PhD Defence: Science with Extreme Mass Ratio Inspirals: Accuracy, Fundamentals, and Beyond Vacuum

PhD Candidate 

Hassan Khalvati

Abstract

This thesis is dedicated to advancing the science of extreme mass-ratio inspirals (EMRIs), a key class of sources for the future space-based gravitational wave observatory, the Laser Interferometer Space Antenna (LISA). EMRIs provide a unique opportunity to probe the spacetime geometry around supermassive black holes, test general relativity in the strong-field regime, and explore interactions with astrophysical and fundamental field environments. Due to the complexity and duration of EMRI signals, accurate and fast waveform models are essential for extracting their full scientific potential.

Chapter 1 introduces the foundational concepts of gravitational waves, LISA, and EMRIs, setting the stage for the results that follow and motivating the necessity of accurate modeling.

Chapter 2 presents the development of fast, fully relativistic EMRI waveforms for circular equatorial orbits in Kerr spacetime, as an extension to the FastEMRIWaveforms (FEW) framework. We demonstrate the importance of relativistic amplitudes for accurately capturing the signal and its scientific content. Additionally, we incorporate two beyond-vacuum effects into the waveforms—accretion disk migration torques and superradiant scalar clouds—modeled relativistically. Our results show that LISA could place meaningful constraints on the parameters of such environmental effects.

Chapter 3 addresses systematic errors that persist even in relativistic EMRI models with fast offline/online architectures. We identify two key sources of bias: inaccuracies in the radiation-reaction flux data and interpolation errors during the transition from offline computations to online waveform generation. We quantify their impact on waveform fidelity and show how they propagate into parameter estimation. We also provide guidance on the sufficient accuracy levels needed for reliable LISA parameter inference.

Chapter 4 investigates whether EMRI gravitational-wave observations can distinguish exotic, non-Kerr compact objects from classical black holes. To this end, we developed a fully numerical ray-tracing code in vacuum, capable of handling arbitrary metrics without assuming spacetime symmetries. This allowed us to directly compare black hole shadow signatures with gravitational-wave phase shifts, demonstrating how EMRIs probe the dynamical strong-field regime and provide complementary, and often more precise, tests of the nature of dark massive bodies.

Examination Committee

• Dr. Robert Wickham, Chair
• Dr. Eric Poisson, Advisor
• Dr. Huan Yang, Advisor
• Dr. Luis Lehner, Associated Graduate Faculty
• Dr. Enrico Barausse, External Examiner (SISSA)