Bouncing Cosmologies: Progress and Problems

TL;DR

This paper reviews bouncing cosmologies as alternatives to inflation, discussing their motivations, how they generate fluctuations, and the challenges faced by different models in explaining early universe observations.

Contribution

It provides a comprehensive overview of the progress, motivations, and problems associated with bouncing cosmologies as early universe models.

Findings

Bouncing models can generate primordial fluctuations.

Various implementations face significant theoretical challenges.

Bouncing cosmologies remain a viable alternative to inflation.

Abstract

We review the status of bouncing cosmologies as alternatives to cosmological inflation for providing a description of the very early universe, and a source for the cosmological perturbations which are observed today. We focus on the motivation for considering bouncing cosmologies, the origin of fluctuations in these models, and the challenges which various implementations face.