Density perturbations in the Ekpyrotic Universe and string-inspired generalizations

TL;DR

This paper investigates density perturbations in string-inspired cosmological models, including the Ekpyrotic scenario and nonsingular string cosmologies, highlighting their tendency to produce blue spectra unlike inflation.

Contribution

It provides a detailed analysis of curvature perturbation spectra in string-inspired models, clarifying conditions for superhorizon enhancement and contrasting with inflationary predictions.

Findings

String-inspired models tend to produce blue spectra.

Superhorizon ${ m R}$ can be enhanced in certain parameter ranges.

Contrast with standard slow-roll inflationary spectra.

Abstract

We study density perturbations in several cosmological models motivated by string theory. The evolution and the spectra of curvature perturbations ${\cal R}$ are analyzed in the Ekpyrotic scenario and nonsingular string cosmologies. We find that these string-inspired models generally exhibit blue spectra in contrast to standard slow-roll inflationary scenarios. We also clarify the parameter range where ${\cal R}$ is enhanced on superhorizon scales.