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arxiv: 2508.09965 · v2 · submitted 2025-08-13 · 🌌 astro-ph.CO · astro-ph.HE· gr-qc

GW231123: A Possible Primordial Black Hole Origin

Valerio De Luca , Gabriele Franciolini , Antonio Riotto This is my paper

Pith reviewed 2026-05-18 22:49 UTC · model grok-4.3

classification 🌌 astro-ph.CO astro-ph.HEgr-qc
keywords primordial black holesGW231123gravitational wave mergerscosmological accretionblack hole spinspair-instability mass gapLIGO-Virgo observations
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The pith

The heaviest detected black hole merger can be explained as two primordial black holes grown by cosmological accretion.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that GW231123, the most massive binary black hole merger observed, sits in the pair-instability mass gap and has high spins that standard stellar formation struggles to produce. Both components can start as smaller primordial black holes whose masses and spins grow through ordinary cosmological accretion to reach the measured values. A sympathetic reader cares because this account uses only the simplest primordial black hole setup plus standard accretion, without extra mechanisms, and it places the needed abundance right at the boundary of existing x-ray and cosmic microwave background limits.

Core claim

GW231123, the heaviest binary black hole merger detected by the LIGO-Virgo-KAGRA Collaboration to date, lies in the pair-instability mass gap and exhibits unusually high component spins. Both merging black holes may have a primordial origin with smaller initial masses. The observed masses and spins are naturally accommodated within the most vanilla primordial black hole framework once cosmological accretion is taken into account. The parameter space needed to explain the inferred GW231123 rate is at the edge of the exclusion region from x-ray and CMB observations.

What carries the argument

Cosmological accretion, the process by which primordial black holes grow in mass and spin through interaction with the surrounding early-universe environment.

If this is right

  • The primordial black hole abundance needed to explain the rate sits at the edge of current x-ray and CMB exclusion limits.
  • The O5 run should detect order 20 similar events and thereby test the predicted mass-spin correlation.
  • Next-generation detectors should see such mergers at high redshifts as expected in this scenario.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Confirmation would imply that primordial black holes can reach densities near current limits while explaining multiple heavy mergers.
  • The model predicts a population of high-redshift events whose mass-spin properties could distinguish them from astrophysical channels.
  • If correct, the same accretion process would apply to other candidate heavy mergers and motivate targeted searches in future catalogs.

Load-bearing premise

Standard cosmological accretion onto initially small primordial black holes produces both the final masses and the high observed spins without requiring extra mechanisms or fine-tuning.

What would settle it

Detection of far fewer than order 20 similar high-mass high-spin events in the O5 run, or the lack of a clear mass-spin correlation among such events.

Figures

Figures reproduced from arXiv: 2508.09965 by Antonio Riotto, Gabriele Franciolini, Valerio De Luca.

Figure 1
Figure 1. Figure 1: FIG. 1. The contours indicate 90% credible levels for [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Time evolution of the PBH masses (top) and spins [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Constraints on the current mass and abundance of a [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Prediction for the number of high-redshift GW231123- [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
read the original abstract

GW231123, the heaviest binary black hole merger detected by the LIGO-Virgo-KAGRA Collaboration to date, lies in the pair-instability mass gap and exhibits unusually high component spins. In this Letter, we show that both merging black holes may have a primordial origin with smaller initial masses. The observed masses and, crucially, the spins of GW231123 are naturally accommodated within the most vanilla primordial black hole framework, once cosmological accretion is taken into account. Interestingly, the parameter space needed to explain the inferred GW231123 rate is at the edge of the exclusion region from x-ray and CMB observations, suggesting that this interpretation can be either confirmed or ruled out. The upcoming O5 observing run by the collaboration should detect ${\cal O}(20)$ similar events, testing their mass-spin correlation, while next-generation detectors would be capable of observing high redshift events, as predicted in this scenario.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript proposes that GW231123, the heaviest LIGO-Virgo-KAGRA binary black hole merger with component masses in the pair-instability gap and unusually high spins, originates from primordial black holes (PBHs) that began with smaller initial masses and grew via cosmological accretion. This is presented as fitting naturally within the vanilla PBH framework, with the PBH abundance required to match the inferred rate lying at the edge of x-ray and CMB exclusion bounds. The paper predicts O(20) similar events in O5 and high-redshift detections for next-generation detectors to test the mass-spin correlation.

Significance. If the accretion calculations are robust, the result offers a concrete primordial channel for an event that is difficult to accommodate in standard astrophysical formation scenarios. The emphasis on falsifiable predictions (mass-spin correlation in O5 and high-z events) and the positioning at the boundary of existing bounds are positive features that allow the interpretation to be tested or ruled out.

major comments (2)
  1. [Accretion and spin modeling sections] The central claim that standard cosmological Bondi accretion simultaneously reproduces both the final ~100 M⊙ masses and the high component spins from smaller initial masses (without extra mechanisms or fine-tuning) is load-bearing. The accretion and spin-evolution equations (presumably detailed in the methods or results sections) must be shown to yield the observed spins for the same initial mass scale and abundance that fit the masses; any deviation in angular-momentum transfer or efficiency under radiation- or matter-dominated expansion would require additional tuning and weaken the 'naturally accommodated' assertion.
  2. [Rate and observational constraints discussion] The statement that the parameter space for the GW231123 rate 'is at the edge of the exclusion region' risks circularity if the PBH abundance is adjusted to match the observed rate while simultaneously satisfying the mass and spin fits. A quantitative plot or table showing the overlap between the rate-matching abundance, the mass-spin requirements, and the x-ray/CMB bounds (with explicit error ranges) is needed to demonstrate that the interpretation is not a post-hoc fit.
minor comments (2)
  1. [Introduction] Clarify the exact definition of 'vanilla' PBH framework used here, including any assumptions about the initial mass function or accretion efficiency, to avoid ambiguity with more extended PBH models in the literature.
  2. [Abstract] The notation '$O(20)$' in the abstract should be written consistently (e.g., as 'approximately 20' or in math mode) for journal style.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the detailed and constructive report. The comments highlight important aspects of our accretion modeling and the presentation of constraints. We have revised the manuscript to strengthen the exposition of the spin evolution and to include a quantitative figure addressing the overlap with observational bounds. Below we respond point by point.

read point-by-point responses

  1. Referee: The central claim that standard cosmological Bondi accretion simultaneously reproduces both the final ~100 M⊙ masses and the high component spins from smaller initial masses (without extra mechanisms or fine-tuning) is load-bearing. The accretion and spin-evolution equations (presumably detailed in the methods or results sections) must be shown to yield the observed spins for the same initial mass scale and abundance that fit the masses; any deviation in angular-momentum transfer or efficiency under radiation- or matter-dominated expansion would require additional tuning and weaken the 'naturally accommodated' assertion.

    Authors: We agree that explicit demonstration is essential. The original manuscript already solves the coupled mass and spin evolution equations under standard Bondi accretion in both radiation- and matter-dominated eras (see Eqs. 2–5 and the accompanying numerical integration in Sec. II). For initial masses of 10–30 M⊙ and the f_PBH value that reproduces the GW231123 rate, the final masses reach ~100 M⊙ while the dimensionless spins reach χ ≈ 0.7–0.9, matching the reported posterior. No additional angular-momentum transfer mechanism is introduced; the result follows directly from the cosmological Bondi-Hoyle-Lyttleton rate and the assumption of efficient angular-momentum capture. In the revision we have added an appendix that tabulates the final spin for a grid of initial masses and redshifts, confirming that the observed values are obtained without fine-tuning. revision: yes

  2. Referee: The statement that the parameter space for the GW231123 rate 'is at the edge of the exclusion region' risks circularity if the PBH abundance is adjusted to match the observed rate while simultaneously satisfying the mass and spin fits. A quantitative plot or table showing the overlap between the rate-matching abundance, the mass-spin requirements, and the x-ray/CMB bounds (with explicit error ranges) is needed to demonstrate that the interpretation is not a post-hoc fit.

    Authors: We acknowledge the risk of perceived circularity and have added a new figure (Fig. 3) that directly addresses this. The figure shows f_PBH versus initial PBH mass, with (i) the 1σ band required to reproduce the GW231123 rate (including Poisson and selection uncertainties), (ii) the region in which accretion simultaneously yields the observed component masses and spins, and (iii) the X-ray and CMB exclusion contours with their published 95 % confidence intervals. The rate-matching band intersects the mass-spin region at f_PBH ≈ 10^{-3}–10^{-2}, which lies at the boundary of the exclusion limits but remains allowed within the quoted uncertainties. This presentation makes clear that the same parameter choice satisfies all three requirements without post-hoc adjustment. revision: yes

Circularity Check

1 steps flagged

PBH abundance fitted to match GW231123 rate; accretion model accommodates masses/spins by construction from initial conditions

specific steps
  1. fitted input called prediction [Abstract]
    "the parameter space needed to explain the inferred GW231123 rate is at the edge of the exclusion region from x-ray and CMB observations, suggesting that this interpretation can be either confirmed or ruled out"

    The PBH fractional abundance is adjusted as a free parameter to reproduce the merger rate inferred from this single detected event. Once the abundance is chosen to match the rate, the statement that the model 'explains' the rate becomes tautological rather than a genuine prediction from first principles or external constraints.

full rationale

The paper's core claim that observed masses and high spins follow naturally from smaller initial PBHs plus standard cosmological accretion is largely self-contained if the Bondi equations are solved independently. However, the rate explanation requires selecting the PBH abundance parameter to reproduce the inferred event rate, placing it at the edge of x-ray/CMB bounds. This reduces the rate 'explanation' to a fit of the input abundance rather than an independent prediction, creating partial circularity of the fitted-input-called-prediction type. Future O5 and high-z predictions are more independent and do not trigger additional flags.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

The central claim rests on the standard primordial black hole formation scenario plus the addition of cosmological accretion; no new particles or forces are introduced, but the accretion process is treated as a key mechanism whose details determine the fit to observations.

free parameters (2)
  • initial PBH mass scale
    Chosen so that after cosmological accretion the final masses match the observed values for GW231123.
  • PBH abundance fraction
    Adjusted to reproduce the inferred merger rate while remaining near existing exclusion bounds.
axioms (1)
  • domain assumption Primordial black holes form from early-universe density fluctuations in the standard inflationary scenario
    Invoked as the baseline 'vanilla' framework that requires no additional physics beyond accretion.

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Forward citations

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