2026-09-21T07:38:20Zhttps://api.repository.cam.ac.uk/server/oai/request

oai:www.repository.cam.ac.uk:1810/3896072025-09-24T01:42:13Zcom_1810_205871com_1810_256064col_1810_206446

Gravitational Waves in String Cosmology https://doi.org/10.17863/CAM.121427 Villa, Gonzalo Quevedo, Fernando 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. University of Cambridge 2025-05-06 Thesis Doctoral Doctor of Philosophy (PhD) eng STFC 2602852 https://www.repository.cam.ac.uk/handle/1810/389607 https://www.repository.cam.ac.uk/bitstreams/965d6819-fabd-4a13-ac2b-e4c2fd7f9279/download 4f5e00e798a685f5e53e40cfda6631db https://www.repository.cam.ac.uk/bitstreams/2c02226a-bfd7-4ac2-b5ea-91f93b5b8547/download 87eda9de84448d1f82354d60eee3eb5f https://creativecommons.org/licenses/by/4.0/ Cosmology Gravitational Waves String Theory