Is the nanohertz background produced by supermassive black hole binaries?
In plain words
The gravitational-wave background seen by pulsar timing arrays fits a population of merging supermassive black holes. It could also come from processes in the very early universe, and the two can be told apart only with more data.
Precise statement
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.
What would settle it
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.
Status in the literature
Unverified note
In 2023 individual arrays reported about $2-4.6\,\sigma$ evidence for the Hellings-Downs correlation (CPTA highest at $4.6\,\sigma$); no combined IPTA DR3 analysis had appeared on arXiv by September 2026.
See also
- Related Is there any confirmed sub-parsec supermassive black hole binary?
- Related Does the background spectrum flatten at the lowest frequencies?
- Related What is the amplitude of primordial gravitational waves?
- Related Is the primordial curvature spectrum enhanced on small scales?
- Related Why is the background louder than most binary population models predict?