# Cosmic acceleration in unimodular gravity

## TL;DR

This paper explores how unimodular gravity with non-conserved energy-momentum can naturally explain late cosmic acceleration without dark energy, linking radiation and the cosmological constant through modified equations and the jerk parameter.

## Contribution

It introduces a modified cosmological framework within unimodular gravity that accounts for late acceleration via non-conservation, connecting radiation and the cosmological constant.

## Key findings

- Derived modified Friedmann and acceleration equations.
- Linked the reionization epoch to the parameter $z_{ini}$ at 11.29.
- Proposed radiation as the source of acceleration, removing the need for dark energy.

## Abstract

We study unimodular gravity in the context of cosmology, particularly some interesting consequences that might be able to describe the background cosmology and the late cosmic acceleration. We focus our attention on the hypothesis of \textit{non conservation of the energy momentum tensor}. This characteristic has an interesting outcome: we can obtain a modified Friedmann equation along with the acceleration equation and also new fluid equations related to a third order derivative of the scale factor, known in cosmography as the jerk parameter. As a consequence of this theory, it seems that radiation and the cosmological constant are intimately related, in agreement with what some authors have called the third coincidence problem. Their connection is the parameter $z_{ini}$, which has a value of $11.29$ and coincide with the reionization epoch. As a result, we are able to explain the late acceleration as a natural consequence of the equations, associating the new fluid to radiation and, thus, eliminating the need for another component (i.e. dark energy). Finally, we interpret the results and discuss the pros and cons of using the cosmological constant under the hypothesis of non conservation of the energy momentum tensor in the unimodular gravity scenario.