REVIEW
Computations of primordial black hole formation
General Relativity and Quantum Cosmology · Submitted 13 Dec 2004 · 47 references
Pith's verdict: A careful, well-designed numerical study that lowers the PBH threshold to δc ≈ 0.43–0.47 for growing-mode initial data; the new number is plausible and independently consistent, but the initial-data prescription behind it is asserted rather than checked.
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Referee report
The paper
The paper claims
The threshold for primordial black hole formation falls to about 0.45 when perturbations begin as pure growing modes.
The reading
What carries the argument
The load-bearing machinery is a two-stage numerical scheme built around the initial-data choice. A super-horizon linear growing mode is specified with density perturbation $��e$ and velocity perturbation $��u = -��e/4$, then evolved with the Misner-Sharp cosmic-time equations until a null slice can be constructed; the collapse is continued with a Hernandez-Misner outgoing-null-slice code, where the lapse $f$ is fixed to unity at the outer FRW observer and evolved inward using $D_k[(��+U)/f] = -4��R(e+p)f$. The null slicing keeps singularity formation from halting the computation, and the paper reports that this alternative $f$ equation, rather than the more direct equation (24), is what preserves the synchronization $D_kR �� 0$ as $f �� 0$.
Extended reading notes
Core claim
The paper's central claim is that the threshold amplitude for primordial black-hole formation is substantially lower than previously reported when the initial perturbation is a pure growing mode. For Mexican-hat perturbations the critical fractional mass excess is $��_c �� 0.43$, and for polynomial perturbations $��_c �� 0.47$, compared with $��_c �� 0.67$ and $0.71$ obtained with horizon-crossing initial data. The authors explain the shift by noting that the earlier initial data contained a decaying-mode component which rapidly disappeared, so the effective growing-mode amplitude available for collapse was smaller than the nominal $��$. The scaling law $M_{��m BH} = K(��-��_c)^�� M_H(t_H)$ survives with $�� �� 0.36$–$0.37$, and adding a positive cosmological constant raises $��_c$ linearly while lowering $��$. For sub-critical perturbations close to threshold, the collapse rebounds, drives strong compression and rarefaction waves, and can produce a second, more violent bounce.
Load-bearing premise
The revised threshold depends on the assumption that a super-horizon perturbation from inflation is well represented by the linear growing mode with velocity perturbation $��u = -��e/4$; if that prescription misstates the actual perturbation, the lower $��_c$ is an artifact of the setup.
Editorial extensions
If this is right
- For a fixed inflationary perturbation spectrum, using $��_c �� 0.43$–$0.47$ instead of $0.67$–$0.71$ raises the predicted number of primordial black holes, because more perturbations cross the collapse threshold.
- The mass scaling $M_{��m BH} \propto (��-��_c)^��$ with $�� �� 0.36$–$0.37$ survives the change in initial data, so near-threshold black-hole masses are controlled mainly by $��-��_c$ rather than by the initial-data convention.
- A positive cosmological constant makes collapse harder: it shifts $��_c$ upward by about $0.98\,y$ and lowers $��$ by about $8.3\,y$ for small vacuum-energy fraction $y$.
- Near-threshold sub-critical perturbations do not fade quietly; they generate strong compression waves and a second, more violent bounce, so the surrounding medium can be dynamically disturbed even when no black hole forms.
Reading between the lines
- If a decaying-mode component can inflate the apparent $��_c$ by roughly 0.2, then threshold values quoted across the literature are not directly comparable unless the initial-data slicing and mode content are specified.
- The shift from $0.67$ to $0.43$ is large enough that primordial-black-hole abundance constraints on the inflationary spectral index should be recomputed with the lower threshold; a constraint derived with the older value may be too conservative.
- The linear $��$ corrections suggest a possible consistency check: any measured primordial-black-hole abundance that requires a nonzero vacuum-energy fraction would also require a correlated change in the scaling exponent, not just in the threshold.
- The second-bounce compression waves in near-critical sub-critical cases may be a source of secondary perturbations or gravitational radiation; simulating them with a shock-capturing code would test whether they leave an observable imprint.
Original abstract
Results are presented from general relativistic numerical computations of primordial black-hole formation during the radiation-dominated era of the universe. Growing-mode perturbations are specified within the linear regime and their subsequent evolution is followed as they become nonlinear. We use a spherically symmetric Lagrangian code and study both super-critical perturbations, which go on to produce black holes, and sub-critical perturbations, for which the overdensity eventually disperses into the background medium. For super-critical perturbations, we confirm the results of previous work concerning scaling-laws but note that the threshold amplitude for a perturbation to lead to black-hole formation is substantially reduced when the initial conditions are taken to represent purely growing modes. For sub-critical cases, where an initial collapse is followed by a subsequent re-expansion, strong compressions and rarefactions are seen for perturbation amplitudes near to the threshold. We have also investigated the effect of including a significant component of vacuum energy and have calculated the resulting changes in the threshold and in the slope of the scaling law.
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
Results are presented from general relativistic numerical computations of primordial black-hole formation during the radiation-dominated era of the universe. Growing-mode perturbations are specified within the linear regime and their subsequent evolution is followed as they become nonlinear. We use a spherically symmetric Lagrangian code and study both super-critical perturbations, which go on to produce black holes, and sub-critical perturbations, for which the overdensity eventually disperses into the background medium. For super-critical perturbations, we confirm the results of previous work concerning scaling-laws but note that the threshold amplitude for a perturbation to lead to black-hole formation is substantially reduced when the initial conditions are taken to represent purely growing modes. For sub-critical cases, where an initial collapse is followed by a subsequent re-expansion, strong compressions and rarefactions are seen for perturbation amplitudes near to the threshold. We have also investigated the effect of including a significant component of vacuum energy and have calculated the resulting changes in the threshold and in the slope of the scaling law.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Results are presented from general relativistic numerical computations of primordial black-hole formation during the radiation-dominated era of the universe. | contribution 0.97 | 0.92 | 0.07 | asserted 0.99 |
| s1 | Growing-mode perturbations are specified within the linear regime and their subsequent evolution is followed as they become nonlinear. | method 0.99 | 0.85 | 0.04 | asserted 0.95 |
| s2 | We use a spherically symmetric Lagrangian code and study both super-critical perturbations, which go on to produce black holes, and sub-critical perturbations, for which the overdensity eventually disperses into the background medium. | method 0.89 | 0.95 | 0.08 | asserted 0.97 |
| s3 | For super-critical perturbations, we confirm the results of previous work concerning scaling-laws but note that the threshold amplitude for a perturbation to lead to black-hole formation is substantially reduced when the initial conditions are taken to represent purely growing modes. | result 1.00 | 0.88 | 0.44 | asserted 1.00 |
| s4 | For sub-critical cases, where an initial collapse is followed by a subsequent re-expansion, strong compressions and rarefactions are seen for perturbation amplitudes near to the threshold. | result 1.00 | 0.93 | 0.05 | asserted 1.00 |
| s5 | We have also investigated the effect of including a significant component of vacuum energy and have calculated the resulting changes in the threshold and in the slope of the scaling law. | contribution 0.54 | 0.97 | 0.13 | asserted 1.00 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:gr-qc/0412063 by the authors.
References
How the literature uses it
158 arXiv papers cite this in 216 sentences
Papers citing it · 128 sentences without a confident label
Uses its method or tool 7 papers
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The numerical code used is the same of Refs. [ref] , which has been fully described previously and therefore we give only a brief outline of it here.
From Primordial Black Holes Abundance to Primordial Curvature Power Spectrum (and back) · Numerical simulations · 0.85
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Assuming Gaussian primordial density fluctuations, the Press–Schechter formalism [ref] relates the initial PBH mass fraction to the curvature power spectrum through [equation] where we adopt the collapse threshold $δ_c≃0.45$ [ref] .
Fine-tuning in mixed Dark Matter models with Primordial Black Hole relics · The Fine-Tuning Definition and Measure of the Model · 0.94
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The spherically symmetric numerical simulations used to compute the threshold $δ_c$, and the mass distribution discussed in the next section, have been performed with a numerical code developed in [ref] that has been widely used and tested [ref] , including also an adaptive mesh refinement (AMR) scheme, which makes the code very flexible.
From inflation to black hole mergers and back again: Gravitational-wave data-driven constraints on inflationary scenarios with a first-principle model of primordial black holes across the QCD epoch · The threshold for PBHs · 1.00
Uses its data or values 38 papers
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The mass of the resulting PBH depends on the horizon mass and the amplitude of the density perturbation $δ$ defined in the comoving gauge from which it formed, and is given by [ref] [equation] where $$ is the horizon mass at the time of horizon re-entry, and during radiation domination the constants have been numerically found to be given by $k=3.3, γ=0.36, δ_c=0.453$ (the values depend upon the radial profile of the perturbations being considered; we use the values given here in order to be concrete).
Primordial black holes with an accurate QCD equation of state · Extended mass function of primordial black hole formation · 0.91
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where $t_PBH$ is the time of production of the PBH, which can be obtained when the density contrast between inside and outside of the Hubble volume exceeds the critical value $δ_c=0.45$ [ref] .
Primordial black holes as a probe of strongly first-order electroweak phase transition · PBH and GW productions from first-order phase transition · 0.86
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We use $δ_c ≃0.42$ [ref] (see also Refs. [ref] ), $k=3.3$ [ref] and $γ=0.36$ [ref] .
Primordial Black Holes from Thermal Inflation · Primordial black holes · 1.00
Compares with it 1 paper
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This corresponds to N = ln(a_m/a_hc) = 1/4ln(ρ_hc/ρ_m)∼0.6 e-folds passing from horizon crossing time up to the onset of the gravitational collapse process at $ρ=ρ_m$, thus being in agreement with the results from PBH numerical simulations [ref] .
Primordial black holes in loop quantum cosmology: The effect on the threshold · Results · 0.82
Cites it as support 2 papers
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This assumption is supported by numerical simulations which show that maximum infall radial velocity remains rather small for a time considerably after horizon crossing, till the perturbation has become highly non-linear [ref] .
The Black Hole Formation -- Null Geodesic Correspondence · Introduction · 0.99
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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 37 papers
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PBHs will form by gravitational collapse at horizon reentry for the Hubble patches where the density contrast $δρ/ρ$ exceeds a threshold $δ_c$ [ref] , which depends on the EoS parameter $w$ at the re-entry moment [ref] .
NANOGrav Hints on Planet-Mass Primordial Black Holes · 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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As it is well known, in the standard scenario of a radiation dominated medium a critical collapse arises [ref] and the mass spectrum of PBHs is characterized by a scaling law [ref] given by [equation] where for $δ-δ_c ≲10^-2$ the critical exponent $γ$ depends only on the parameter of the equation of state, i.e the value of $w$, completely independent on the initial configuration of the initial conditions, given by the initial profile of $ζ(r)$, which affect instead the value of $𝒦$.
From inflation to black hole mergers and back again: Gravitational-wave data-driven constraints on inflationary scenarios with a first-principle model of primordial black holes across the QCD epoch · The mass spectrum for PBHs · 0.92
Discussion
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