Comparing the data with

The Hellings-Downs curve is a theoretical tool used to establish the telltale signature that a galactic-scale pulsar timing array has detected gravitational waves, typically of wavelengths . The method entails searching for spatial correlations of the timing residuals from pairs of pulsars and comparing the data with the Hellings-Downs curve. When the data fit exceeds the standard 5 sigma threshold, the pulsar timing array can declare detection of gravitational waves. More precisely, the Hellings-Downs curve is the expected correlations of the timing residuals from pairs of pulsars as a function of their angular separation on the sky as seen from Earth. This theoretical correlation function assumes Einstein's general relativity and a gravitational wave background that is isotropic.