thermodynamic consistency of Rastall gravity in a flat FLRW universe, showing it behaves similarly to ΛCDM at late times but differs thermodynamically, especially with entropy and chemical potential considerations.

## Contribution

It provides a thermodynamic analysis of Rastall gravity, linking the Rastall parameter to cosmological behavior and exploring entropy and chemical potential effects.

## Key findings

- Rastall gravity's Hubble parameter resembles ΛCDM at late times.
- Entropy growth remains positive with convexity maintained.
- Thermodynamics remains consistent despite matter production and chemical potential effects.

## Abstract

In this work we study some aspects of the Rastall gravity, being the thermodynamics consistency of the model the core of this paper, for this purpose we will consider the dynamical equations of Rastall model in a flat FLRW geometry. Under a holographic description can be seen that this scenario for gravity contributes to the energy density of the fluid with an extra term that can be related to the deceleration parameter, providing a way to estimate the value of the Rastall parameter, termed as $\xi \lambda$, at present time. By adopting a specific Ansatz for the $\xi \lambda$ term it is possible to determine that the behaviour of the Hubble parameter in Rastall gravity has a similar aspect to the $\Lambda$CDM model at late times, but at thermodynamics level differs from the standard cosmology since the adiabatic behaviour for the entropy depends on the value of the parameter state, $\omega$. However, the entropy has a positive growth and simultaneously its convexity condition can be guaranteed; when other contributions are considered such as matter production and chemical potential, the adiabatic expansion can not be achieved, but the theory keeps its thermodynamics consistency. The chemical potential seems to have an interesting role since at effective level we could have a cosmological constant or phantom expansion in the model.