What produces the excess of merging black holes near 35 solar masses?
In plain words
Merging black holes pile up near 35 solar masses, more than a smooth distribution would give; this could mark the pile-up just below the pair-instability gap or a feature of one formation route.
Precise statement
Population analyses of LIGO-Virgo-KAGRA catalogs find a peak near $35\,M_{\mathrm{sun}}$ in the primary-mass distribution, below the $\sim 40\text{ to }50\,M_{\mathrm{sun}}$ where standard pulsational pair-instability models put the pile-up. Determine whether the peak is the pair-instability pile-up shifted by nuclear-rate or wind physics, a signature of a channel such as globular-cluster dynamics, or a core-collapse fallback feature. Answer: an identification that predicts the peak location, width and any redshift dependence.
What would settle it
Measurement of the peak location versus redshift and spin with a catalog of order 1e3 events, compared with channel predictions.