The study of supermassive binary black holes (BBHs) is of great importance for various topics in physics and astrophysics, including detection of gravitational waves, formation and evolution of galaxies and supermassive black holes (SMBHs). Formation of BBHs in the galactic centers is an inevitable consequence of hierarchical mergers of galaxies in the standard LambdaCDM cosmology. BBHs may be abundant in active galactic nuclei (AGNs) and Quasars, which are believed to be triggered by galaxy mergers. However, direct observational evidence for the existence of BBHs in AGNs and Quasars remains elusive. I will review the orbital evolution of BBHs and the characteristic observational features of BBHs at different evolutionary stages and how these features can be used to identify BBHs, including a few methods that we proposed. I will also summarize previous and ongoing efforts in searching for BBHs and discuss how the current observations, specifically on the frequency of dual AGNs, can be used to constrain the evolution of BBHs and the triggering mechanism of nuclear activities.
Bio: Youjun Lu is a professor at National Astronomical Observatory of China. He graduated from Anhui University in 1992 and obtained Ph.D. in Astrophysics from University of Science and Technology of China (USTC) in 1997. His research interests include black hole physics, active galactic nuclei and QSOs, galactic dynamics, galaxy formation and evolution, and cosmology.