Density Perturbations in the Ekpyrotic Scenario

TL;DR

This paper analyzes the generation and matching of density perturbations in the ekpyrotic universe model, including gravitational effects, and finds a nearly scale-invariant spectrum with small amplitude, consistent with observations.

Contribution

It provides a detailed method for matching perturbations across the bounce in ekpyrotic models, including gravitational backreaction effects, and confirms the generation of a nearly scale-invariant spectrum.

Findings

Perturbation spectrum is nearly scale-invariant.

Perturbation amplitude is small, matching observations.

Includes gravitational backreaction in the analysis.

Abstract

We study the generation of density perturbations in the ekpyrotic scenario for the early universe, including gravitational backreaction. We expose interesting subtleties that apply to both inflationary and ekpyrotic models. Our analysis includes a detailed proposal of how the perturbations generated in a contracting phase may be matched across a `bounce' to those in an expanding hot big bang phase. For the physical conditions relevant to the ekpyrotic scenario, we re-obtain our earlier result of a nearly scale-invariant spectrum of energy density perturbations. We find that the perturbation amplitude is typically small, as desired to match observation.