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Accepted · first revision · record

Title: Why is the background louder than most binary population models predict?
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.
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.
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.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
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.
Posed since: 2023