Non-Gaussianity excess problem in classical bouncing cosmologies

TL;DR

This paper investigates how classical bouncing cosmologies can generate excessive non-Gaussianities during the bounce, potentially invalidating perturbative approaches and posing challenges for model viability.

Contribution

It demonstrates that classical bounce models can produce large non-Gaussianities, highlighting a generic problem that must be addressed in bouncing cosmology scenarios.

Findings

Post-bounce $f_{NL}$ can exceed observational limits

Bounce acts as a source of non-Gaussianity

Non-Gaussianity production may depend only on bounce behavior

Abstract

The simplest possible classical model leading to a cosmological bounce is examined in the light of the non-Gaussianities it can generate. Concentrating solely on the transition between contraction and expansion, and assuming initially purely Gaussian perturbations at the end of the contracting phase, we find that the bounce acts as a source such that the resulting value for the post-bounce $f_{\mathrm{NL}}$ may largely exceed all current limits, to the point of potentially casting doubts on the validity of the perturbative expansion. We conjecture that if one can assume that the non-Gaussianity production depends only on the bouncing behavior of the scale factor and not on the specifics of the model examined, then many realistic models in which a nonsingular classical bounce takes place could exhibit a generic non-Gaussianity excess problem that would need to be addressed for each case.