Quantify the fractional contributions of binary black-hole formation channels

Determine the relative contributions of isolated binary evolution (including common-envelope evolution, chemically homogeneous evolution, and Population III stars) and dynamical formation (including triple interactions, von Zeipel–Kozai–Lidov oscillations, binary–binary and binary–single interactions in clusters) to the observed population of binary black holes detected by ground-based gravitational-wave observatories, using measurements such as masses, spins, eccentricity, and merger rates.

Background

The paper reviews multiple proposed formation channels for binary black holes, spanning isolated binary evolution paths and dynamical interactions in various stellar environments. It emphasizes that simultaneous operation of different channels complicates the attribution of observed events to specific origins.

Establishing the fraction arising from each channel is vital because identifying eccentricity, spins, and other properties can serve as tracers of formation mechanisms, thereby constraining astrophysical models and informing population studies.

References

It is unclear what fraction of BBHs arise from each formation channel.

— Evidence for eccentricity in the population of binary black holes observed by LIGO-Virgo-KAGRA  (2404.14286 - Gupte et al., 2024) in Introduction, Section 1

Given these large uncertainties in the different theoretical models for BBH formation, as well as the unknown relative importance of each mechanism, it appears natural to look for alternative probes able to test these hypotheses.

— Breaking binary formation mechanism degeneracies with gravitational wave clustering  (2609.09083 - Bellomo et al., 8 Sep 2026) in Section 3, immediately before Section 3.1

Though some channels seem more likely than others, for now the origin of binary black holes in their mid-thirties remains unsolved .

— The School on Gravity: from motion to commotion  (2610.13514 - Cardoso et al., 9 Oct 2026) in Chapter 1, Section 1.4.4, subsection “The $\sim$35$M$ feature”