Evolution of curvature and anisotropy near a nonsingular bounce

TL;DR

This paper analyzes how curvature and anisotropy evolve during a nonsingular bounce in ekpyrotic cosmology, revealing growth of perturbations that challenge the bounce's stability and scale-invariance.

Contribution

It demonstrates that curvature and anisotropy grow exponentially during the bounce, undermining the nonsingular bounce scenario.

Findings

Curvature and anisotropy grow exponentially during the bounce.

Adiabatic curvature perturbation develops a blue spectrum.

Anisotropy becomes nonlinear, disrupting the bounce.

Abstract

We consider bouncing cosmologies in which an ekpyrotic contraction phase with w >> 1 is followed by a bouncing phase with w < -1 that violates the null energy condition. The bouncing phase, induced by ghost condensation, is designed to produce a classically nonsingular bounce at a finite value of the scale factor. We show that the initial curvature and anisotropy, though diluted during the ekpyrotic phase, grow back exponentially during the bouncing phase. Moreover, curvature perturbations and anisotropy are generated by quantum fluctuations during the ekpyrotic phase. In the bouncing phase, however, an adiabatic curvature perturbation grows to dominate and contributes a blue spectrum that spoils the scale-invariance. Meanwhile, a scalar shear perturbation grows nonlinear and creates an overwhelming anisotropy that disrupts the nonsingular bounce altogether.