Future Prospects in Gravitational Waves: From Testing Fundamental Physics to Instruments beyond LIGO

Citation

Seymour, Brian Christopher (2025) Future Prospects in Gravitational Waves: From Testing Fundamental Physics to Instruments beyond LIGO. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ask9-zk26. https://resolver.caltech.edu/CaltechTHESIS:06032025-041244872

Abstract

In this thesis, we study the prospects for gravitational wave astronomy in the future. We focus on a couple of areas for gravitation waves beyond LIGO: improving measurement techniques of cosmological parameters, developing new waveforms for environmental effects, probing fundamental physics in waveforms, and high frequency gravitational wave detectors.

In the first part of this thesis, we develop two methods to constrain cosmological parameters using gravitational-wave observations. The first approach employs the statistical dark siren method, where the observed distribution of binary black hole events---whose luminosity distances are directly measured---is matched against astrophysical population models. By analyzing the Fisher information in the event distribution, we derive the Cram\'er-Rao bounds to quantify both statistical uncertainties and potential biases arising from unmodeled features in the merger rate and mass distribution. The second approach leverages the benefits of multiband observations with decihertz detectors, which dramatically improve host galaxy identification by refining source localization. This enhanced capability benefits reduces systematic errors in the measurement of the Hubble constant and other cosmological parameters. Together, these methods pave new pathways for precision cosmography using gravitational waves.

In the second part of the thesis, we investigate gravitational-wave signatures arising from binary black holes merging in the vicinity of supermassive black holes (SMBHs). One study focuses on hierarchical triple systems where the orbital motion around an SMBH imprints striking modulations on the gravitational waveforms. In our work, gravitational lensing is highlighted as a pivotal effect---alongside Doppler shifts and de Sitter precession---that is crucial for breaking parameter degeneracies. A complementary analysis considers eccentric orbits, incorporating orbital pericenter precession alongside Doppler and precession effects to further refine parameter estimation. Together, these investigations demonstrate that dynamic lensing and orbital modulations can be leveraged to probe SMBH properties and their environments with unprecedented precision, underscoring the importance of incorporating these environmental effects into waveform models.

In the third work, we explore inspiral tests of general relativity by examining the phase evolution of gravitational-wave signals from coalescing binary systems. First, we test Giddings' non-violent non-locality proposal, which posits that quantum information is transferred via a non-local interaction that generates metric perturbations around black holes by creating an effective-one-body waveform. We show that this can be captured by parameterized tests of general relativity waveforms. In the second half, we assess the robustness of post-Newtonian coefficients against unmodeled deviations by introducing parameterized tests that exploit the inherent geometry of the waveform. We show that the tests of general relativity are intimately related to the geometry of the signal manifold and propose a new singular value decomposition method to search for deviations for testing the predictions of general relativity and probing potential modifications to gravitational dynamics.

In the fourth part of this thesis, we explore optimizing the GEO600 detector for high-frequency gravitational wave detection. Although GEO600 is less sensitive than LIGO in the conventional 50–400 Hz band, we demonstrate that by detuning the signal-recycling mirror its sensitivity can be enhanced at tens of kHz. Using simulations with Finesse 3.0, we show that the sensitive point can be effectively scanned across various frequencies by adjusting the detuning angle. This tuning enables GEO600 to better target monochromatic sources, such as boson clouds arising from superradiance, thereby opening a promising new window for high-frequency gravitational wave astronomy.

Thesis Files