Aasim Jan Final Defense

Abstract: Gravitational-wave astronomy has opened a new window onto the Universe, enabling direct studies of compact binary mergers and providing new opportunities to probe astrophysics, cosmology, and strong-field gravity. Extracting reliable science from gravitational-wave observations requires waveform models with sufficient accuracy and physical completeness to support robust parameter estimation. In this dissertation, I investigate how inaccuracies and missing physics in current waveform models affect gravitational-wave inference and identify the origin of waveform-modeling systematics in gravitational-wave observations, with the broader goal of guiding waveform development for the era of precision gravitational-wave astronomy. I present detailed analyses of the two exceptionally massive binary black hole mergers observed by the LIGO--Virgo--KAGRA Collaboration, GW190521 and GW231123, both of which probe regimes where existing waveform models are expected to be less reliable and waveform-modeling inaccuracies become increasingly significant. For GW190521, I show that the event is consistent with a highly eccentric interpretation and demonstrate that previously reported discrepancies in inferred source properties can be explained by the presence of highly eccentric features that are not captured by standard quasicircular analyses. For GW231123, I show that differences in the inferred source parameters are driven primarily by waveform-modeling systematics associated with strong spin precession. I also present a detailed analysis of GW200105, the first confidently detected neutron star--black hole merger, and show that the signal is consistent with an eccentric interpretation, while previously reported discrepancies in the inferred source properties can be attributed to waveform-modeling systematics. In addition to these event analyses, I develop tools aimed at enabling robust inference for future gravitational-wave observatories. I introduce LISA-RIFT, a rapid Bayesian inference framework designed for preparatory studies of massive black hole binaries observed by the Laser Interferometer Space Antenna (LISA), systems expected to be both exceptionally loud and astrophysically rich, while placing unprecedented accuracy requirements on waveforms. Finally, I investigate the impact of numerical relativity waveform inaccuracies on gravitational-wave parameter estimation and establish numerical accuracy requirements for future, high sensitivity observatories such as Cosmic Explorer and LISA. Together, the results presented in this dissertation demonstrate that waveform systematics already impact the interpretation of current gravitational-wave observations and are expected to become increasingly important for future gravitational-wave detectors.