The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background
Using 15 years of pulsar timing data from a 67-pulsar array, the NANOGrav Collaboration reported evidence for a stochastic gravitational-wave background at nanohertz frequencies. The team detected the characteristic spatial cross-correlation (Hellings-Downs) signature expected for such a background across pulsar pairs, at a significance of roughly 3 to 4 sigma. The signal is consistent with a population of inspiraling supermassive black hole binaries, though exotic cosmological sources are not excluded.