Sourcing curvature modes with entropy perturbations in non-singular bouncing cosmologies

TL;DR

This paper investigates how entropy perturbations can generate curvature modes during the transition from slow contraction to bounce in non-singular cosmologies, highlighting the role of kinetic energy in this process.

Contribution

It demonstrates that curvature modes are naturally sourced during the graceful exit in non-singular bouncing cosmologies, emphasizing the kinetic energy's role.

Findings

Curvature modes are generated during the transition to bounce.

Kinetic energy components drive the evolution towards the bounce.

Entropy perturbations source primordial curvature modes.

Abstract

The observed temperature fluctuations in the cosmic microwave background can be traced back to primordial curvature modes that are sourced by adiabatic and/or entropic matter perturbations. In this paper, we explore the entropic mechanism in the context of non-singular bouncing cosmologies. We show that curvature modes are naturally generated during `graceful exit,' i.e., when the smoothing slow contraction phase ends and the universe enters the bounce stage. Here, the key role is played by the kinetic energy components that come to dominate the energy density and drive the evolution towards the cosmological bounce.