Abstract: Every massive galaxy is thought to host a supermassive black hole at its center. We currently have an incomplete understanding of how black holes are seeded, how they grow, and how their growth affects their host galaxies. In this thesis, I present a theoretical study of supermassive black hole seeding and growth across cosmic time, using a combination of “N-body simulations” and “semi-analytic modeling” techniques. First, I simulate the fallback of stellar debris during the tidal disruption of a star by a massive black hole binary, which reveals observational signatures that can help us identify future sources of gravitational waves. Next, I develop a semi-analytic model for black hole growth across cosmic time and explore models for the fueling of black holes by their host galaxies. In the work that follows, I explore the impact of seeding in detail and study the unique electromagnetic and gravitational wave signatures of different black hole seeding models. I then apply this framework to test the hypothesis that early black hole growth is reflected in the fluctuations of the cosmic infra-red background by developing a formalism for the clustering of undetected populations. Finally, I turn to the cosmological Romulus simulations to study the connection between black holes and their galaxies in the most realistic environments currently computationally accessible. This work is constrained by the current multi-wavelength census of accreting black holes over cosmic time, and makes predictions for a variety of next-generation instruments including LSST, Lynx, and LISA.
