Perturbations in bouncing cosmologies: dynamical attractor vs scale invariance

TL;DR

This paper investigates bouncing cosmologies, demonstrating that density perturbations can be predicted independently of bounce details via a dynamical attractor, but resulting spectra are not scale invariant and incompatible with observations.

Contribution

It shows that dynamical attractors in bouncing cosmologies allow for predictions of density perturbations independent of bounce specifics, highlighting differences from inflationary models.

Findings

Perturbations can be evolved at non-linear level assuming bounce physics is stable.

The resulting spectrum is not scale invariant.

In synchronous gauge, perturbations remain small and attractor behavior is clear.

Abstract

For bouncing cosmologies such as the ekpyrotic/cyclic scenarios we show that it is possible to make predictions for density perturbations which are independent of the details of the bouncing phase. This can be achieved, as in inflationary cosmology, thanks to the existence of a dynamical attractor, which makes local observables equal to the unperturbed solution up to exponentially small terms. Assuming that the physics of the bounce is not extremely sensitive to these corrections, perturbations can be evolved even at non-linear level. The resulting spectrum is not scale invariant and thus incompatible with experimental data. This can be explicitly shown in synchronous gauge where, contrary to what happens in the commonly used Newtonian gauge, all perturbations remain small going towards the bounce and the existence of the attractor is manifest.