{"id":{"repo_id":"cambridge","oai_identifier":"oai:www.repository.cam.ac.uk:1810/389607"},"canonical_url":"https://search.dev.ndltd.org/etd/cambridge/oai:www.repository.cam.ac.uk:1810/389607","repository":{"repo_id":"cambridge","name":"Cambridge University","base_url":"https://api.repository.cam.ac.uk/server/oai/request"},"display":{"title":"Gravitational Waves in String Cosmology","abstract":"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.","abstract_html":"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.","abstract_has_math":false,"creators":["Villa, Gonzalo"],"institution":"University of Cambridge","degree_name":"Doctor of Philosophy (PhD)","degree_level":"Doctoral","degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Quevedo, Fernando"],"committee_chairs":[],"committee_members":[],"year":2025,"date_issued":"2025-05-06","date_published":"2025-05-06","updated_at":"2026-07-22T22:24:17Z","subjects":["Cosmology","Gravitational Waves","String Theory"],"languages":["eng"],"rights":[],"rights_urls":["https://www.repository.cam.ac.uk/bitstreams/2c02226a-bfd7-4ac2-b5ea-91f93b5b8547/download","https://creativecommons.org/licenses/by/4.0/"],"identifier_entries":[]},"links":{"outbound_url":"https://doi.org/10.17863/CAM.121427","outbound_label":"DOI","outbound_source":"dc:identifier.doi"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Quevedo, Fernando"]},{"key":"dc:contributor.sponsor","label":"Sponsor","values":["STFC 2602852"]},{"key":"dc:creator","label":"Author","values":["Villa, Gonzalo"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.issued","label":"Date","values":["2025-05-06"]},{"key":"dc:publisher.institution","label":"Dc Publisher Institution","values":["University of Cambridge"]},{"key":"dc:relation.isreferencedby.uri","label":"Dc Relation Isreferencedby URI","values":["https://www.repository.cam.ac.uk/handle/1810/389607"]},{"key":"dc:type","label":"Dc Type","values":["Thesis"]},{"key":"dc:type.qualificationlevel","label":"Dc Type Qualificationlevel","values":["Doctoral"]},{"key":"dc:type.qualificationname","label":"Dc Type Qualificationname","values":["Doctor of Philosophy (PhD)"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Cosmology","Gravitational Waves","String Theory"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["eng"]},{"key":"dc:rights","label":"Dc Rights","values":["https://www.repository.cam.ac.uk/bitstreams/2c02226a-bfd7-4ac2-b5ea-91f93b5b8547/download","https://creativecommons.org/licenses/by/4.0/"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.doi","label":"DOI","values":["https://doi.org/10.17863/CAM.121427"]},{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://www.repository.cam.ac.uk/bitstreams/965d6819-fabd-4a13-ac2b-e4c2fd7f9279/download"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["We study the production of cosmological backgrounds of gravitational waves in string theory models of the early universe. 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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. 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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. 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