Where Dead Stars Collide: Unveiling Compact Binary Formation Environments with Gravitational Waves

Gravitational waves (GWs) from colliding black holes and neutron stars are transforming our understanding of the Universe. The LIGO-Virgo-KAGRA Collaboration’s latest update reports 90 compact-binary coalescences (CBCs), and hundreds more are expected over the next six years. Yet despite their evident abundance, the process by which CBCs form is unknown. My research addresses the following questions: How and where do merging compact binaries form? What can GWs tell us about the environments in which compact binaries reside? CBCs may form in isolation, with two stars born into a binary evolving into a tightly-bound stellar remnant pair, or dynamically, with the remnants becoming bound inside densely-populated environments like globular star clusters (GCs) or active galactic nuclei (AGN). Isolated and dynamical CBCs can be distinguished through the shapes of their orbits: while isolated binaries have circular orbits at detection, dynamical formation can produce mergers with significant orbital eccentricities. Eccentricity can therefore reveal how and where a binary formed, but complexities in GW modelling have---until recently---prevented its measurement. From 2019-2022, I circumvented these complexities to make the first measurements of eccentricity in CBCs detected with GWs, revealing a potentially dynamically-formed contingent within the population. Our ability to resolve CBC formation environments is still hindered by a strong caveat: it is not yet possible to disentangle measurements of eccentricity from measurements of spin-induced orbital precession, as there are no waveform approximants that contain both effects. Through this Fellowship, I will tease apart measurements of eccentricity and spin-induced precession to discern CBC formation environments. I will perform large-scale simulation campaigns, use cutting-edge waveform approximants, and progress novel analysis techniques to break the degeneracy of these parameters. The current generation of GW detectors will yield ~1000 CBC detections before the end of this Fellowship, enabling CBC formation environments to be identified. To utilise CBC signals as engines for astrophysics in their own right, however, we need detectors capable of capturing millions of signals per year, or observing earlier stages of CBC lifetimes. In the 2030s, existing detectors will be upgraded to “A#” sensitivity, yielding more than 104 detections per year out to redshifts z~5. Future observatories like the Einstein Telescope will detect all CBCs in the Universe out to z ? 20. Such plentiful detections have the power to probe CBC environments with unsurpassed accuracy. In 2021, I found that observations of dynamically-formed CBCs with future detectors will provide unrivalled constraints on GC formation epochs. Due to launch in 2035, upcoming space-based detector LISA will observe stellar-mass binary black holes years before they merge, when their GW emission retains fingerprints of other nearby masses. In 2023 and 2024, I devised and demonstrated uniquely efficient methods for inferring the properties of masses that perturb stellar-mass binary black holes with future detectors. Through this Fellowship, I will deliver frameworks for illuminating CBC environments with GWs over the next decade. I will lay the groundwork for GWs as environmental indicators, incorporating simulations of GCs, AGN, and field triples, and refining my strategies for probing these scenarios with synthetic multimessenger studies. By the end of this Fellowship, I will have positioned myself with a unique stance as a GW astronomer driving efforts to probe CBC environments via GWs.