Two ocean liners sail at night without lights. One was built in a shipyard from steel plates, the other seems to have grown by itself from foam and waves. By ear they are almost indistinguishable, but an experienced hydroacoustician will catch the difference in the hum of the propellers and the contours of the hull. That's roughly how astrophysicists look at black holes: some are born from dying stars, others from random clumps of matter in the very first moments after the Big Bang. Both types merge, sending tremors through the fabric of space, but their signatures are different.
Mergers of black holes leave ripples — gravitational waves. When many such events accumulate, their indistinct chorus blends into a continuous hum, known as the stochastic gravitational-wave background (stochastic means random, noise-like). The authors of the new study took the catalog of signals GWTC-4, collected by the LIGO detectors, and using Bayesian statistics (a method that estimates the probability of a hypothesis given new data) calculated what fraction of this chorus might belong to primordial black holes — those "foam-born" objects that emerged before the first stars. Then they modeled the background separately for stellar and primordial holes, accounted for the gravitational lensing effect — where a massive body in the wave's path bends it, like a magnifying glass changes the view of an ant — and overlaid the result on sensitivity curves of ground-based antennas.
A curious thing emerged. The current LIGO observatory can distinguish one type of hole from another, but only in a narrow frequency window and by a single method. Lensing shifts this window, thus complicating the task. The future Einstein Telescope (ET) — an underground triangle with ten-kilometer arms — will prove far sharper. It will be able to separate the populations over a much larger range of parameters, without relying on a single technique. It's like comparing tuning a radio during a thunderstorm versus on a clear day: the first catches one station, the second hears the entire airwaves.
Thus humanity's gravitational hearing is becoming ever more acute. If we learn to confidently separate the chorus of stellar and primordial holes, we will be able to eavesdrop on the history of those times when not a single star yet burned in the Universe.
A primordial black hole with the mass of a small asteroid could be the size of an atomic nucleus and pass straight through Earth without causing noticeable harm — only a microscopic trace.