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Black Hole Binary Formation in the Expanding Universe --- Three Body Problem Approximation ---

Astrophysics · Submitted 2 Jul 1998 · 32 references

Pith's verdict: Real three-body numerics that confirm rather than overturn the 1997 estimate; the rate's error bar is a stack of worst cases, not a propagated uncertainty.

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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  4. T3 Formal statement The claims written out in a formal math language a computer can read.
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arXiv IDastro-ph/9807018 (version 1)
Pith ID7HUH5PZP, Pith's permanent record of this paper and its review
CitationKunihito Ioka, Takahiro Tanaka, Takashi Nakamura, Takeshi Chiba. "Black Hole Binary Formation in the Expanding Universe --- Three Body Problem Approximation ---." arXiv:astro-ph/9807018 (1998). https://pith.science/paper/7HUH5PZP
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Referee report
The paper

The paper claims

Numerical three-body calculations confirm that black-hole MACHO binaries form with semimajor axes and eccentricities close to earlier order-of-magnitude estimates, giving a coalescence rate of about 0.05 events per year per galaxy.

The reading

If the dark matter in our galaxy's halo is made of black holes (the MACHO hypothesis), those black holes would have formed very early, about ten microseconds after the Big Bang. Most would fly apart with the cosmic expansion, but occasionally three would come close enough that two of them lose energy to the third and settle into a bound pair. The paper works out how tight and how eccentric those pairs would be. The authors solve the Newtonian three-body problem for black holes embedded in an expanding universe, running about a hundred numerical experiments for each direction of the third body. They find that the earlier back-of-envelope formulas are right in form: the pair's size scales as the fourth power of the initial separation, and its shape (how elongated the orbit is) scales as the cube of the ratio of the pair separation to the distance of the nearest neighbor. The numerical coefficients come out near one, so the earlier estimates were good to about 50 percent. Assuming the black holes are scattered at random, the authors then derive the full distribution of pair sizes, eccentricities, and merger times. They fold in corrections for the third body sometimes staying bound, for the gravitational pull of more distant black holes, and for the angle at which the third body arrives. The net estimate is about one merger per twenty galaxies per year, with a factor-of-two uncertainty either way. That is enough, they argue, for the first generation of gravitational-wave interferometers to see several events per year out to the Virgo cluster, which would settle whether the MACHOs are black holes.
Extended reading notes

Core claim

The coalescence rate of black hole MACHO binaries is about 5 x 10^-2 x 2^(+/-1) events per year per galaxy, implying at least several coalescences per year within 15 Mpc detectable by the first LIGO/VIRGO interferometers (abstract; Sections 5B and 8). If correct, the MACHO dark matter is made of black holes and gravitational-wave detectors will see a guaranteed source population.

Load-bearing premise

The binary population is generated by Eq. 5.1, P(x,y) = 9 x^2 y^2 / xbar^6 exp(-y^3/xbar^3), which assumes black holes form at rest with respect to the Hubble flow, at random uncorrelated positions, with a delta-function mass function and Omega = Omega_BHM (Sections 2 and 5A). The entire event rate scales with the number of binaries this distribution produces; clustering or a broad mass function would change it by more than the quoted factor of two. The authors acknowledge this in Section 8 and sketch an unequal-mass extension in Appendix A but do not compute it.

Original abstract

We study black hole MACHO binary formation through three-body interactions in the early universe at $t\sim 10^{-5}$s. The probability distribution functions of the eccentricity and the semimajor axis of binaries as well as of the coalescence time are obtained assuming that the black holes are randomly formed in space. We confirm that the previous order-of-magnitude estimate for the binary parameters is valid within $\sim 50%$ error. We find that the coalescence rate of the black hole MACHO binaries is $\sim 5 \times 10^{-2} \times 2^{\pm 1}$ events/year/galaxy taking into consideration several possible factors which may affect this estimate. This suggests that the event rate of coalescing binary black holes will be at least several events per year within 15 Mpc. The first LIGO/VIRGO interferometers in 2001 will be able to verify whether the MACHOs are black holes or not.

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, 5,020 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.04 · states limitation 0.78 · human participants 0.01 · controlled comparison 0.08 · randomized assignment 0.05 · states quantitative result 0.98 · compares with prior results 0.93 · mentions code or data availability 0.02.

The abstract, sentence by sentence

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

We study black hole MACHO binary formation through three-body interactions in the early universe at $t\sim 10^{-5}$s. The probability distribution functions of the eccentricity and the semimajor axis of binaries as well as of the coalescence time are obtained assuming that the black holes are randomly formed in space. We confirm that the previous order-of-magnitude estimate for the binary parameters is valid within $\sim 50%$ error. We find that the coalescence rate of the black hole MACHO binaries is $\sim 5 \times 10^{-2} \times 2^{\pm 1}$ events/year/galaxy taking into consideration several possible factors which may affect this estimate. This suggests that the event rate of coalescing binary black holes will be at least several events per year within 15 Mpc. The first LIGO/VIRGO interferometers in 2001 will be able to verify whether the MACHOs are black holes or not.

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 study black hole MACHO binary formation through three-body interactions in the early universe at $t\sim 10^{-5}$s. contribution 1.00 0.89 0.05 asserted 0.87
s1 The probability distribution functions of the eccentricity and the semimajor axis of binaries as well as of the coalescence time are obtained assuming that the black holes are randomly formed in space. method 0.42 0.91 0.06 asserted 0.98
s2 We confirm that the previous order-of-magnitude estimate for the binary parameters is valid within $\sim 50%$ error. result 0.96 0.87 0.11 asserted 1.00
s3 We find that the coalescence rate of the black hole MACHO binaries is $\sim 5 \times 10^{-2} \times 2^{\pm 1}$ events/year/galaxy taking into consideration several possible factors which may affect this estimate. result 1.00 0.94 0.45 asserted 1.00
s4 This suggests that the event rate of coalescing binary black holes will be at least several events per year within 15 Mpc. interpretation 0.98 0.72 0.32 hedged possibility 1.00
s5 The first LIGO/VIRGO interferometers in 2001 will be able to verify whether the MACHOs are black holes or not. future work 0.81 0.09 0.07 hedged possibility 0.67

What kind of paper the classifier reports this is: theoretical result 0.61 · evidence from simulation or computation 0.75.

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. The trajectories of the second body (thick curve) and
Figure 5. Fig.5. The angle dependence of

Showing 2 of the paper's 5 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:astro-ph/9807018 by the authors.

References
How later papers used it

93 later papers cite it

Evidence: substantial. 90 of the citing papers share no author with it, and 21 of those carry a confident label from Pith's citation classifier.

Pith read 16 of the 166 citing sentences, choosing only those whose citation markers point at this paper alone, and wrote the paraphrases from them. The summary, the counts, the evidence level, the links and the quotes come from code; the summary counts the papers listed below. A later paper is listed under a label when Pith's citation classifier gives one of its sentences that label at probability 0.8 or more (0.95 for support), Pith's reading finds that the citing paper itself did it, and a second check confirms the paraphrase against the sentence. This section lists uses only. It does not list criticism or disagreement, so their absence here says nothing about how the paper has been received. Papers that share an author with this one are listed apart, under "By the same authors". A quote is the citing authors' own sentence, shown when their paper's licence is CC BY, CC BY-SA or CC0. JSON

Uses its method or tool 1 of 8 papers with this label
Uses its data or values 2 of 4 papers with this label
How the literature uses it

83 arXiv papers cite this in 143 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 citing it · 96 sentences without a confident label

Builds on it 3 papers
Uses its method or tool 6 papers
Uses its data or values 4 papers
Compares with it 6 papers
  • The decoupling condition, Eq.([ref]), is very similar to the one used previously [ref] , but differs in one important aspect: it involves the approximate time-dependent Misner-Sharp mass, $ma=m/(1+z)$, instead of the static ADM mass $m$.

    Eliminating the LIGO bounds on primordial black hole dark matter · Introduction · 0.84

  • Solving the equation ([ref]) and ([ref]) for any $x$ (different values of $y$ are taken into account in ([ref]) by the scaling of $ξ$) and comparing with ([ref]), we obtain the following correction factors in ([ref]) [equation] In our calculations these factors are independent of $x$ with high accuracy, however [ref] obtained $α≃0.4$, $β≃0.8$ and weak dependence on the fraction $x/y$.

    Gravitational waves from primordial black holes collisions in binary systems · The evolution of binaries at the stage of radiation domination · 0.98

  • This implies an angular momentum j ≈1.5 x^3/y^3 (x̂ ·ŷ) (x̂ ×ŷ), with magnitude $j ≈0.8 (x/y)^3 sin(2 θ)$, where $θ$ is the angle between $x̂$ and $ŷ$, consistent with the results of Ref. [ref] .

    The merger rate of primordial-black-hole binaries · Torques by other PBHs · 0.95

Cites it as support 2 papers
Cites it as background 20 papers

Discussion

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