The NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background

We report multiple lines of evidence for a stochastic signal that is correlated among 67 pulsars from the 15-year pulsar-timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. The correlations follow the Hellings–Downs pattern expected for an isotropic stochastic gravitational-wave background. The presence of such a gravitational-wave background with a power-law–spectrum is favored over a model with only independent pulsar noises with a Bayes factor in excess of 10^{14}, and this same model is favored over an uncorrelated common power-law–spectrum model with Bayes factors of 200–1000, depending on spectral modeling choices. We have built a statistical background distribution for these latter Bayes factors using a method that removes inter-pulsar correlations from our data set, finding p = 10^{−3} (approx. 3σ) for the observed Bayes factors in the null no-correlation scenario. A frequentist test statistic built directly as a weighted sum of inter-pulsar correlations yields p = 5×10^{−5}–1.9×10^{−4} (approx. 3.5–4σ). Assuming a fiducial f^{−2/3} characteristic strain spectrum, as appropriate for an ensemble of binary supermassive black-hole inspirals, the strain amplitude is 2.4+0.7−0.6 × 10^{−15} (median + 90% credible interval) at a reference frequency of 1 yr^{−1}. The inferred gravitational-wave background amplitude and spectrum are consistent with astrophysical expectations for a signal from a population of supermassive black-hole binaries, although more exotic cosmological and astrophysical sources cannot be excluded. The observation of Hellings–Downs correlations points to the gravitational-wave origin of this signal.