The Gravitational Wave Background Per NANOGrav

By carefully observing ultra-precise pulsar timing data over fifteen years and correlating irregularities from different sources, they have observed a shared random background gravitational wave signal that follows a power law. The signal seen is what would be expected from the gravitational waves created by multiple supermassive binary black hole systems.

We report multiple lines of evidence for a stochastic signal that is correlated among 67 pulsars from the 15 yr pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. 

The correlations follow the Hellings–Downs pattern expected for a 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 the latter Bayes factors using a method that removes interpulsar correlations from our data set, finding p = 10^−3 (≈3σ) for the observed Bayes factors in the null no-correlation scenario. A frequentist test statistic built directly as a weighted sum of interpulsar correlations yields p = 5 × 10^−5 to 1.9 × 10^−4 (≈3.5σ–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 (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.

Gabriella Agazie, et al., "The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background" 951(1) The Astrophysical Journal Letters L8 (June 29, 2023) DOI 10.3847/2041-8213/acdac6