How many ways do black hole pairs actually form?

Analyzing all binary black hole mergers in LIGO/Virgo's fourth catalog (GWTC-4), the BBH-Genesis pipeline finds the data are best explained by two separate formation channels, with hints of a third tied to active galactic nuclei. The tell is in spin and mass-ratio patterns: black holes born together in isolated binaries look subtly different from those paired up in crowded galactic environments. More detections from next-generation detectors should settle whether that third channel is real.