Nonparametric analysis of correlations in the binary black hole population with LIGO-Virgo-KAGRA data

Formation channels of merging compact binaries imprint themselves on the distributions and correlations of their source parameters, but current understanding of this population is hindered by simplified parametric models. We overcome such limitations using PixelPop [J. Heinzel, M. Mould, S. \'Alvarez-L\'opez, and S. Vitale, companion paper, Phys. Rev. D 111, 063403 (2025)]---our Bayesian nonparametric multidimensional population model. We analyze data from the first three LIGO-Virgo-KAGRA observing runs and make high resolution, minimally modeled measurements of the pairwise distributions of binary black hole masses, redshifts, and spins. We find no evidence that the mass spectrum evolves over redshift and show that such measurements are fundamentally limited by the detector horizon. We find support for correlations of the spin distribution with binary mass ratio and redshift, but at reduced significance compared to overly constraining parametric models. Confident data-driven conclusions about population-level correlations with flexible models like PixelPop will require more informative gravitational-wave catalogs.