Ekpyrotic Non-Gaussianity -- A Review

TL;DR

This review discusses ekpyrotic cosmological models that involve a contracting universe phase, highlighting their predictions of non-Gaussian density fluctuations and their testability with upcoming experiments.

Contribution

It provides a comprehensive overview of non-Gaussianity predictions in ekpyrotic models and their observational implications.

Findings

Some ekpyrotic models are nearly ruled out by current data.

Predicted non-Gaussianity levels are within reach of near-future experiments.

Ekpyrotic models can produce nearly scale-invariant spectra with significant non-Gaussian corrections.

Abstract

Ekpyrotic models and their cyclic extensions solve the standard cosmological flatness, horizon and homogeneity puzzles by postulating a slowly contracting phase of the universe prior to the big bang. This ekpyrotic phase also manages to produce a nearly scale-invariant spectrum of scalar density fluctuations, but, crucially, with significant non-gaussian corrections. In fact, some versions of ekpyrosis are on the borderline of being ruled out by observations, while, interestingly, the best-motivated models predict levels of non-gaussianity that will be measurable by near-future experiments. Here, we review these predictions in detail, and comment on their implications.