Probing Astrophysics, Cosmology, and Nuclear Physics with Gravitational Waves from Black Holes and Neutron Stars

Abstract

Gravitational waves now serve as a powerful tool for studying physics of compact objects, including black holes and neutron stars. When two compact objects merge, they emit gravitational waves that encode information about their masses, spins, and orbital dynamics. Ground-based detectors capture these signals, allowing us not only to measure the properties of individual mergers but also to characterize the population properties of black holes and neutron stars. In this thesis, I present a collection of works using real and simulated gravitational wave observations of compact binary coalescences to study the physics of black holes and neutron stars, and the implications these observations have on our broader understanding of astrophysics and fundamental physics.

The first part of this thesis is background material reviewing some of the theory behind gravitational waves. The second part focuses on measuring the physical properties of a compact binary coalescence detected in gravitational wave data. This includes the methods and models used in parameter estimation and a presentation of the properties of detections in the fourth Gravitational Wave Transient Catalog (GWTC-4). The third part of this thesis turns to measuring and extracting astrophysical information from the population properties of compact binaries. This features the astrophysical distributions of binary black holes as inferred from GWTC-3 and GWTC-4. I also present studies measuring specific aspects of the binary black hole mass and spin distributions, and the implications these results have for understanding binary black hole formation channels and stellar astrophysics. This section additionally features applications of population inference to studies of large-scale structure and predictions for the gravitational wave stochastic background, as well as technical discussions of the methods and custom libraries used to implement population analyses and potential biases associated with commonly-used methods. The fourth part explores how properties of dense nuclear matter are encoded in observations of neutron stars. This section includes studies using our knowledge of the nuclear equation of state to classify low-mass compact binary mergers, and results from using gravitational waves and electromagnetic observations of neutron stars to measure the equation of state and neutron star population properties.