Non-Singular Bouncing Cosmology: Consistency of the Effective Description

TL;DR

This paper confirms that non-singular bouncing cosmologies are consistent as effective theories, demonstrating their viability through ghost condensation and analyzing perturbations to resolve big-bang singularities.

Contribution

It provides a detailed calculation confirming the consistency of ghost-condensate bounce models and explores their implications for cosmological perturbations and singularity resolution.

Findings

Ghost-condensate bounce remains trustworthy within the effective theory regime.

Perturbation modes are controlled and well-behaved during the bounce.

Healthy non-singular bouncing cosmologies can resolve big-bang singularities.

Abstract

We explicitly confirm that spatially flat non-singular bouncing cosmologies make sense as effective theories. The presence of a non-singular bounce in a spatially flat universe implies a temporary violation of the null energy condition, which can be achieved through a phase of ghost condensation. We calculate the scale of strong coupling and demonstrate that the ghost-condensate bounce remains trustworthy throughout, and that all perturbation modes within the regime of validity of the effective description remain under control. For this purpose we require the perturbed action up to third order in perturbations, which we calculate in both flat and co-moving gauge -- since these two gauges allow us to highlight different physical aspects. Our conclusion is that there exist healthy descriptions of non-singular bouncing cosmologies providing a viable resolution of the big-bang singularities in cosmological models. Our results also suggest a variant of ekpyrotic cosmology, in which entropy perturbations are generated during the contracting phase, but are only converted into curvature perturbations after the bounce.