Constraints on supermassive black hole binaries from NANOGrav

Earlier this year NANOGrav, along with other pulsar timing arrays, announced strong evidence for a stochastic gravitational wave (GW) background at nanohertz frequencies. For decades, such a signal has been predicted from binaries of supermassive black holes (SMBHs). I will present NANOGrav's recent data and our interpretation of the signal as produced by SMBH binaries. I will show that these GWs encode a wealth of new information about SMBH formation and evolution. Now, the race is on for the next expected measurements: detection of anisotropy in the GW background, and individual loud binaries. These measurements would confirm the origin of the GWs, provide a crucial testbed for the future LISA mission, and open a new multi-messenger window into the Universe.