2026ApJ...999L..10S

The growing number of binary black hole mergers detected through gravitational waves offers unprecedented insight into their underlying population, yet their astrophysical formation channels remain unresolved. We present a new method to distinguish binary black hole formation channels using their spatial clustering at cosmological scales. Employing the cosmological hydrodynamic simulation Illustris, we trace the distribution of mergers across cosmic time and compare them with the underlying matter distribution associated with three candidate origins: isolated binary stellar evolution, binaries embedded in active galactic nucleus (AGN) disks, and primordial black holes within dark matter halos. Using mock catalogs for next-generation facilities such as Cosmic Explorer, we find that their sensitivities could enable differentiation of these formation pathways out to redshift z ∼ 5 within the first decade of observations. This approach provides a new framework to link gravitational-wave populations with the large-scale structure of the Universe. By treating black hole mergers as cosmological tracers, our results demonstrate how cross-correlations between gravitational-wave catalogs and the cosmic matter field can constrain the relative contribution of stellar, AGN, and primordial channels, offering a complementary probe to population inference studies. These findings underscore the emerging potential of gravitational-wave cosmology to reveal where and how black holes form and merge across cosmic history.