Princeton University Thunch Talk

Pulsar timing array experiments around the world have reported evidence of a stochastic gravitational wave background (GWB) at nanohertz frequencies. This background is thought to be sourced by a cosmic population of supermassive black hole binaries (SMBHBs) — systems of two gravitationally bound supermassive black holes emitting gravitational waves as they co-orbit each other at sub-parsec separations. Quasars have long been associated with galaxy mergers, suggesting a link with SMBHBs. In this talk, I will show how we can constrain the mass, volume, and local number density of the SMBHB population using a multi-messenger, quasar-based SMBHB population model. I will then talk about how the discrete nature of SMBHBs can manifest as excursions from, and breaks in, the expected fGW-2/3 power-law behavior of the GWB strain spectrum. I will also discuss how we can use SMBHB population models to interpret the observed spectrum of the GWB. Finally, I will show how candidate binary quasars — identified via periodicities in their light curves — can be used to constrain the fraction of quasars hosting a SMBHB. I will also show how this compares to the predicted fraction of galaxies hosting a SMBHB.