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 discuss this discovery and its implications for SMBHB evolution as well as the next expected milestone, which is the detection of individually 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.

Speaker: Maria Charisi (Washington State University)

The 2026 Spring Colloquium talks are held on Wednesdays at 3:00 PM. This colloquium is hosted by STScI and will be held as an in-person and virtual event.

You may join in person at STScI’s John N. Bahcall Auditorium or virtually on the STScI Research YouTube channel.

Please direct questions or comments to contact above. The 2026 Spring Colloquium members are: Nimisha Kumari (STScI), Elena Manjavacas (STScI), Jack Neustadt (JHU), Kevin Schlaufman (JHU), Adam Smercina (STScI), Ethan Vishniac (JHU).