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Title: Is the nanohertz background produced by supermassive black hole binaries?
Plain: 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: 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.
Settled by: 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.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: open
Status 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.
Posed since: 2023