Evolution of fluctuations during graceful exit in string cosmology Original Research Article Pages 41-46

We study the evolution of fluctuations in a universe dominated by a scalar field coupled to the Gauss–Bonnet term. During the graceful exit, we found non-negligible enhancements of both curvature perturbation and gravitational wave in the long wavelength limit, and we also found a short wavelength instability for steep background superinflation just after the completion of the graceful exit. This result for one possible graceful exit mechanism would provide a significant implication on the primordial spectrum from the string cosmology.