Gravitational Waves from the Hagedorn Phase

TL;DR

This paper investigates the gravitational wave spectrum produced during a Hagedorn phase in the early universe, dominated by excited string states, using a thermodynamic approach and toy models to compare with standard predictions.

Contribution

It introduces an analytic framework for computing gravitational waves from a string-dominated early universe phase, highlighting the dominance of stringy signals over field theory predictions.

Findings

String-dominated phase produces larger gravitational wave signals than standard models.

Long, open strings are the primary source of gravitational waves during the Hagedorn phase.

The model provides insights into reheating and out-of-equilibrium dynamics in string cosmology.

Abstract

We illustrate the main points behind the computation of the gravitational wave spectrum originated from a phase in the early universe where the energy density is dominated by highly excited fundamental string degrees of freedom - a Hagedorn phase. This phase is described using the Boltzmann equation approach to string thermodynamics, which we illustrate via a toy model which permits analytic computations for the evolution of perturbations. This window into the out-of-equilibrium regime allows us to compute equilibration rates and conclude that, in realistic scenarios, long, open strings dominate the ensemble, source gravitational waves and provide a succesful reheating. Furthermore, we compare the results against the Standard Model prediction for a gravitational wave background of thermal origin and conclude that the string prediction is larger. This is one of the few cases in which a signal of stringy origin dominates over the field theory analogue. This work is based on arXiv:2310.11494 and arXiv:2408.13803 and is a contribution to the Proceedings of the 29th symposium on Particles, Strings and Cosmology (PASCOS 2024).