Generating Ekpyrotic Curvature Perturbations Before the Big Bang

TL;DR

This paper presents a mechanism within 4d effective field theory for generating nearly scale-invariant curvature perturbations during a contracting pre-big bang phase, compatible with cyclic and ekpyrotic models.

Contribution

It introduces a novel entropic perturbation conversion process in contracting cosmologies, expanding the theoretical framework for pre-big bang scenarios.

Findings

Achieves correct perturbation amplitude.

Predicts scalar spectral tilt n between 0.97 and 1.02.

Tilt range is slightly blue-shifted compared to inflation models.

Abstract

We analyze a general mechanism for producing a nearly scale-invariant spectrum of cosmological curvature perturbations during a contracting phase preceding a big bang, that can be entirely described using 4d effective field theory. The mechanism, based on first producing entropic perturbations and then converting them to curvature perturbations, can be naturally incorporated in cyclic and ekpyrotic models in which the big bang is modelled as a brane collision, as well as other types of cosmological models with a pre-big bang phase. We show that the correct perturbation amplitude can be obtained and that the scalar spectral tilt n tends to range from slightly blue to red, with 0.97 < n < 1.02 for the simplest models, a range compatible with current observations but shifted by a few per cent towards the blue compared to the prediction of the simplest, large-field inflationary models.