Gravitational waves from black hole mergers have become a routine tool, but GW231123 stands out even among previous record holders. Its components, 137 and 103 solar masses, fall into the so-called "pair-instability gap": a range where ordinary stellar evolution should not leave black holes. Related papers on the archive actively discuss alternative pathways: hierarchical mergers in dense clusters, exotic channels in active galactic nuclei, even "ghost" black holes. This paper joins that conversation but bets on primordial black holes, objects born in the first instants after the Big Bang rather than from stars.
The authors do not invent new physics; they assemble a well-tested model of primordial black holes with accretion. They start with small initial masses (on the order of tens of solar masses) and small spins, then include growth from gas falling in from the surrounding environment. Accretion is described within a thin disk, where angular momentum is efficiently transferred to the black hole, spinning it up to the observed values. The key parameter is the cutoff redshift $z_{cuto}$ff, after which accretion is considered inefficient; for GW231123 they choose $z_{cuto}$ff = 24. The merger rate is calculated using standard formulas for early binary systems, corrected for mass growth and orbital evolution.
With $z_{cuto}$ff = 24, the initial masses of GW231123's components were 85.5 and 59.1 solar masses, noticeably smaller than observed. Accretion not only increased the masses but also spun up both black holes to spins of 0.90 and 0.80, consistent with LIGO-Virgo-KAGRA measurements. To explain the event rate, the fraction of primordial black holes in dark matter needs to be about $10^{-3}$, a value right at the edge of regions excluded by X-ray and relic observations. A forecast for the future: in the upcoming O5 run, about 20 similar events are expected, and the Einstein Telescope could see mergers at redshifts z > 15, which is impossible for stellar scenarios.
The work shows that GW231123 does not require exotic astrophysical mechanisms: the most "vanilla" primordial black hole scenario with accretion handles the masses and spins. This sets the paper apart from neighboring works that emphasize hierarchical mergers in clusters or active galactic nucleus disks. Here, for the first time for such a massive event, consistency with the predicted mass-spin correlation characteristic of accretion growth is demonstrated. If the interpretation is correct, primordial black holes could make up a significant fraction of dark matter in the tens of solar mass range.
The topic could develop in several directions. First, a more detailed Bayesian comparison of primordial and astrophysical scenarios, accounting for all uncertainties. Second, including radiation feedback during accretion, which could slow growth and alter spins. Third, searching for electromagnetic flares from tidal disruption of accretion disks during mergers, which would provide independent confirmation. Finally, data from future third-generation detectors will test the prediction of a high merger rate at large redshifts.
The results will affect interpretations of the black hole population in gravitational-wave catalogs, constraints on the fraction of primordial black holes in dark matter, and planning of observational strategies for future detectors.
Conduct a Bayesian analysis varying the mass function and accretion parameters; include radiative feedback effects; search for electromagnetic counterparts in future events with similar parameters.
For cosmology and gravitational-wave astronomy, a key unsolved problem is the origin of black holes in the "pair-instability gap" and the nature of their spins. This work directly links that problem to the question of the existence and abundance of primordial black holes, as well as uncertainties in accretion models in the early Universe.
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
The authors interpret event GW231123 as a merger of primordial black holes that grew by accreting gas in the early Universe. They show that initial masses of 85.5 and 59.1 solar masses at z_cutoff = 24 increase to the observed 137 and 103, while spins spin up to 0.90 and 0.80. The merger rate is consistent with primordial black holes making up about 10^-3 of dark matter, and future observations by LIGO-Virgo-KAGRA and the Einstein Telescope could test these predictions.
Where its strength is
The work demonstrates that the standard primordial black hole scenario with accretion naturally explains both the masses and spins of GW231123 without invoking exotic astrophysical mechanisms. Particularly valuable is the consistency with the mass–spin correlation characteristic of accretion-driven growth, which sets the paper apart from alternative explanations.
Where it could go next
Worth trying
Why this may matter
If the interpretation of GW231123 as a primordial black hole merger is confirmed, it would provide strong evidence for a population of primordial black holes in the tens of solar mass range and their contribution to dark matter. Observations at high redshifts with the Einstein Telescope could definitively distinguish this scenario from stellar-mass models.
Nearby works in our archive
The closest work in the archive sits at 0.64 by meaning — the usual level for our corpus.
The initial masses of GW231123 (85.5 and 59.1 solar masses) grew through accretion to 137 and 103 solar masses, meaning the smaller black hole "caught up" with the larger one, increasing the mass ratio from 0.69 to 0.75.
The idea of primordial black holes as hidden mass echoes Greg Egan's novel "Schild's Ladder," where exotic compact objects from the early Universe play a key role in cosmology.