# Non-linear interactions in cosmologies with energy exchange

## TL;DR

This paper explores non-linear interactions between two fluids in cosmology, presenting new exact solutions and analyzing their implications for the universe's evolution, including addressing the flatness problem.

## Contribution

It introduces a realistic non-linear interaction term between fluids, extending previous models and providing new exact solutions in cosmological contexts.

## Key findings

- New exact solutions for scale factor in interacting fluid cosmologies
- Non-linear interaction term avoids zero interaction at zero Hubble parameter
- Energy exchange between radiation and scalar field can alleviate flatness problem

## Abstract

We investigate the case of two interacting fluids in homogeneous and isotropic cosmologies with a non-linear interaction term. The interaction term avoids the unrealistic form generally used in the literature, beginning with Tolman, in which the interaction is zero when the Hubble parameter vanishes. A variety of exact solutions for the scale factor are found and describe a range of new behaviors. We also extend the analysis of possible cosmological solutions with interacting fluids when curvature is taken into consideration. We use an example of energy exchange between radiation and scalar field to alleviate the flatness problem.