Gravitational Waves in String Cosmology

We study the production of cosmological backgrounds of gravitational waves in string theory models of the early universe. After a discussion of stochastic gravitational wave backgrounds, we describe the Cosmic Gravitational Wave Background (CGWB), its sensitivity to ultra-violet physics, and provide arguments supporting the claim that the peak amplitude cannot be made parametrically larger in field theory for a given reheating temperature. We then develop a Boltzmann equation approach to string thermodynamics and study a scenario where the early universe features a Hagedorn phase, with the energy density dominated by highly excited fun damental strings. We argue that the Hagedorn phase produces a gravitational wave background with a hierarchically larger amplitude and similar peak frequency to the CGWB. As an em bedding of the Hagedorn phase into a cosmologically viable setup, we describe brane-antibrane inflation and propose a mechanism that avoids the well-known η problem. Lastly, we study a complementary scenario where the byproduct of brane-antibrane inflation is a network of cosmic superstrings, where the background evolution induces variations of the string tension. We argue that the network dynamics features a scaling attractor and compute the associated gravitational wave spectrum. The spectrum can explain the signal measured by Pulsar Timing Arrays and a large class of models make concrete predictions for a spectral tilt in the LISA band. Our discussions provide further evidence that gravitational wave backgrounds (in particular at high frequencies) are a potential observational window into high-energy physics.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Villa, Gonzalo

Advisor dc:contributor.advisor

• Quevedo, Fernando

• Cosmology
• Gravitational Waves
• String Theory

Licence

• https://www.repository.cam.ac.uk/bitstreams/2c02226a-bfd7-4ac2-b5ea-91f93b5b8547/download
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.121427

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/389607