The Dark Side of the Gravitational Force: Lessons from Astrophysics on Gravity, Black Holes, and Dark Matter

Abstract

There remain several unanswered questions about gravity in our universe: Does general relativity accurately describe gravity? Do supermassive black holes ever merge? What is the dark matter? Can it be explained by an alternative theory of gravity? This dissertation develops several astrophysical tests that serve to further our understanding of gravity, black holes, and dark matter. The first section of the thesis develops two tests using gravitational waves: 1) using a recent gravitational wave event, GW170817, to place limits on extra spatial dimensions; 2) using a newly discovered supermassive black hole binary to place limits on the gravitational wave background and its implications for the `final parsec problem'. The second half of this thesis develops three tests of alternative theories for the dark matter: 1) self-interacting dark matter using galaxy warps; 2) a specific modified gravity theory, Verlinde's Emergent Gravity theory, using isolated dwarf galaxy kinematics; 3) general modified gravity theories using the cosmic microwave background polarization spectrum and the low-redshift galaxy correlation function. As we enter the next era of galaxy survey data and gravitational wave observations, these tests and others like them will hopefully bring us closer to answering these questions about gravity and our universe.