Searching for Supermassive Black Hole Binaries: Gas, Gravity, and Gravitational Waves

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? I will discuss progress in modeling accretion onto supermassive black hole binaries with focus on implications for binary orbital evolution and the prospects for finding them with upcoming time-domain surveys and gravitational wave observatories.