Supermassive binary black holes and current status of their multimessenger observations

Supermassive binary black holes (SMBBHs) are natural products of the hierarchical mergers of galaxies with central black holes in the Λ cold dark matter cosmogony. We briefly introduce the formation and evolution processes of SMBBHs and population synthesis modeling of SMBBHs across cosmic time. Both the semi-analytical analysis and numerical simulations suggest that close SMBBHs are abundant in the Universe, with rich electromagnetic signatures and enormous gravitational wave radiation. However, observational evidence for their existence is still uncertain. We summarize the current status of the electromagnetic searches and observations of these binaries, focusing on their morphological signatures, continuum spectra, line properties, and periodic variations modulated by their orbital motions. We review pulsar timing array observations of nanohertz gravitational waves from these SMBBHs, including from gravitational wave signals from individual SMBBH sources and the stochastic background from the whole population of SMBBHs. Finally, we discuss the prospects of multimessenger studies for SMBBHs.