Is the nanohertz background produced by supermassive black hole binaries?

The pulsar-timing common red process has characteristic strain $h_{c}(f) = A (f/f_{\mathrm{ref}})^{-2/3}$ with A of approximately $2.4x10^{-15}$ at $f_{\mathrm{ref}} = 1/\mathrm{yr}$ (NANOGrav 15-yr). Determine whether it is produced by a supermassive black hole binary population or by a cosmological source (first-order phase transition, cosmic strings, scalar-induced gravitational waves, domain walls). The answer is an identification supported by spectral shape, anisotropy, excess variance between frequency bins, and resolvable individual binaries.

Detection of anisotropy, non-Gaussian excess variance or individual continuous-wave sources in combined pulsar timing data (IPTA DR3 and successors), whose level is predicted for a finite binary population.