LIGO and Einstein Telescope can tell stellar black holes from primordial ones

Black holes formed in the early universe from density fluctuations leave a different imprint on the stochastic gravitational-wave background than those born from collapsing stars, mainly because primordial black holes merge at much higher redshifts. Using hierarchical Bayesian inference on GWTC-4 catalog data, the authors infer primordial black hole population parameters and compute both backgrounds separately, including the effect of gravitational lensing on signal amplitude. Comparing these predictions against LIGO and Einstein Telescope sensitivity curves, they find LIGO can discriminate between the two populations only in a narrow frequency window, while the Einstein Telescope does so across a substantially broader parameter space.