String Gas Cosmology after Planck

TL;DR

String Gas Cosmology offers an alternative to inflation, predicting specific spectral tilts for cosmological perturbations and gravitational waves, with current data supporting its consistency and future observations poised to distinguish it further.

Contribution

This paper reviews the status of String Gas Cosmology post-Planck data, highlighting its predictions and consistency with observations, and discusses future tests to differentiate it from inflation.

Findings

String gas cosmology predicts a nearly scale-invariant scalar spectrum with a slight red tilt.

It predicts a scale-invariant gravitational wave spectrum with a slight blue tilt.

The model's consistency relations align well with Planck data.

Abstract

We review the status of String Gas Cosmology after the 2015 Planck data release. String gas cosmology predicts an almost scale-invariant spectrum of cosmological perturbations with a slight red tilt, like the simplest inflationary models. It also predicts a scale-invariant spectrum of gravitational waves with a slight blue tilt, unlike inflationary models which predict a red tilt of the gravitational wave spectrum. String gas cosmology yields two consistency relations which determine the tensor to scalar ratio and the slope of the gravitational wave spectrum given the amplitude and tilt of the scalar spectrum. We show that these consistency relations are in good agreement with the Planck data. We discuss future observations which will be able to differentiate between the predictions of inflation and those of string gas cosmology.