Cosmological perturbations in bouncing cosmologies and the case of the Pre-Big Bang scenario

TL;DR

This paper reviews how cosmological perturbations evolve in bouncing cosmologies, especially the Pre-Big Bang scenario, highlighting their potential as alternatives to inflation despite challenges in generating scale-invariant spectra.

Contribution

It provides a comprehensive overview of perturbation evolution in bouncing models and discusses historical attempts to align their predictions with observations.

Findings

Bouncing cosmologies can potentially solve horizon and curvature problems.

Generating a nearly scale-invariant spectrum remains a challenge in these models.

Pre-Big Bang scenario is a viable alternative to inflation despite complexities.

Abstract

The Pre-Big Bang cosmology inspired generations of cosmologists in attempts to cure the initial Big Bang singularity using a fundamental length scale as proposed by String Theory. The existence of a phase of collapse/inflation with increasing curvature followed by a cosmic bounce has been proposed as an alternative to standard inflation in the solution of the horizon and curvature problems. However the generation of a nearly scale-invariant spectrum of perturbations is not an automatic prediction of such scenarios. In this paper I review some general statements about the evolution of perturbations in bouncing cosmologies and some historically significant attempts to reconcile the predicted spectra with the observations. Bouncing cosmologies and in particular the Pre-Big Bang scenario stand as viable, although more complicated, alternatives to inflation that may still help solve current theoretical and observational tensions.