Converting entropy to curvature perturbations after a cosmic bounce

TL;DR

This paper investigates how entropy perturbations in two-field bouncing cosmologies can grow during a non-singular bounce and convert into curvature perturbations, potentially explaining observed cosmic microwave background fluctuations.

Contribution

It demonstrates that entropy perturbations can be amplified during a bounce and converted into curvature perturbations, impacting cosmological models and observations.

Findings

Entropy perturbations grow significantly during the bounce phase.

Converted entropy perturbations can dominate CMB temperature fluctuations.

The mechanism relaxes constraints on ekpyrotic potentials and suppresses non-Gaussianity.

Abstract

We study two-field bouncing cosmologies in which primordial perturbations are created in either an ekpyrotic or a matter-dominated contraction phase. We use a non-singular ghost condensate bounce model to follow the perturbations through the bounce into the expanding phase of the universe. In contrast to the adiabatic perturbations, which on large scales are conserved across the bounce, entropy perturbations can grow significantly during the bounce phase. If they are converted into adiabatic/curvature perturbations after the bounce, they typically form the dominant contribution to the observed temperature fluctuations in the microwave background, which can have several beneficial implications. For ekpyrotic models, this mechanism loosens the constraints on the amplitude of the ekpyrotic potential while naturally suppressing the intrinsic amount of non-Gaussianity. For matter bounce models, the mechanism amplifies the scalar perturbations compared to the associated primordial gravitational waves.