Generalized Galileons: instabilities of bouncing and Genesis cosmologies and modified Genesis

TL;DR

This paper investigates generalized Galileon theories and finds that typical bouncing and Genesis cosmologies within these models are plagued by instabilities or singularities, but proposes a modified Genesis scenario that can be stable.

Contribution

The paper demonstrates that standard bouncing and Genesis models in generalized Galileon theories are inherently unstable or singular, and introduces a modified Genesis model that avoids these issues.

Findings

Bouncing models have gradient or ghost instabilities or singularities.

Original Genesis models are also unstable or singular.

A modified Genesis model can be stable and connect to kination.

Abstract

We study spatially flat bouncing cosmologies and models with the early-time Genesis epoch in a popular class of generalized Galileon theories. We ask whether there exist solutions of these types which are free of gradient and ghost instabilities. We find that irrespectively of the forms of the Lagrangian functions, the bouncing models either are plagued with these instabilities or have singularities. The same result holds for the original Genesis model and its variants in which the scale factor tends to a constant as $t\to -\infty$. The result remains valid in theories with additional matter that obeys the Null Energy Condition and interacts with the Galileon only gravitationally. We propose a modified Genesis model which evades our no-go argument and give an explicit example of healthy cosmology that connects the modified Genesis epoch with kination (the epoch still driven by the Galileon field, which is a conventional massless scalar field at that stage).