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

TL;DR

This paper demonstrates how entropy perturbations during slow contraction can generate a nearly scale-invariant curvature spectrum during a non-singular bounce, aligning with observational data.

Contribution

It introduces a mechanism where entropic perturbations source curvature modes during a graceful exit, a novel approach in bounce cosmology models.

Findings

Nearly scale-invariant curvature spectrum achieved

Amplitude consistent with observations

Entropic fluctuations become negligible after transition

Abstract

In this Letter, 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 non-singular bounce (the `graceful exit' phase). The sourcing occurs naturally through higher-order scalar field kinetic terms common to classical (non-singular) 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.