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Dynamics of Primordial Black Hole Formation

Astrophysics · Submitted 21 Jan 1999 · 23 references

Pith's verdict: A solid numerical study that puts the PBH formation threshold near 0.7 and shows critical-collapse scaling survives in an expanding FRW background; the headline comparison to 1/3 is the shakiest part of the framing.

T0 · AI review

How this review was made: An AI referee (selfhosted/deepseek-v4.1-flash) read the paper on October 29, 2026 and answered a fixed list of questions about it. T0 is the starting level every paper gets. The levels describe how deeply Pith has checked the paper, never the authors.

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arXiv IDastro-ph/9901292 (version 1)
Pith IDNWISGXYG, Pith's permanent record of this paper and its review
CitationJ.C. Niemeyer (UC Chicago), K. Jedamzik (MPA Garching). "Dynamics of Primordial Black Hole Formation." arXiv:astro-ph/9901292 (1999). https://pith.science/paper/NWISGXYG
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Referee report
The paper

The paper claims

The density contrast needed to collapse a horizon-size fluctuation into a primordial black hole is about 0.7, not the 1/3 long assumed, and the same value holds for three different perturbation shapes.

The reading

This paper asks a bookkeeping question with large consequences: how big must a density fluctuation be, at the moment it enters the horizon in the radiation-dominated early universe, for gravity to beat pressure and crush it into a primordial black hole? The standard answer, inherited from a toy model that pastes a closed Friedmann universe onto a flat one, is roughly one third of the horizon mass in excess. The authors instead evolve the full general relativistic fluid equations in spherical symmetry, for three different perturbation shapes, and find the threshold nearer $0.7$. That number is the hinge on which estimates of how many primordial black holes exist, and therefore constraints on small-scale primordial power and on evaporating black holes, turn.

What carries the argument

The load-bearing technical choice is the Hernandez–Misner formulation of the spherically symmetric hydrodynamic equations, in which the time coordinate is constant along outgoing null rays. Because the grid points sit on null surfaces rather than on slices of constant time, the computation never has to cross the event horizon, so the authors can follow a newly formed black hole for arbitrarily long times without hitting a coordinate singularity — the thing earlier Misner–Sharp-based simulations could not do, and the reason they could not see the post-formation bounce or measure late accretion. Surrounding this sits the definition of the control parameter: $��$ is the excess gravitational mass inside the initial horizon volume relative to the unperturbed Friedmann solution, and the black hole mass is read off as the gravitational mass enclosed by the innermost shell whose lapse has fallen below $10^{-10}$.

Extended reading notes

Core claim

The paper's central claim is that the threshold for primordial black hole formation, defined as the excess mass within the initial horizon volume in uniform Hubble gauge, is $��_{c} = 0.67$ for a Mexican-hat profile, $0.70$ for a Gaussian, and $0.71$ for a sixth-order polynomial, i.e. $��_{c} �� 0.7$ rather than the generally employed $��_{c} �� 1/3$. Two further results ride along. First, black holes that form close to threshold are surrounded by a fluid bounce: material just outside the event horizon is pushed outward in a compression wave, followed by a rarefaction that shuts off accretion almost completely, whereas black holes far above threshold keep accreting at a slowly declining rate; in neither case is the accreted mass significant. Second, the near-critical scaling $M_{bh} = K M_h (�� - ��_{c})^{��}$ known from asymptotically flat collapse carries over to an expanding Friedmann background, with $�� = 0.34$ to $0.37$ and $K$ between about $2.4$ and $12$ depending on shape.

Load-bearing premise

The claim that $��_{c} �� 0.7$ is a number cosmology can use rests on three hand-picked, spherically symmetric density profiles bracketing the space of perturbations inflation actually produces at horizon crossing; the paper treats the narrow spread it finds across them as evidence of universality rather than as a measured systematic.

Editorial extensions

If this is right

  • Cosmological PBH abundance estimates built on $��_{c} �� 1/3$ are wrong by a large factor for any steeply falling fluctuation spectrum, since the number of fluctuations above threshold is exponentially sensitive to where the threshold sits.
  • Upper bounds on the spectral index of the primordial power spectrum on small scales, derived from PBH overproduction and from evaporating black holes, loosen correspondingly.
  • Accretion can be dropped from the calculation of final PBH masses for the perturbation types studied, so $M_{bh}$ is effectively fixed at formation.
  • The initial PBH mass function acquires a sub-horizon tail from critical scaling, with $�� �� 0.36$, changing which masses dominate the evaporation-era phenomenology.
  • The same numerical machinery can be pointed at the cosmological QCD transition to test whether a softened equation of state really lowers the formation threshold.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Because $��$ here is a mass excess inside the horizon volume in uniform Hubble gauge, thresholds quoted elsewhere under a different gauge or a linear extrapolation of the density contrast are not directly comparable; a reader comparing numbers across the literature should check the definition before concluding there is a disagreement.
  • The near-agreement of the threshold across one uncompensated and two mass-compensated profiles hints that $��_{c}$ is set mainly by the equation of state and the background expansion rather than by the perturbation's shape, which is what makes a QCD-era drop in $��_{c}$ a clean prediction rather than a model-dependent hope.
  • That the fitted exponent $��$ matches the asymptotically flat radiation-fluid value suggests the critical solution barely notices the expanding background, which would make the resulting mass function largely insensitive to the cosmology of the formation epoch.
  • All three families are spherically symmetric and non-rotating; if shape-robustness holds only within that class, realistic three-dimensional perturbations could move the threshold, and that is the obvious next computation.
Original abstract

We present a numerical investigation of the gravitational collapse of horizon-size density fluctuations to primordial black holes (PBHs) during the radiation-dominated phase of the Early Universe. The collapse dynamics of three different families of initial perturbation shapes, imposed at the time of horizon crossing, is computed. The perturbation threshold for black hole formation, needed for estimations of the cosmological PBH mass function, is found to be $\delta_{\rm c} \approx 0.7$ rather than the generally employed $\delta_{\rm c} \approx 1/3$, if $\delta$ is defined as $\Delta M/\mh$, the relative excess mass within the initial horizon volume. In order to study the accretion onto the newly formed black holes, we use a numerical scheme that allows us to follow the evolution for long times after formation of the event horizon. In general, small black holes (compared to the horizon mass at the onset of the collapse) give rise to a fluid bounce that effectively shuts off accretion onto the black hole, while large ones do not. In both cases, the growth of the black hole mass owing to accretion is insignificant. Furthermore, the scaling of black hole mass with distance from the formation threshold, known to occur in near-critical gravitational collapse, is demonstrated to apply to primordial black hole formation.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 5,768 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.31 · human participants 0.01 · controlled comparison 0.13 · randomized assignment 0.02 · states quantitative result 0.99 · compares with prior results 0.91 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

We present a numerical investigation of the gravitational collapse of horizon-size density fluctuations to primordial black holes (PBHs) during the radiation-dominated phase of the Early Universe. The collapse dynamics of three different families of initial perturbation shapes, imposed at the time of horizon crossing, is computed. The perturbation threshold for black hole formation, needed for estimations of the cosmological PBH mass function, is found to be $\delta_{\rm c} \approx 0.7$ rather than the generally employed $\delta_{\rm c} \approx 1/3$, if $\delta$ is defined as $\Delta M/\mh$, the relative excess mass within the initial horizon volume. In order to study the accretion onto the newly formed black holes, we use a numerical scheme that allows us to follow the evolution for long times after formation of the event horizon. In general, small black holes (compared to the horizon mass at the onset of the collapse) give rise to a fluid bounce that effectively shuts off accretion onto the black hole, while large ones do not. In both cases, the growth of the black hole mass owing to accretion is insignificant. Furthermore, the scaling of black hole mass with distance from the formation threshold, known to occur in near-critical gravitational collapse, is demonstrated to apply to primordial black hole formation.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 We present a numerical investigation of the gravitational collapse of horizon-size density fluctuations to primordial black holes (PBHs) during the radiation-dominated phase of the Early Universe. contribution 1.00 0.94 0.05 asserted 0.93
s1 The collapse dynamics of three different families of initial perturbation shapes, imposed at the time of horizon crossing, is computed. contribution 0.65 0.95 0.03 asserted 0.98
s2 The perturbation threshold for black hole formation, needed for estimations of the cosmological PBH mass function, is found to be $\delta_{\rm c} \approx 0.7$ rather than the generally employed $\delta_{\rm c} \approx 1/3$, if $\delta$ is defined as $\Delta M/\mh$, the relative excess mass within the initial horizon volume. result 1.00 0.95 0.58 asserted 1.00
s3 In order to study the accretion onto the newly formed black holes, we use a numerical scheme that allows us to follow the evolution for long times after formation of the event horizon. method 0.98 0.94 0.03 asserted 0.99
s4 In general, small black holes (compared to the horizon mass at the onset of the collapse) give rise to a fluid bounce that effectively shuts off accretion onto the black hole, while large ones do not. result 1.00 0.90 0.09 asserted 0.98
s5 In both cases, the growth of the black hole mass owing to accretion is insignificant. result 1.00 0.92 0.15 asserted 1.00
s6 Furthermore, the scaling of black hole mass with distance from the formation threshold, known to occur in near-critical gravitational collapse, is demonstrated to apply to primordial black hole formation. result 0.99 0.88 0.15 asserted 1.00

What kind of paper the classifier reports this is: simulation study 1.00 · evidence from simulation or computation 1.00.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. FIG. 1. Shapes of the critical perturbations as imposed at th
Figure 2. FIG. 2. Time evolution of a near-critical Mexi
Figure 4. FIG. 4. Time evolution of a near-critical Gaus
Figure 7. FIG. 7. Growth rate of
Figure 8. FIG. 8. Black hole masses as a function of the distance to the fo

Showing 5 of the paper's 8 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:astro-ph/9901292 by the authors.

References
How the literature uses it

144 arXiv papers cite this in 192 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers whose main result relates to this one's

Found among the papers closest to this one by title and main claim. Pith's classifier compared each pair's main results and a second model family confirmed the relation; checked against two further families, shown conflicts agreed 118 times in 120 and shown same results 218 times in 239. The quoted sentence is the other paper's main claim.

States the same result 1 paper

Papers citing it · 127 sentences without a confident label

Contests it 1 paper
Builds on it 1 paper
  • Niemeyer & Jedamzik [ref] demonstrated that near-critical behaviour applies to PBH formation, extending the results of Evans & Coleman [ref] to an asymptotic FLRW background.

    Cosmology using numerical relativity · Primordial black holes · 0.92

Uses its method or tool 4 papers
  • We employ the Misner-Sharp [ref] and Hernandez-Misner [ref] formalisms, building upon the ideas of Niemayer and Jedamzik [ref] and Polnarev et al. [ref] .

    Formalism for Primordial Black Hole Formation in Spherical Symmetry · Introduction · 0.87

  • By using the Press-Schechter theory and smoothing on the scale $M$ with a Gaussian, the fractional energy density of PBH in the Universe is [equation] where the critical density perturbation is assumed to be in the range $δ_c=0.07-0.7$ [ref] , the density contrast $δ$ is related with the primordial curvature perturbation $ζ$ as $δ=4ζ/9$ during radiation domination, and the power spectrum for the primordial scalar perturbation is [equation] where we choose $M_Pl^-2=8πG=1$, the slow-roll parameter $ϵ=1/2(V_ϕ/V)^2$, $V_ϕ=dV/dϕ$, and the $ϕ_*$ is the value of $ϕ$ at the time when a given scale $k_*$ exits the horizon.

    Primordial black holes and second order gravitational waves from ultra-slow-roll inflation · Primordial black holes from inflation · 0.86

  • The fraction of horizon patches that collapse is [equation] and the present abundance, normalized to the dark matter, follows by the standard redshift factor [ref] , [equation] We take a collapse threshold $δ_c=0.414$, representative of the radiation-era value [ref] , and quantify the sensitivity to this choice below.

    One Feature, Three Clocks: Phase-Locked Gravitational Waves, Primordial Black Holes, and Non-Gaussianity from Periodic Warm Inflation · Primordial black holes · 0.86

Uses its data or values 16 papers
Compares with it 1 paper
Cites it as support 2 papers
Cites it as background 40 papers

Discussion

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