Classically stable non-singular cosmological bounces

TL;DR

This paper demonstrates that within cubic Galileon theories, it is possible to construct non-singular cosmological bounces that are free of instabilities and singularities, challenging previous claims of their impossibility.

Contribution

The authors provide a method to explicitly build stable, non-singular bouncing cosmologies in cubic Galileon models, avoiding ghost and gradient instabilities.

Findings

Constructed explicit examples of stable bounces

Maintained sub-luminal sound speed during NEC violation

Challenged previous claims of unavoidable instabilities

Abstract

One of the fundamental questions of theoretical cosmology is whether the universe can undergo a non-singular bounce, i.e., smoothly transit from a period of contraction to a period of expansion through violation of the null energy condition (NEC) at energies well below the Planck scale and at finite values of the scale factor such that the entire evolution remains classical. A common claim has been that a non-singular bounce either leads to ghost or gradient instabilities or a cosmological singularity. In this letter, we examine cubic Galileon theories and present a procedure for explicitly constructing examples of a non-singular cosmological bounce without encountering any pathologies and maintaining a sub-luminal sound speed for co-moving curvature modes throughout the NEC violating phase. We also discuss the relation between our procedure and earlier work.