REVIEW 3 major objections 4 minor 3 cited by
Even a trace of primordial black holes would cap the WIMP annihilation cross-section.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-03 21:03 UTC pith:L7AUHEJS
load-bearing objection Useful analytical tool for PBH minispikes, honestly benchmarked; the headline '±15%' is conditional on the adopted plateau model and the claimed new cross-section bound overlaps with prior work. the 3 major comments →
In-depth analysis of the clustering of dark matter particles around primordial black holes. Part II. Analytical prescriptions for spikes
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
On the paper's own terms, the central discovery is that the minispike annihilation rate is both computable analytically and powerful enough to decide the fate of the simplest WIMPs. Once the radial profile is approximated by the soft prescription—and the inner region is treated as a saturation plateau with density ρ_sat = m_χ/(⟨σ v⟩ Δt)—the annihilation rate factorizes into simple scalings: Γ_BH ∝ M_BH^3 for light PBHs (with no redshift dependence), Γ_BH ∝ M_BH for heavy PBHs, with the transition set by a critical mass M_T. Feeding these into gamma-ray and CMB constraints yields the mutual-exclusivity picture: a PBH fraction f_BH above roughly 10^-8–10^-6 and below near 1 is excluded for s-w
What carries the argument
The load-bearing tool is the 'soft approximation' (Eq. 2.46): instead of integrating the orbital phase space, one writes 1/ρ_soft = Σ_a 1/ρ_a over the four asymptotic regimes (slopes 3/4, 3/2, 9/4, with two distinct 3/2 origins), then applies surface corrections (Eq. 3.39) from the companion analysis. Annihilation is grafted on via a saturation plateau (Eq. 3.9), ρ(t,r) = ρ_sat ρ(r)/(ρ_sat+ρ(r)), with ρ_sat = m_χ/(⟨σ v⟩Δt); this converts the spike into a flat core plus the surviving power-law envelope, and the phase diagram (critical masses M_1, M_2, M_T) organizes the integral into the closed-form scalings (3.28), (3.33), (3.36).
Load-bearing premise
The load-bearing premise is that annihilation reshapes the spike's inner region into a flat saturation plateau, an assumption equivalent to circular orbits; minispike orbits are highly eccentric, and the paper's own Appendix B.2 shows that replacing the plateau by a slope-1/2 cusp changes the annihilation integral K_BH by up to 50%, so the quoted ±15% and all derived constraints inherit at least this modeling uncertainty.
What would settle it
A phase-space simulation that tracks annihilating dark-matter particles on realistic eccentric orbits around a PBH from kinetic decoupling onward would settle the plateau-vs-cusp question; if the inner profile develops a slope-1/2 cusp, the annihilation integral K_BH exceeds the plateau value by up to 50%, making the paper's central ±15% accuracy claim fail for the reshaped profile and tightening the derived ⟨σv⟩ limits correspondingly.
If this is right
- If the soft approximation holds, spike phenomenology no longer needs costly numerical integration: density profiles and annihilation rates for any PBH mass and WIMP parameters reduce to a few algebraic lines, enabling fast scans of other observables.
- If sub-solar PBHs are discovered at a DM fraction ≳10^-7, s-wave annihilating thermal DM with mass below ~700 GeV is essentially ruled out, a constraint that does not depend on kinetic decoupling temperature for heavy PBHs.
- Light PBH minispikes would shine with an annihilation luminosity that stays constant as the universe ages; such non-evolving compact sources would be a distinctive signature distinguishing PBH spikes from astrophysical transients.
- The mutual-exclusivity result means PBHs and s-wave annihilating WIMPs cannot both make comparable contributions to dark matter: f_BH must be either very small or close to unity.
- The ±15% accuracy claim applies in the parameter range tested; the worst case (1 MeV WIMPs) stays within -10% to +22% error on Γ_BH even though Γ_BH spans tens of orders of magnitude.
Where Pith is reading between the lines
- If the saturation plateau is replaced by the weak slope-1/2 cusp suggested by earlier spike studies, the paper's Appendix B.2 implies annihilation rates—and therefore all derived limits on ⟨σv⟩—tighten by up to 50%; a dedicated eccentric-orbit phase-space treatment would settle which shape is physical and could extend the results to p-wave annihilation.
- The parameter-free low-mass scaling Γ∝M_BH^3 with zero redshift drift suggests a concrete observational test: search time-domain gamma-ray or multi-wavelength surveys for compact sources whose flux does not evolve, which astrophysical transients would not mimic.
- Because the soft approximation is purely algebraic, it should transfer to other gravitational-capture problems—spikes around intermediate-mass black holes, or around seeds formed in matter domination—where the same phase-space integrals appear, providing fast predictions for gravitational-wave dephasing or neutrino signals.
- The claimed ±15% is for the annihilation rate, not the density profile itself; density errors are larger (up to a factor ~2 at some radii), so any constraint that depends on the profile shape rather than the integrated rate will need a separate error budget.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper (Part II of a series) constructs fast analytical approximations for the dark-matter density profiles ('minispikes') that form around primordial black holes during the radiation era, extending the asymptotic analysis of Part I. The authors introduce a 'soft approximation' for the post-collapse density, combine it with a saturation-plateau model for self-annihilation, and derive closed-form scaling laws for the annihilation rate Γ_BH in three mass regimes. They then apply these results to observational constraints, recasting gamma-ray and CMB limits into bounds on the PBH fraction f_BH and, assuming a discovered sub-solar PBH population, on the s-wave annihilation cross-section ⟨σ_ann v⟩. The paper's central claims are that the approximate density reproduces the full numerical integration of Eq. (2.4) to within ±15% for the annihilation rate over a wide parameter range, and that the resulting constraints on WIMP properties are stringent and partly new.
Significance. If the central claims hold, the fast analytic formulae in Eqs. (3.28), (3.33), and (3.36) would provide a valuable replacement for expensive numerical integration in spike phenomenology, and the derived cross-section constraints would be an important new result for mixed PBH-DM scenarios. The paper's strengths are its honest quantitative error reporting, its derivation (rather than fitting) of the scaling laws, its careful treatment of the phase diagram, and its explicit statement of the modeling assumptions and their limitations. The Appendix B sensitivity study is a particularly commendable example of transparent uncertainty assessment. The significance is therefore high within the scope of JCAP, provided the model-dependence of the annihilation-rate normalization is addressed or clearly qualified.
major comments (3)
- [§3.1–§3.2 and Appendix B.2] The headline precision claim is conditional on the validity of the saturation-plateau model of Eq. (3.9), which follows from Eq. (3.3) under the assumption v⊥∇ρ (circular orbits). The authors themselves state that minispike orbits are highly eccentric and that a dedicated phase-space treatment is beyond the scope of this work. Appendix B.2 then shows that replacing the plateau by the slope-1/2 cusp of [28,29] changes K_BH, and hence Γ_BH, by up to 50% (Eqs. B.9–B.14 vs B.1–B.8). Since Γ_BH is the quantity validated at the ±15% level and is the input to all constraints in §4, the true model uncertainty in the annihilation rate is larger than the quoted numerical error. The paper should either provide a treatment of the eccentric-orbit case, or explicitly reframe the abstract and conclusions so that the ±15% claim refers to the accuracy of the soft approximation relative to the integral (2
- [Abstract and §3.4, Fig. 6] The abstract states: 'Our approximate density yields the correct annihilation rate within ±15% precision.' This is not literally supported by the body. In Fig. 6 (right panel), the corrected soft approximation for the 1 MeV case gives discrepancies between −10% and +22%, and the text acknowledges this. The conclusions (§5) correctly say 'almost always, within ±15%', but the abstract omits the qualification. This overstatement matters because the abstract is the primary summary of the paper's contribution. The claim should be reworded to state the range of errors actually found, e.g., 'within ±15% over most of parameter space, with a maximum deviation of +22% in the light-WIMP regime'.
- [§4.2.3 and Eq. (4.11)] The recasting of the Ando–Ishiwata decaying-DM limits into the inequality (4.11) is defended with a heuristic 'decaying particle twice as massive' correspondence, but the authors themselves note in §4.1.1 that the time dependence of minispike annihilation differs from standard dilution and that a rigorous recasting is left to future work. Given that the minispike rate is redshift-dependent in a way that is not simply ∝(1+z)^6, the translation of a decaying-DM lifetime bound into a fixed constraint on f_BH Γ_BH at z≃0 is an uncontrolled approximation. This does not invalidate the methodology, but the resulting f_BH and ⟨σ_ann v⟩ bounds should be flagged as illustrative estimates rather than definitive limits, with the associated systematic caveat stated in the abstract or conclusions.
minor comments (4)
- [Throughout] There are several typographical errors and awkward phrasings: 'showed' should be 'shown' (e.g., §2.1, §2.2), 'decrases' should be 'decreases' (§3.2), 'currrently' should be 'currently' (§5), and 'acually' should be 'actually' (§4.1.2). These do not affect the science but should be cleaned up.
- [Eq. (3.39) and caption of Fig. 6] The 'corrected soft approximation' is introduced by replacing I^asy_{3/2} and I^asy_{9/4} with the radius-dependent expressions (3.39). It would improve clarity to define explicitly which expression is used in each panel of Fig. 6 and to state in the caption that the right panel already includes this correction. The current text refers to 'expansions (3.39)' but does not specify whether the correction is applied uniformly to all asymptotic pieces.
- [§4.1.1, Eq. (4.11)] The product f_BH Γ_BH is said to vanish for f_BH = 0 or 1; while true for f_BH=0, for f_BH=1 the DM fraction f_DM vanishes, so Eq. (4.2) suppresses Γ_BH, but the phrase could be misinterpreted. A short explanatory clause would help the reader.
- [§4.2.1, Eq. (4.37)] The lower bound on m_χ in Eq. (4.37) is derived using the scaling (4.35) that assumes the lower-right asymptotic regime. It would be useful to state explicitly that this estimate is valid only within that regime and may be modified near the transition to the lower-left boundary.
Circularity Check
No significant circularity: the approximate density and scaling laws are constructed from analytically derived asymptotics and validated against an independent numerical integration.
full rationale
The paper's central approximate density (2.46) is an explicit harmonic combination of the analytically derived asymptotic densities of Part I; no parameter is fitted to the benchmark density or annihilation rate. The ±15% claim is checked in Figs. 3 and 6 against the full numerical integration of the fundamental integral (2.4), which is stated and used in this paper independently of the approximations. The surface corrections (3.39) are taken from the authors' previous work [26], but since the comparison target is the numerical integral (2.4), the self-citation functions as a source of analytic expansions rather than as the evidence that the approximation works. Scaling relations (3.28), (3.33), (3.36) follow algebraically from the saturation-plateau model and the asymptotic densities; no fitted parameter is renamed as a prediction. The saturation plateau itself is an openly declared heuristic (Eq. 3.9, derived under v⊥∇ρ), with an explicit sensitivity test (Appendix B.2) showing up to 50% changes in K_BH if a slope-1/2 cusp is used; this is a modeling uncertainty, not a circular use of the result being predicted. The abstract's unqualified '±15%' is slightly stronger than the body's 'almost always within ±15%' with -10% to +22% for a 1 MeV WIMP, but this is an accuracy/claim-scope mismatch, not circularity. No self-definitional, fitted-input-as-prediction, or self-citation-load-bearing structure is present.
Axiom & Free-Parameter Ledger
free parameters (1)
- f_eff =
0.1
axioms (5)
- domain assumption s-wave only, constant ⟨σ_ann v⟩, Majorana fermions (factor 1/2 in Γ_BH)
- domain assumption Spike formation stops at matter-radiation equality; inner spike unaffected by later non-linear infall
- domain assumption Saturation plateau shape (3.9) accurately describes annihilation reshaping
- domain assumption Decaying-DM lifetime bounds [37] can be recast as f_BH Γ_BH ≤ (M_BH/2m_χ)(1/τ_dm^min)
- ad hoc to paper The soft-approximation combination 1/ρ_soft = Σ 1/ρ_a (Eq 2.46) is a valid interpolation
read the original abstract
Primordial black holes (PBHs) are very appealing dark matter (DM) candidates. It is highly plausible though, should they exist, that they would not make up all of the DM. Several studies showed that if the rest of DM is made of thermal particles, then these should accumulate around such PBHs, leading to the formation of very dense spikes in the radiation era. We contributed a detailed analytical study about this phenomenon, providing clear explanations as for the origin of scaling relations in the form of power-law density profiles with up to 3 different spectral indices, i.e. $3/4$, $3/2$, and $9/4$, and 4 asymptotic regimes. Here, we further derive an approximate analytical solution that enables fast numerical predictions for the density profiles of these spikes. We also address the specific case of self-annihilating DM species and derive new approximate analytical formulae. Our approximate density yields the correct annihilation rate within $\pm 15\%$ precision. We then focus on indirect detection in the cosmic microwave background and in extragalactic gamma-rays. We shed new and subtle light on how mutually exclusive PBHs and self-annihilating DM species can really be. In particular, the discovery of a population of sub-solar PBHs would set stringent constraints on the $s$-wave annihilation cross-section of these particles, a point so far missed in the literature.
Forward citations
Cited by 3 Pith papers
-
Constraints on Primordial Black Hole Dressed by Dark Matter Halo from Microlensing Effect of Fast Radio Bursts
A new conversion formula turns monochromatic bare-PBH microlensing bounds into extended-mass dressed-PBH bounds, and a 10^5-FRB forecast places f_PBH near 10^-4.
-
Constraining the Coexistence of Primordial Black Holes and Particle Dark Matter with Neutrino Observations
A refined halo model turns neutrino-telescope data into upper limits on the primordial-black-hole fraction f_PBH, reaching ~10^-8 in some mass ranges for mixed WIMP/FIMP scenarios.
-
In-depth analysis of the clustering of dark matter particles around primordial black holes. Part III: CMB constraints
CMB data limits the s-wave annihilation cross section of thermal dark matter particles to ≲ 10^{-30} cm³/s scaled by PBH fraction and mass for PBHs heavier than ~10^{-10} solar masses.
Reference graph
Works this paper leans on
-
[1]
Hawking,Gravitationally collapsed objects of very low mass,MNRAS152(Jan., 1971) 75
S. Hawking,Gravitationally collapsed objects of very low mass,MNRAS152(Jan., 1971) 75
1971
-
[2]
B. J. Carr and S. W. Hawking,Black holes in the early Universe,MNRAS168(Aug., 1974) 399–416
1974
-
[3]
S. W. Hawking,Particle Creation by Black Holes,Commun. Math. Phys.43(1975) 199–220
1975
-
[4]
G. Dvali, L. Eisemann, M. Michel and S. Zell,Black hole metamorphosis and stabilization by memory burden,Phys. Rev. D102(Nov., 2020) 103523, [2006.00011]
Pith/arXiv arXiv 2020
-
[5]
G. F. Chapline,Cosmological effects of primordial black holes,Nature253(Jan., 1975) 251–252
1975
-
[6]
B. J. Carr, K. Kohri, Y. Sendouda and J. Yokoyama,Constraints on primordial black holes, Reports on Progress in Physics84(Nov., 2021) 116902, [2002.12778]
Pith/arXiv arXiv 2021
-
[7]
A. M. Green and B. J. Kavanagh,Primordial black holes as a dark matter candidate,Journal of Physics G Nuclear Physics48(Apr., 2021) 043001, [2007.10722]
Pith/arXiv arXiv 2021
-
[8]
Eros-2 Collaboration, Tisserand, L
P. Eros-2 Collaboration, Tisserand, L. Le Guillou et al.,Limits on the macho content of the galactic halo from the eros-2 survey of the magellanic clouds,A&A469(July, 2007) 387–404, [astro-ph/0607207]
Pith/arXiv arXiv 2007
-
[9]
A. M. Green,Microlensing and dynamical constraints on primordial black hole dark matter with an extended mass function,Phys. Rev. D94(Sept., 2016) 063530, [1609.01143]
Pith/arXiv arXiv 2016
-
[10]
M. Petaˇ c, J. Lavalle and K. Jedamzik,Microlensing constraints on clustered primordial black holes,Phys. Rev. D105(Apr., 2022) 083520, [2201.02521]
Pith/arXiv arXiv 2022
-
[11]
S. Bird, I. Cholis, J. B. Mu˜ noz, Y. Ali-Ha ¨ ımoud, M. Kamionkowski, E. D. Kovetz et al.,Did LIGO Detect Dark Matter?,Phys. Rev. Lett.116(May, 2016) 201301, [1603.00464]
Pith/arXiv arXiv 2016
-
[12]
M. Sasaki, T. Suyama, T. Tanaka and S. Yokoyama,Primordial Black Hole Scenario for the Gravitational-Wave Event GW150914,Phys. Rev. Lett.117(Aug., 2016) 061101, [1603.08338]
Pith/arXiv arXiv 2016
-
[13]
M. Raidal, V. Vaskonen and H. Veerm¨ ae,Formation of primordial black hole binaries and their merger rates,arXiv e-prints(Apr., 2024) arXiv:2404.08416, [2404.08416]
Pith/arXiv arXiv 2024
-
[14]
G. H¨ utsi, M. Raidal, V. Vaskonen and H. Veerm¨ ae,Two populations of ligo-virgo black holes,J. Cosmology Astropart. Phys.2021(Mar., 2021) 068, [2012.02786]
Pith/arXiv arXiv 2021
-
[15]
G. Arcadi, D. Cabo-Almeida, M. Dutra, P. Ghosh, M. Lindner, Y. Mambrini et al.,The waning of the wimp: endgame?,European Physical Journal C85(Mar., 2025) 152, [2403.15860]
Pith/arXiv arXiv 2025
-
[16]
N. Bernal, M. Heikinheimo, T. Tenkanen, K. Tuominen and V. Vaskonen,The dawn of FIMP Dark Matter: A review of models and constraints,International Journal of Modern Physics A 32(Sept., 2017) 1730023–274, [1706.07442]
Pith/arXiv arXiv 2017
-
[17]
L. Di Luzio, M. Giannotti, E. Nardi and L. Visinelli,The landscape of qcd axion models, Phys. Rep.870(July, 2020) 1–117, [2003.01100]
Pith/arXiv arXiv 2020
-
[18]
P. S. Cole, A. D. Gow, C. T. Byrnes and S. P. Patil,Primordial black holes from single-field inflation: a fine-tuning audit,J. Cosmology Astropart. Phys.2023(Aug., 2023) 031, [2304.01997]
Pith/arXiv arXiv 2023
-
[19]
Ricotti,Bondi accretion in the early universe,ApJ662(June, 2007) 53–61, [0706.0864]
M. Ricotti,Bondi accretion in the early universe,ApJ662(June, 2007) 53–61, [0706.0864]
Pith/arXiv arXiv 2007
-
[20]
K. J. Mack, J. P. Ostriker and M. Ricotti,Growth of Structure Seeded by Primordial Black Holes,ApJ665(Aug., 2007) 1277–1287, [astro-ph/0608642]
Pith/arXiv arXiv 2007
-
[21]
M. Ricotti, J. P. Ostriker and K. J. Mack,Effect of primordial black holes on the cosmic microwave background and cosmological parameter estimates,ApJ680(June, 2008) 829–845, [0709.0524]. – 57 –
Pith/arXiv arXiv 2008
-
[22]
B. C. Lacki and J. F. Beacom,Primordial Black Holes as Dark Matter: Almost All or Almost Nothing,ApJ720(Sept., 2010) L67–L71, [1003.3466]
Pith/arXiv arXiv 2010
-
[23]
E. Barausse, V. Cardoso and P. Pani,Can environmental effects spoil precision gravitational-wave astrophysics?,Phys. Rev. D89(May, 2014) 104059, [1404.7149]
Pith/arXiv arXiv 2014
-
[24]
B. J. Kavanagh, D. A. Nichols, G. Bertone and D. Gaggero,Detecting dark matter around black holes with gravitational waves: Effects of dark-matter dynamics on the gravitational waveform,Phys. Rev. D102(Oct., 2020) 083006, [2002.12811]
Pith/arXiv arXiv 2020
-
[25]
Y. N. Eroshenko,Dark matter density spikes around primordial black holes,Astronomy Letters 42(June, 2016) 347–356, [1607.00612]
Pith/arXiv arXiv 2016
-
[26]
M. Boudaud, T. Lacroix, M. Stref, J. Lavalle and P. Salati,In-depth analysis of the clustering of dark matter particles around primordial black holes. Part I. Density profiles,J. Cosmology Astropart. Phys.2021(Aug., 2021) 053, [2106.07480]
Pith/arXiv arXiv 2021
-
[27]
J. Adamek, C. T. Byrnes, M. Gosenca and S. Hotchkiss,WIMPs and stellar-mass primordial black holes are incompatible,Phys. Rev. D100(July, 2019) 023506, [1901.08528]
Pith/arXiv arXiv 2019
-
[28]
Vasiliev,Dark matter annihilation near a black hole: Plateau vs
E. Vasiliev,Dark matter annihilation near a black hole: Plateau vs. weak cusp,Phys. Rev. D 76(2007) 103532, [0707.3334]
Pith/arXiv arXiv 2007
-
[29]
S. L. Shapiro and J. Shelton,Weak annihilation cusp inside the dark matter spike about a black hole,Phys. Rev. D93(2016) 123510, [1606.01248]
Pith/arXiv arXiv 2016
-
[30]
S. M. Boucenna, F. K¨ uhnel, T. Ohlsson and L. Visinelli,Novel constraints on mixed dark-matter scenarios of primordial black holes and WIMPs,J. Cosmology Astropart. Phys. 2018(July, 2018) 003, [1712.06383]
Pith/arXiv arXiv 2018
-
[31]
B. Carr, F. K¨ uhnel and L. Visinelli,Black holes and WIMPs: all or nothing or something else, MNRAS506(Sept., 2021) 3648–3661, [2011.01930]
Pith/arXiv arXiv 2021
- [32]
-
[33]
E. U. Gin´ es, O. Mena and S. J. Witte,Revisiting constraints on WIMPs around primordial black holes,Phys. Rev. D106(Sept., 2022) 063538, [2207.09481]
Pith/arXiv arXiv 2022
-
[34]
Planck Collaboration et al.,Planck 2018 results. VI. Cosmological parameters,A&A641 (Sept., 2020) A6, [1807.06209]
Pith/arXiv arXiv 2018
-
[35]
D. J. Fixsen,The Temperature of the Cosmic Microwave Background,ApJ707(Dec., 2009) 916–920, [0911.1955]. [36]Fermi-LATcollaboration, M. Ackermann et al.,The spectrum of isotropic diffuse gamma-ray emission between 100 MeV and 820 GeV,ApJ799(2015) 86, [1410.3696]
Pith/arXiv arXiv 2009
-
[37]
S. Ando and K. Ishiwata,Constraints on decaying dark matter from the extragalactic gamma-ray background,J. Cosmology Astropart. Phys.2015(May, 2015) 024–024, [1502.02007]
Pith/arXiv arXiv 2015
-
[38]
C. Blanco and D. Hooper,Constraints on Decaying Dark Matter from the Isotropic Gamma-Ray Background,JCAP03(2019) 019, [1811.05988]
Pith/arXiv arXiv 2019
-
[39]
D. P. Finkbeiner, S. Galli, T. Lin and T. R. Slatyer,Searching for dark matter in the cmb: A compact parametrization of energy injection from new physics,Phys. Rev. D85(Feb., 2012)
2012
-
[40]
N. Esser, C. Filion, S. De Rijcke, N. Kallivayalil, H. Richstein, P. Tinyakov et al.,Constraints on asteroid-mass primordial black holes in dwarf galaxies using Hubble Space Telescope photometry,A&A698(2025) A290, [2503.03352]. – 58 –
Pith/arXiv arXiv 2025
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.