Merging Galaxies & Supermassive Black Hole Binaries: Multi-Messenger Astrophysics with Pulsar Timing Arrays

Pulsar timing arrays (PTAs) are galactic-scale low-frequency (nHz - μHz) gravitational wave (GW) observatories, which aim to directly detect GWs from supermassive black hole (SMBH) binaries (≥ 10⁷ M_⊙). SMBH binaries are predicted products of galaxy mergers and are a crucial step in galaxy formation theories. The primary source of gravitational radiation in the nHz regime is expected to be a stochastic background formed from the cosmic population of SMBH binaries. In this talk, I will discuss the current state-of-the-art detection approaches to searching for a gravitational wave background in pulsar timing data, including a novel approach to account for errors in Solar System ephemerides, and present the results obtained by analyzing the newest data release from the North American Observatory for Gravitational Waves (NANOGrav). Our analysis shows a strong preference for a stochastic process with a common spectrum in all pulsars. However, the evidence is only slightly higher for a spatially correlated process. Spatial correlations are considered the "smoking gun" of gravitational wave background detection, thus the analysis of this data set remains inconclusive. Assuming the signal is indeed an astrophysical background, I will end my talk with a discussion of the expected growth in future data sets, as well as the astrophysics we expect to constrain in the coming years.