Density perturbations in the ekpyrotic Universe and string-inspired generalizations

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