Density perturbations in the Ekpyrotic Universe and string-inspired generalizations

Abstract

We study density perturbations in several cosmological models motivated by string theory. The evolution and the spectra of curvature perturbations $\mathcal{R}$ 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 $\mathcal{R}$ is enhanced on superhorizon scales.

Comments: 5 pages, 3 figures, some discussions and references are added