{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"17bbf4615b4dcec4f18961638797eb40aedcb2c79ec3d829d5b551ff1cbb138c","created":"2026-10-03T07:17:55Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"fadebbfb70c6c667de8b0829e381a288c47d36fea1a68ea4d3ff387b1366f783","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"astro.smbh-binaries.gwb-origin","kind":"well-posed","literature_status":"open","n":"1","parents":[],"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.","posed_since":"2023","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.","problem_ref":null,"references":"","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.","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.","title":"Is the nanohertz background produced by supermassive black hole 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