Institute for Advanced Study / Princeton University Joint Astrophysics Colloquium

Supermassive black hole binaries (SMBHBs) should be common in galactic nuclei as a result of frequent galaxy mergers. Yet they remain undetectable, especially during the final stage of their evolution. At this stage, SMBHBs are expected to emit bright electromagnetic (EM) radiation and could be identified as quasars with periodic variability in time-domain surveys. I will describe systematic searches for quasar periodicity that have revealed promising candidates, as well as the challenges in confirming these candidates and prospects for discoveries with the Rubin Observatory. SMBHBs are also promising sources of low-frequency gravitational waves (GWs). In fact, Pulsar Timing Arrays recently found evidence for a GW background, likely produced by a population of unresolved SMBHBs. I will discussthis discovery and its implications for SMBHB evolution as well as the next expected milestone, which is the detection ofindividually resolved binaries.Finally, I will discuss recent work which combines electromagnetic and gravitational-wave data and aims to deliver the first multi-messenger detection of a SMBHB.