Inflation Free, Stringy Generation of Scale-Invariant Cosmological Fluctuations in D = 3 + 1 Dimensions

TL;DR

This paper presents a string gas cosmology model where thermal fluctuations during the Hagedorn era generate scale-invariant cosmological perturbations without requiring inflation, with results consistent with observed cosmic structures.

Contribution

It introduces a novel inflation-free mechanism for generating scale-invariant fluctuations via thermal string gas fluctuations in three dimensions.

Findings

Scalar power spectrum is scale-invariant for closed strings.

Open strings produce a red-tilted spectrum.

Fluctuation amplitude depends on the ratio of Planck and string lengths.

Abstract

We propose an alternative scenario to cosmic inflation for producing the initial seeds of cosmic structures. The cosmological fluctuations are generated by thermal fluctuation of the energy density of the ideal string gas in three compact spatial dimensions. Statistical mechanics of the strings reveals that scalar power spectrum of the cosmological fluctuations on cosmic scales is scale-invariant for closed strings and inclines towards red for open strings in three compact spatial dimensions. This generation of thermal fluctuations happens during the Hagedorn era of string gas cosmology and without invoking an inflationary epoch the perturbations enter the radiation-dominated era. The amplitude of the fluctuations is proportional to the ratio of the two length scales in the theory, i.e., the Planck length over the string length. Since modes with the shorter wavelengths exit the Hubble radius at the end of the Hagedorn phase at later times compare to the modes with long wavelengths, the scalar fluctuations gain mild tilt towards red.