Nearly scale-invariant curvature modes from entropy perturbations during the graceful exit phase

Abstract

In this paper we describe how a spectrum of entropic perturbations generated during a period of slow contraction can source a nearly scale-invariant spectrum of curvature perturbations on length scales larger than the Hubble radius during the transition from slow contraction to a classical nonsingular bounce (the "graceful exit"phase). The sourcing occurs naturally through higher-order scalar field kinetic terms common to classical (nonsingular) bounce mechanisms. We present a concrete example in which, by the end of the graceful exit phase, the initial entropic fluctuations have become negligible and the curvature fluctuations have a nearly scale-invariant spectrum with an amplitude consistent with observations.