{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"a8ece6c536494032f6b1ae5894504e1612ed55d49dff89ed6e60989a18c8e103","created":"2026-10-03T07:17:55Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"1a45a525cfe2ea41a382f3c8bf8b4e7f0eed49bebeba2f380de82a1434a3f687","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"astro.smbh-binaries.gwb-amplitude","kind":"phenomenon","literature_status":"open","n":"1","parents":[],"plain":"Models of how many massive black hole pairs exist predict a somewhat weaker background than is measured. Either there are more or heavier black holes than thought, or part of the signal comes from something else.","posed_since":"2023","precise":"Population models built from local black hole mass to bulge mass relations, galaxy stellar mass functions and galaxy merger rates typically predict $A$ below the measured value of approximately $2.4x10^{-15}$ at $f = 1/\\mathrm{yr}$. Determine whether the difference is due to a higher black hole mass function at $M_{\\mathrm{BH}} > 10^9\\,M_{\\mathrm{sun}}$, higher merger rates, shorter merger times, or a non-binary contribution, with a population model consistent with both the amplitude and independent electromagnetic constraints.","problem_ref":null,"references":"","settled_by":"A joint fit of the gravitational-wave amplitude and spectrum with independently measured black hole mass functions and galaxy pair fractions that either closes the gap or quantifies it at high significance.","status_note":"Since 2023 the measured amplitude has been found at the upper end of, or above, most population-model predictions; no explanation is agreed.","title":"Why is the background louder than most binary population models 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