Using gravitational waves to study neutron stars in general relativity and alternative theories of gravity

Abstract

LIGO's detection of gravitational waves emitted by a binary black hole merger in 2015 opened a new window into our universe. The era of multi-messenger astronomy began in 2017 with the detection of binary neutron star merger GW170817 and its electromagnetic counterpart GRB170817A. Gravitational wave observations have become a valuable tool for studying diverse areas of physics. Gravitational waves are particularly suited to tests of general relativity in the strong field regime and to studies of nuclear matter in extreme conditions. The work presented in this thesis uses gravitational wave data to contribute to our understanding of neutron stars, nuclear matter, and general relativity, explore the capabilities of current and future detectors, and provide a foundation for future studies of alternate theories of gravity.

Binary neutron star merger GW170817 offered new insights into nuclear physics, astrophysics, gravitational physics, and many other disciplines. The first study in this thesis combines the multi-messenger signals from GW170817 with state-of-the-art nuclear theory to place tight constraints on neutron star radii and tidal deformabilities, improving the radius measurement by a factor of 2. This study also constrains the nuclear equation of state and predicts that future neutron star-black hole mergers are unlikely to be disrupted and thus unlikely to have an electromagnetic counterparts.

The second study in this thesis builds upon the first and focuses on the capabilities of aLIGO-Virgo, LIGO A+, LIGO Voyager, the Einstein Telescope, and Cosmic Explorer to study neutron star-black hole mergers without electromagnetic counterparts. The results demonstrate that neither the present LIGO-Virgo detector network nor its near-term upgrades are likely to distinguish between neutron star-black hole and binary black hole mergers. Third-generation instruments such as Cosmic Explorer may be able to make the distinction given an event with favorable parameters. This result emphasizes the need for third-generation detectors.

The third study widens the scope of this thesis and analyzes data from all gravitational wave events detected to date to test for birefringence. Birefringence occurs when a wave's left- and right-handed polarizations propagate along different equations of motion. Bayesian inference was performed on the 4th-Open Gravitational-wave Catalog (4-OGC) using a parity-violating waveform. The vast majority of events show no deviation from general relativity, but the two most massive events (GW190521 and GW191109) support the birefringence hypothesis. Excluding these two events, the constraint on the parity-violating energy scale is an improvement over previous results by a factor of five. Future detections of massive binary black hole mergers will help shed light on the origin of this apparent birefringence.

The final study in this thesis provides a foundation that will improve future tests of general relativity using neutron stars. Any gravitational waveform for a system with a neutron star must include tidal effects. Even though tidal deformabilities in alternative theories can differ from their general relativistic counterparts, tests of general relativity seldom take this into account. The tidal deformabilities for static neutron stars in scalar-tensor theories with a focus on spontaneous scalarization are derived. The results demonstrate that tidal deformabilities can differ significantly between theories. Future analyses can apply these results alone or combine them with the parameter estimation methods developed in the first part of this thesis for more accurate tests of scalar modes in gravitational waves from neutron star mergers.