On the new string theory inspired mechanism of generation of cosmological perturbations

TL;DR

This paper critically examines a string theory-inspired non-inflationary model for generating cosmological perturbations, finding it produces a spectrum inconsistent with observations and thus not a viable alternative to inflation.

Contribution

The paper analyzes the proposed string gas cosmology mechanism and demonstrates it yields a blue spectrum with spectral index n=5, conflicting with empirical data.

Findings

The model produces a blue spectrum with spectral index n=5.

The predicted spectrum strongly disagrees with observational data.

The model, in its current form, is not a viable alternative to inflation.

Abstract

Recently a non-inflationary mechanism of generation of scale-free cosmological perturbations of metric was proposed by Brandenberger, Nayeri, and Vafa in the context of the string gas cosmology. We discuss various problems of their model and argue that the cosmological perturbations of metric produced in this model have blue spectrum with a spectral index n = 5, which strongly disagrees with observations. We conclude that this model in its present form is not a viable alternative to inflationary cosmology.