REVIEW
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
Why Pith says this
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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
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
- 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.
The abstract, sentence by sentence
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
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| 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 was examined, and what was not
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Figures
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
Papers whose main result relates to this one's
States the same result 1 paper
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The threshold for black hole formation, \delta_{c} \approx 0.7, is found to be nearly identical for all perturbation families if the control parameter, \delta, is chosen as the total excess mass within the initial horizon volume.
Numerical investigation of the threshold for primordial black hole formation · 0.87
Papers citing it · 127 sentences without a confident label
Contests it 1 paper
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Disregard for this issue affected early work on this subject [ref] , as pointed out in Ref. [ref] .
Formalism for Primordial Black Hole Formation in Spherical Symmetry · Initial Conditions · contest 0.77
Builds on it 1 paper
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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
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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
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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
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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
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In GR, the mass of a primordial black hole near the threshold obeys the critical scaling relation $M_PBH∝(δ-δ_c)^γ$, with $γ_GR≃0.36$ for a radiation fluid [ref] .
Primordial Black Hole Formation in $f(R)=R+\alpha R^2$ Gravity: Perturbative and Non-Perturbative Analysis · Einstein–Frame Formulation for the Non–Perturbative Regime · 0.84
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Horizon patches with $δ>δ_c$ collapse to PBHs with mass [ref] M=M_HK(δ-δ_c)^γ_r, where $M_H$ is the horizon mass as a function of $R$, and we use $δ_c=0.55$, $K=10$ and $γ_r=0.36$ [ref] .
Pinning down the primordial black hole formation mechanism with gamma-rays and gravitational waves · PBHs from curvature perturbations · 0.92
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Here $$ gives the minimum density contrast required for a perturbation to later collapse into an object of type $X∈{PBHs$, UCMHs$}$; $PBH∼1/3$ [ref] , while $UCMH$ is a scale-dependent quantity [ref] , although typically $𝒪(10^-2-10^-3)$ [ref] .
Number Counts and Non-Gaussianity · Non-Gaussian Probability Distributions · 0.87
Compares with it 1 paper
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The horizon mass can be easily estimate as follows [equation] This estimation for $M_PBH$ also differs from the one in [ref] , where the critical scaling model is used instead [ref] .
Primordial black holes through preheating instabilities in $\alpha$-attractor models · PBH mass · 0.95
Cites it as support 2 papers
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Numerical general relativistic simulations [ref] have demonstrated that the scaling of black hole mass with the proximity to the formation threshold, known to occur in near-critical gravitational collapse, also applies to PBH formation.
Primordial black holes through preheating instabilities in $\alpha$-attractor models · PBH mass · 0.96
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We do not include the effect of the critical collapse [ref] for simplicity because Ref. [ref] implies that it does not modify the PBH abundance so much.
NANOGrav results and LIGO-Virgo primordial black holes in axion-like curvaton model · Introduction · 0.97
Cites it as background 40 papers
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The population of primordial black holes that formed in the early Universe depends on the Hubble rate and the total mass within the Hubble horizon [ref] .
Pion condensation in the early Universe at nonvanishing lepton flavor asymmetry and its gravitational wave signatures · Introduction · 0.80
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After these pioneering works, the collapse of initial perturbations in the form of primordial curvature fluctuations, was numerically studied in Refs. [ref] , followed some years afterwards by an extensive numerical analysis by one of us using an explicit Lagrangian hydrodynamics code developed and used in Refs. [ref] and, recently, by other authors in Refs. [ref] .
From Primordial Black Holes Abundance to Primordial Curvature Power Spectrum (and back) · Primordial black holes formation · 0.91
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Our present considerations do not apply to such situation.] A rough criterion for the formation of PBHs was first proposed by Carr [ref] , and much numerical work has been done in search of a more accurate criterion [ref] .
PBH abundance from random Gaussian curvature perturbations and a local density threshold · Introduction · 0.96
Discussion
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