Thermal Fluctuations and Bouncing Cosmologies

TL;DR

This paper investigates how thermal fluctuations in non-singular bouncing cosmologies can produce scale-invariant spectra of cosmological perturbations, analyzing different gas models and their non-Gaussianities.

Contribution

It identifies specific conditions and equations of state for various gases that lead to scale-invariant spectra in bouncing cosmologies, including thermal non-Gaussianity analysis.

Findings

Point particle gases require w=7/3 for scale invariance.

Gibbons-Hawking radiation yields scale invariance in matter-dominated backgrounds.

Non-Gaussianities are suppressed for point particle gases but large for Gibbons-Hawking radiation.

Abstract

We study the conditions under which thermal fluctuations generated in the contracting phase of a non-singular bouncing cosmology can lead to a scale-invariant spectrum of cosmological fluctuations at late times in the expanding phase. We consider point particle gases, holographic gases and string gases. In the models thus identified, we also study the thermal non-Gaussianities of the resulting distribution of inhomogeneities. For regular point particle radiation, we find that the background must have an equation of state $w = 7/3$ in order to obtain a scale-invariant spectrum, and that the non-Gaussianities are suppressed on scales larger than the thermal wavelength. For Gibbons-Hawking radiation, we find that a matter-dominated background yields scale-invariance, and that the non-Gaussianities are large. String gases are also briefly considered.