Abstract
Understanding gravity is at the heart of some of the biggest questions in mod- ern physics. While General Relativity (GR) is a theoretically unique and exper- imentally well-tested framework, it remains important to question whether it accurately describes gravity at all scales. This motivates the exploration of broader theories of gravity, particularly in regimes where deviations from GR might still arise. Black holes provide ideal natural laboratories for testing grav- itational theories in the strong-field regime, and the recent advent of gravi- tational wave (GW) astronomy has opened a new observational window into these extreme environments. In particular, the final stage of a compact binary merger—the ringdown phase—is of great interest. Here, under the so-called black hole spectroscopy program, the study of quasinormal modes (QNMs) offers a powerful tool to probe the fundamental nature of gravity and to extract in- trinsic properties of black holes.This thesis investigates black hole solutions and their QNM spectra within scalar-tensor (ST) theories of gravity—well-motivated extensions of GR that include an additional scalar degree of freedom. In particular, it focuses on ‘stealth’ black holes, where scalar hair exists without altering the background metric but can modify the QNM spectrum. By analysing perturbations around such spacetimes, we derive forecasted constraints on beyond-GR parameters for current and future GW detectors.
Three main investigations are presented: (i) a novel method to constrain the speed of gravity using ringdown signals alone; (ii) a stability and QNM analysis of black holes with linearly time-dependent scalar hair; and (iii) a general clas- sification of stealth solutions in higher-order scalar-tensor (HOST) theories, including a stability analysis and identification of ringdown observational sig- natures.
Together, these studies contribute new theoretical tools and observational fore- casts that advance our understanding on fundamental gravitational physics in the era of GW astronomy.
| Date of Award | 22 Jul 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Ian Harry (Supervisor), Johannes Noller (Supervisor) & Andrew Lundgren (Supervisor) |