  Pulsar timing arrays (PTAs) are on the verge of detecting low-frequency
gravitational waves (GWs) from supermassive black hole binaries (SMBHBs). With
continued observations of a large sample of millisecond pulsars, PTAs will
reach this major milestone within the next decade. Already, SMBHB candidates
are being identified by electromagnetic surveys in ever-increasing numbers;
upcoming surveys will enhance our ability to detect and verify candidates, and
will be instrumental in identifying the host galaxies of GW sources.
Multi-messenger (GW and electromagnetic) observations of SMBHBs will
revolutionize our understanding of the co-evolution of SMBHs with their host
galaxies, the dynamical interactions between binaries and their galactic
environments, and the fundamental physics of accretion. Multi-messenger
observations can also make SMBHBs 'standard sirens' for cosmological distance
measurements out to $z\simeq0.5$. LIGO has already ushered in breakthrough
insights in our knowledge of black holes. The multi-messenger detection of
SMBHBs with PTAs will be a breakthrough in the years $2020-2030$ and beyond,
and prepare us for LISA to help complete our views of black hole demographics
and evolution at higher redshifts.

High Energy Astrophysical Phenomena