Low-frequency Gravitational Waves from Massive Black Hole Binaries

The NANOGrav Pulsar Timing Array (PTA) detected a low-frequency (nanohertz) common process in our 12.5yr data that is consistent with expectations for a stochastic gravitational wave (GW) background. Such a background has long been predicted from the ensemble of millions of massive black hole (MBH) binaries distributed throughout the Universe. Our upcoming 15yr dataset provides far more leverage on the nature of this signal, allowing for unprecedented constraints on its possible origin. In this talk, I will discuss the wealth of information encoded within this GW cacophony, highlighting our group's new simulation frameworks to comprehensively model MBH populations and make astrophysical constraints. I will also present future prospects for complementing GW measurements from PTAs with electromagnetic counterparts from surveys of active galactic nuclei, and the unique insights that these low-frequency multi-messenger signals will unveil in the coming years. In addition to its own significant breakthroughs, I will mention how the nanohertz regime serves as a crucial testbed for the LISA mission, which will transform our understanding of MBHs both nearby and out to the early Universe.