On adiabatic perturbations in the ekpyrotic scenario

TL;DR

This paper critically examines a proposed mechanism for generating adiabatic perturbations in a contracting universe, arguing that the methods used are not applicable for describing the evolution and perturbation generation in the ekpyrotic scenario.

Contribution

It demonstrates the inapplicability of previous methods for modeling perturbations in the ekpyrotic scenario, highlighting limitations in the proposed approach.

Findings

Previous methods are inapplicable for the scenario

Challenges in transmitting perturbations to the expanding phase

Issues with the rapid change in the equation of state

Abstract

In a recent paper arXiv:0910.2230, Khoury and Steinhardt proposed a way to generate adiabatic cosmological perturbations with a nearly flat spectrum in a contracting Universe. To produce these perturbations they used a regime in which the equation of state exponentially rapidly changed during a short time interval. Leaving aside the singularity problem and the difficult question about the possibility to transmit these perturbations from a contracting Universe to the expanding phase, we will show that the methods used in arXiv:0910.2230 are inapplicable for the description of the cosmological evolution and of the process of generation of perturbations in this scenario.