Running of the Spectrum of Cosmological Perturbations in String Gas Cosmology

TL;DR

This paper calculates the running of cosmological perturbation spectra in String Gas Cosmology, revealing it shares the same sign as inflation but is parametrically larger, proportional to (1 - n_s).

Contribution

It introduces a method to compute the spectrum running in String Gas Cosmology, highlighting differences from inflationary models.

Findings

The running has the same sign as in inflation models.

Its magnitude is proportional to (1 - n_s).

It is parametrically larger than in inflationary cosmology.

Abstract

We compute the running of the spectrum of cosmological perturbations in String Gas Cosmology, making use of a smooth parametrization of the transition between the early Hagedorn phase and the later radiation phase. We find that the running has the same sign as in simple models of single scalar field inflation. Its magnitude is proportional to $(1 - n_s)$ ($n_s$ being the slope index of the spectrum), and it is thus parametrically larger than for inflationary cosmology, where it is proportional to $(1 - n_s)^2$.