For a whole year, LIGO, Virgo, and KAGRA listened to the trembling of spacetime. Thousands of mergers of black holes and neutron stars left clear signatures. But hidden among them was another signal — a barely audible whisper. That’s how astronomers hunt for collisions of objects lighter than the Sun. Stellar evolution doesn’t produce black holes below a certain mass, and neutron stars of that mass would require exotic physics. But primordial black holes, born in the hellish cauldron of the young Universe, naturally fall into this range. The idea of such objects was developed back in the 1970s by Stephen Hawking.
In the fourth observing run (O4a), data sifting yielded seven of the “loudest” candidate triggers. No direct detection yet — false alarms mask the signal. To dig deeper, the research team armed themselves with a fast Bayesian algorithm called QuickCBC. It iteratively cleans the data of glitches while refining noise properties and recovering masses, spins, and distances. For each trigger, the analysis used between 32 and 512 seconds of data — extraordinarily long signals for such lightweight systems.
The result is striking. Out of the seven triggers, five yielded reliable parameter estimates. Three events — from July 19, September 14, and October 14, 2023 — if real, contain a component with mass 0.22–0.27 solar masses. The other two more closely resemble a neutron star merging with a black hole. Additional checks against data from individual detectors confirm consistency. In other words, three “whispers” persistently point toward primordial black holes.
If even one candidate is confirmed, it would be the first direct evidence of primordial black holes. Even without a formal discovery, the estimates themselves breathe life into models that predict a peak in their masses right around solar mass — due to the QCD phase transition. The work also highlights challenges: long signals drown in noise, demanding ever more sophisticated cleaning methods. Future runs of LIGO with improved sensitivity will provide more statistics. Then coherent Bayesian analysis might extract not just a whisper, but a clear voice from the dawn of time.
The lightest candidate in the sample has a mass of about 0.22 solar masses – that's less than many known neutron stars, and it doesn't fit into standard astrophysics.
In science fiction, primordial black holes often appear as sources of colossal energy or portals to other universes – for instance, in Greg Egan's novel 'Schild's Ladder'.