A Multi-messenger Exposé of the Biggest Black Hole Pairs in the Universe

At the center of nearly every galaxy in the Universe resides a supermassive black hole. When galaxies collide, their supermassive black holes sink to the center of the newly forming galaxy. There in this nascent galactic nucleus a supermassive black hole binary is formed. Supermassive binaries are the subject of a long-standing mystery in astrophysics: will these monstrous black holes merge and what can that tell us about the extreme environments that shape them? Their mutual evolution with galaxies? And the low-frequency gravitational wave sky? To solve this problem, we need to advance binary population models that are currently limited by uncertainties in environment-assisted binary orbital evolution, and we importantly need to couple model predictions with observational constraints on the supermassive black hole binary population using both electromagnetic and gravitational tracers. I will discuss work on both of these fronts, and how in a few years a wealth of data from both gravitational wave (Pulsar Timing Arrays and in the next decade LISA) and time-domain electromagnetic observations (e.g., with the Vera C. Rubin Observatory) could usher in a golden era of discovery for supermassive black hole binaries and their astrophysical environments.