Astrophysics in low-frequency gravitational waves

The NANOGrav pulsar timing array experiment has recently found evidence for low-frequency (nanohertz) gravitational waves (GWs). Binaries of supermassive black holes (SMBHs) are expected to produce GWs in this regime, but the existence of such sources has yet to be confirmed. The detection of GWs from SMBH binaries can provide independent constraints on black hole masses, constrain the dynamical co-evolution of SMBHs and galactic nuclei, and be used as probes of the cosmological evolution of the Universe. The current NANOGrav signal is consistent with a stochastic GW background produced by the ensemble of thousands or millions of SMBH binaries across the Universe. In this talk, I will discuss how features of the GW background encode detailed information about SMBH properties and SMBH binary evolution. I will also highlight prospects for detecting individual, loud SMBH binaries which are promising sources for identifying electromagnetic counterparts (as AGN/quasars), and opening low-frequency GWs to multi-messenger astrophysics.