Why is the background louder than most binary population models predict?
In plain words
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 statement
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.
What would settle it
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 in the literature
Since 2023 the measured amplitude has been found at the upper end of, or above, most population-model predictions; no explanation is agreed.