Spotting a compact object with a mass below one solar mass would be a sensation. Stellar evolution doesn't produce black holes lighter than the Chandrasekhar limit, and neutron stars in that mass range would demand exotic equations of state. But primordial black holes (PBHs), forged in the hellish cauldron of the early Universe, naturally fill this gap. The very concept of PBHs goes back to the work of Stephen Hawking in the early 1970s. That's what scientists were hunting for in data from the fourth observing run (O4a) of the LIGO–Virgo–KAGRA detectors, instruments built by the efforts of many, including Nobel laureate Rainer Weiss. No direct detections have been made yet, but the seven 'loudest' candidate triggers were made public. A new Bayesian analysis of these events brings us closer to answering the question: are signals from primordial black holes lurking in the detector noise, possibly making up dark matter?
The authors employed the fast Bayesian algorithm QuickCBC, which can clean glitches from the data and iteratively estimate the noise power spectral density directly on the segment being analyzed. For each trigger, they constructed posterior distributions for the masses (in the source frame), chirp masses in the detector frame, effective spins, and distances. Depending on the expected total mass of the system, the length of the analyzed data ranged from 32 to 512 seconds. They also performed single-detector analyses to check the consistency of the results.
Of the seven O4a triggers, five were amenable to Bayesian parameter estimation. For these, the recovered chirp masses are in good agreement with the initial estimates from the search pipelines. Three events – on 14 September 2023, 19 July 2023, and 14 October 2023 – if real, contain a component with a mass of about 0.22–0.27 M⊙, squarely in the subsolar range. Two other triggers look more like mergers of neutron stars with black holes. The analysis also covered three additional candidates from other studies, and for three of them, a subsolar nature of the components is also not ruled out. Single-detector checks agreed with the joint analysis, boosting confidence in the results.
If even one of these candidates is confirmed, it would be the first direct hint of primordial black holes – natural candidates for dark matter. Even without a formal detection, the fact that Bayesian estimates stubbornly point to masses below one solar mass reinvigorates theoretical models predicting a peak in the PBH mass spectrum near the solar mass due to the QCD phase transition. Moreover, the work exposes the practical difficulties of analyzing long signals and sets challenges to improve cleaning algorithms and parameter estimation.
As the detectors become more sensitive in future runs (O5), similar events could be detected with greater statistical significance. Developing coherent Bayesian analysis methods that can handle even longer signals and automatically deal with non-stationary noise will be key to teasing out faint 'whispers' from the data. Population studies will also be needed to distinguish PBHs from neutron stars via tidal effects.
The results impact cosmology (the nature of dark matter), compact-object astrophysics, and gravitational-wave astronomy, setting the direction for future searches for primordial black holes.
The next steps are to accumulate more data with upgraded detectors and to perform a joint analysis of many near-threshold triggers to look for population patterns.
This work is directly connected to the unsolved problem of the nature of dark matter and the question of the existence of primordial black holes, as well as the puzzle of early supermassive black hole formation, hinted at by JWST observations.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This study presents a Bayesian analysis of seven O4a triggers using QuickCBC, aimed at finding subsolar-mass components—potential primordial black holes. For five events, parameter estimates were obtained, and in three cases, the recovered mass of one component falls within the range of 0.22–0.27 M⊙. Additionally, three candidates from other studies were analyzed, for which a subsolar nature is also not ruled out.
Where its strength is
The fast Bayesian algorithm QuickCBC allowed for rapid estimation of parameters for long-duration signals, iteratively dealing with noise and glitches. The resulting posterior mass distributions for several triggers consistently point to subsolar values, providing concrete targets for further testing of the primordial black hole hypothesis.
Where it could go next
Worth trying
Why this may matter
Confirming a subsolar component in even one event would be the first direct evidence for the existence of primordial black holes, fundamentally changing our understanding of dark matter and the early Universe.
Nearby works in our archive
The closest work in the archive sits at 0.67 by meaning — the usual level for our corpus.
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'.