On adiabatic perturbations in the ekpyrotic scenario

TL;DR

This paper critically analyzes Khoury & Steinhardt’s pre-ekpyrotic mechanism for producing adiabatic, nearly scale-invariant perturbations in a contracting universe. It shows that achieving the desired perturbation spectrum would require super-Planckian curvatures and ultra-short timescales, placing the evolution outside the domain of classical gravity. Moreover, the linear perturbation theory used to derive the perturbation spectrum breaks down because energy-density perturbations become large, and non-linear effects cannot be neglected for physically relevant parameter values. Consequently, the proposed mechanism does not provide a reliable, internally consistent prediction, highlighting unresolved issues with the background dynamics, the perturbative treatment, and the singularity problem in ekpyrotic scenarios.

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.