The universe hums like an orchestra: its notes are gravitational waves — ripples in spacetime predicted by Albert Einstein. The LIGO observatories, made exquisitely sensitive thanks to Rainer Weiss, strive to catch these whispers. Most often, we hear thunderous crashes — the mergers of ordinary black holes born from stars. But astrophysicists tried to pick out quiet, barely-there notes — signals from primordial black holes that emerged right after the Big Bang.
These tiny specks (some lighter than an asteroid) might actually be dark matter — the invisible glue holding galaxies together. The idea of such ghost holes was fleshed out by Stephen Hawking. Scientists choreographed computer dances of pairs of such holes and compared them with real data. The result was stunning: the orchestra played not a single new note. Almost all the chords we heard belong to ordinary black holes.
as written by the authors arXiv:2605.15749
We analyse primordial black hole (PBH) populations using state-of-the-art modelling of PBH binaries, deriving the strongest bounds on PBH abundance in the $$0.6$$-$$100\,M_\odot$$ range from LIGO-Virgo-KAGRA O4a data and demonstrating sensitivity in the $$10^{-4}$$-$$10^4\,M_\odot$$ range, for both monochromatic and log-normal mass functions. The constraints are dominated by resolvable PBH mergers, while the associated gravitational wave background provides complementary but weaker limits. To obtain limits that are agnostic about the astrophysical black hole (ABH) population, we devise two new methods, data-driven methods for statistical inference on scenarios in which PBHs account for a subset of the catalogued events. Allowing for this possibility relaxes slightly the bounds in the solar mass range $$2$$-$$20\, M_\odot$$. Our bounds are independent of the assumptions about the astrophysical black holes population and represent the most stringent constraints on the PBH abundance in the solar mass range to date.
If you squeezed Earth down to the size of a grape, it would become a black hole. Primordial black holes can be trillions of times lighter, yet their gravitational whisper is still detectable.
In science fiction, tiny black holes sometimes serve as fuel for starships — for instance, in Larry Niven’s Ringworld series.