From black holes to the Big Bang: astrophysics and cosmology with gravitational waves and their electromagnetic counterparts

The growing catalog of gravitational-wave signals from compact object mergers has allowed us to study the properties of black holes and neutron stars more precisely than ever before. Population-level studies of compact-objects mergers can reveal how these systems form and evolve. Multi-messenger observations of these events can shed light on the properties of their electromagnetic counterparts, such as short gamma-ray bursts and kilonovae. Finally, observations of the stochastic gravitational-wave background can constrain early-universe physics inaccessible with other means.

In my thesis, I demonstrate how we can leverage observations of gravitational waves and their electromagnetic counterparts to learn about astrophysics and cosmology. The first part focuses on methods for facilitating the detection of electromagnetic counterparts and the simultaneous analysis of gravitational-wave and electromagnetic data for mergers including a neutron star. I then transition to a detailed study of black hole spin, characterizing the measurability of spin in individual systems with current gravitational-wave detectors and presenting novel population-level analyses including a simultaneous search for the primordial stochastic background and the binary black hole foreground. Such analyses will be critical to the interpretation of the compact-object binaries and stochastic backgrounds accessible with present and future detectors.