Cosmic Bounces and Cyclic Universes

TL;DR

This paper reviews cyclic and bouncing cosmological models, especially ekpyrotic and cyclic models, highlighting their potential to address cosmological puzzles and generate primordial perturbations without inflation, with connections to string theory.

Contribution

It provides a comprehensive discussion of the theoretical foundations, observational predictions, and distinctions between singular and non-singular bounce models in the context of string theory.

Findings

Ekpyrotic and cyclic models can naturally fit into string theory.

These models can address standard cosmological puzzles without inflation.

Predictions for upcoming observations can test these models.

Abstract

Cosmological models involving a bounce from a contracting to an expanding universe can address the standard cosmological puzzles and generate "primordial" density perturbations without the need for inflation. Some such models, in particular the ekpyrotic and cyclic models that we focus on, fit rather naturally into string theory. We discuss a number of topics related to these models: the reasoning that leads to the ekpyrotic phase, the predictions for upcoming observations, the differences between singular and non-singular models of the bounce as well as the predictive and explanatory power offered by these models.