Probing Fundamental Physics with Gravitational Waves

The explosive coalescence of two black holes 1.3 billion light years away has for the very first time allowed us to peer into the extreme gravity region of spacetime surrounding these events. With these maximally compact objects reaching speeds up to 60% the speed of light, collision events such as these create harsh spacetime environments where the fields are strong, non-linear, and highly dynamical -- a place yet un-probed in human history. On September 14, 2015, the iconic chirp signal from such an event was registered simultaneously by both of the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors -- by an unparalleled feat of modern engineering. Dubbed "GW150914", this gravitational wave event paved the way for an entirely new observing window into the universe, providing for the unique opportunity to probe fundamental physics from an entirely new viewpoint. Since this historic event, the LIGO/Virgo collaboration (LVC) has further identified ten additional gravitational wave signals in its first two observing runs, composed of a myriad of different events. Important among these new cataloged detections is GW170817, the first detection of gravitational waves from the merger of two neutron stars, giving way to new insight into the supranuclear physics resident within.

This thesis explores this new unique opportunity to harness the information encoded within gravitational waves in regards to their source whence they came, to probe fundamental physics from an entirely new perspective. Part A focuses on probing nuclear physics by way of the tidal information encoded within gravitational waves from binary neutron star mergers. By finding correlations between this tidal information and fundamental nuclear matter parameters, we find new constraints on the latter with both current and future gravitational wave observations. Finally, by making use of constraints on the nuclear matter equation of state from GW170817, we develop improved universal relations between neutron star observables, which assist in better parameter estimation for future observations.

Another enticing subject one might consider is the validity of Einstein's general relativity. While observationally confirmed in every spacetime region reachable over the last century, it has yet to be probed in extreme gravity environments, such as those outside binary black hole mergers. Part B focuses on testing general relativity from such events by way of the remnants of such spacetime encoded within the gravitational wave signal. By considering both parameterized tests and by testing the consistency between the inspiral and merger-ringdown signals, we find strong constraints on several alternative theories of gravity with both current and future observations, including the combination of multiple events and with the multi-band detections between both space-based and ground-based detectors. Finally, we devise a new general spacetime metric which is parameterized beyond the Kerr one that describes black holes in general relativity. We find corrections to several astrophysical phenomena in the new beyond-Kerr metric which could be observed with future observations by e.g. the Event Horizon Telescope.

This is a dissertation presented to the graduate faculty of the University of Virginia in candidacy for the degree of Doctor of Philosophy. All of the work presented within this dissertation has been published in Physical Review D, Classical and Quantum Gravity, Classical and Quantum Gravity Letters, and MDPI Proceedings.