Fundamental physics in extreme-gravity environments

Extreme gravity sourced by compact objects and their coalescences is a great avenue for testing Einstein’s general relativity and is deeply interconnected with fundamental physics. In this dissertation, we discuss three topics on the interplay between extreme gravity and fundamental physics driven by recent observational advancements. The first topic concerns making predictions about black holes in modified gravity. We prove that the spacetimes of isolated black holes in a broad class of modified gravity theories must be circular, justifying the long-existing use of a circular ansatz to simplify black hole solutions in these theories. We then analytically calculate the observables of the Blandford–Znajek process around a supermassive black hole in quadratic gravity. The calculation reveals a degeneracy between the black hole’s spin and the quadratic coupling, which hinders such an effect from constraining quadratic gravity unless the black hole is fast-spinning. The second topic concerns deciphering fundamental physics implications in gravitational-wave data from observations of compact binary coalescences. We search over the current gravitational-wave transient catalog for activated dipolar emission from massive scalar fields nonminimally coupled to gravity. Our Bayesian analysis suggests no evidence for these fields and places the most stringent upper-bound constraints on their coupling strengths. We then generalize the above search and combine it with the LIGO-Virgo-KAGRA parametrized inspiral tests of general relativity. This is achieved by extending the parametrized post-Einsteinian framework behind these tests using neural networks. We find that the resulting new framework leads to more theory-agnostic and more efficient tests of general relativity using gravitational waves. The third topic concerns improving gravitational-wave measurements of compact binary coalescences using domain knowledge from nuclear astrophysics. We show that modeling binary neutron star signals with binary Love relations breaks the distance-inclination degeneracy and improves the measurement of the neutron star masses. We forecast the decrease in the measurement error in the era of third-generation detectors, and we relax the assumptions behind our approach to prove the robustness of our forecasts.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Xie, Yiqi

Contributors dc:contributor

• Yunes, Nicolás
• Gammie, Charles F.
• Holder, Gilbert
• Narayan, Gautham

• Compact Objects
• Gravitational Waves
• Tests Of General Relativity
• Fundamental Physics
• Phenomenology
• Data Analysis

Statement dc:rights

• Copyright 2025 Yiqi Xie

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129843

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129843