Stability of multi-field cosmological solutions

TL;DR

This paper analyzes the stability of multi-field cosmological solutions, showing that assisted inflation models are generally stable without slow-roll assumptions and that fields behave independently at linear order.

Contribution

It provides a stability analysis of assisted inflation multi-field solutions, including noninteracting and interacting fields, without relying on slow-roll approximation.

Findings

Noninteracting fields' stability condition is less restrictive than quintessence tracking.

Fields are unaware of each other's existence at linear order in homogeneous perturbations.

Results extend to interacting fields and high-energy braneworld scenarios.

Abstract

We explore the stability properties of multi-field solutions of assisted inflation type, where several fields collectively evolve to the same configuration. In the case of noninteracting fields, we show that the condition for such solutions to be stable is less restrictive than that required for tracking in quintessence models. Our results, which do not rely on the slow-roll approximation, further indicate that to linear order in homogeneous perturbations the fields are in fact unaware of each other's existence. We end by generalizing our results to some cases of interacting fields and to other background solutions and dynamics, including the high-energy braneworld.