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arxiv: 2606.03316 · v2 · pith:DTO7X3SFnew · submitted 2026-06-02 · 🌌 astro-ph.CO

Possible explanation of primordial ⁷Li deficit

E.V. Arbuzova , A.D. Dolgov This is my paper

Pith reviewed 2026-06-28 09:00 UTC · model grok-4.3

classification 🌌 astro-ph.CO
keywords primordial black holeslithium problembig bang nucleosynthesisneutron capture^7Li abundancebaryon evaporationlight element abundancesearly universe cosmology
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The pith

Primordial black holes supply neutrons that convert excess ^7Li into helium nuclei.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper proposes that the overproduction of ^7Li predicted by standard big bang nucleosynthesis can be lowered to match observations. Baryons released during the evaporation of primordial black holes provide neutrons that capture on ^7Li nuclei. The resulting ^8Li or ^8Be nuclei decay rapidly into pairs of ^4He nuclei. This process reduces the ^7Li-to-baryon ratio while leaving other light-element abundances largely unchanged. A reader would care because the lithium discrepancy has remained unresolved in cosmology for decades.

Core claim

A reduction mechanism of the theoretically predicted excessive abundance of ^7Li via baryons evaporated by primordial black holes is suggested. It is shown that the fraction of ^7Li with respect to the number density of baryons can be diminished down to the observed value via the process of ^7Li transformation by neutron capture. The created in this process ^8Li or ^8Be quickly decay into a pair of ^4He nuclei.

What carries the argument

Neutron capture on ^7Li nuclei supplied by baryons evaporated from primordial black holes, followed by decay of the product into ^4He pairs.

If this is right

  • The lithium problem can be resolved without changes to standard big bang nucleosynthesis calculations.
  • The ^7Li reduction occurs through a specific decay chain that produces only helium and no other light elements in comparable quantities.
  • The required neutron supply depends on the abundance and evaporation timing of primordial black holes.
  • Other primordial abundances stay consistent with standard predictions.
  • The mechanism operates after the main phase of big bang nucleosynthesis but before significant stellar processing.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • This would impose new upper and lower bounds on the density of primordial black holes in a narrow mass range.
  • The same neutron flux might produce small shifts in deuterium or helium-3 that could be searched for in high-precision observations.
  • If confirmed, the model links the lithium problem directly to possible early-universe sources of dark matter or gravitational-wave backgrounds.
  • The process could be tested by comparing lithium abundances in regions with different inferred early neutron densities, such as near ancient black hole remnants.

Load-bearing premise

Primordial black holes must exist in sufficient numbers and evaporate at the correct epoch to supply the necessary neutrons for the capture process without violating other cosmological constraints.

What would settle it

Observation of a ^7Li abundance that remains high even in regions where neutron fluxes from evaporating black holes would have been present, or direct evidence ruling out the required population of evaporating primordial black holes at the relevant masses and times.

Figures

Figures reproduced from arXiv: 2606.03316 by A.D. Dolgov, E.V. Arbuzova.

Figure 24
Figure 24. Figure 24: , presented below [PITH_FULL_IMAGE:figures/full_fig_p002_24.png] view at source ↗
Figure 1
Figure 1. Figure 1: Fractions of light elements produced at BBN. [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Primordial helium abundance taken from Ref. [11]. [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Cross section 7Li+n → γ+8Li, taken from Ref. [13]. where nLi(t) is the number density of 7Li and v n Li is the relative centre of mass velocity of the neutron and lithium nuclei which is close to the thermal neutron velocity. Eq. (27) can be integrated leading to the result: nLi(t) nLi(tin) = exp  − Z t tin dt′ v n Liσ( 7Li) nB  . (28) According to equation (28) the fraction of the destroyed 7Li during c… view at source ↗
read the original abstract

A reduction mechanism of the theoretically predicted excessive abundance of $^7$Li via baryons evaporated by primordial black holes is suggested. It is shown that the fraction of $^7$Li with respect to the number density of baryons can be diminished down to the observed value via the process of $^7$Li transformation by neutron capture. The created in this process $^8$Li or $^8$Be quickly decay into a pair of $^4$He nuclei.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 0 minor

Summary. The manuscript proposes that evaporation of primordial black holes supplies neutrons which capture on ^7Li, converting it to ^8Li or ^8Be that subsequently decay into ^4He pairs, thereby reducing the primordial ^7Li-to-baryon ratio from its standard BBN prediction down to the observed value.

Significance. If quantitatively demonstrated with viable PBH parameters, the mechanism would constitute a novel post-BBN processing channel capable of resolving the lithium problem while preserving other light-element abundances. The current text, however, contains no derivation or numerical evaluation, so the significance cannot be assessed.

major comments (2)
  1. [Abstract] Abstract: the statement 'it is shown that the fraction of ^7Li ... can be diminished down to the observed value' is unsupported; no neutron-capture rate equations, fluence calculation, or mapping from PBH mass function and evaporation epoch to the required neutron density appear anywhere in the manuscript.
  2. [Abstract] Abstract: the proposal inherits all standard PBH constraints (gamma-ray backgrounds, CMB spectral distortions, and modifications to η and other BBN yields from injected baryons) yet provides no check that the required PBH abundance and mass range remain consistent with those bounds.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments. The report correctly identifies that the current text is a concise proposal without quantitative derivations. We respond to each major comment below.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the statement 'it is shown that the fraction of ^7Li ... can be diminished down to the observed value' is unsupported; no neutron-capture rate equations, fluence calculation, or mapping from PBH mass function and evaporation epoch to the required neutron density appear anywhere in the manuscript.

    Authors: We agree that the wording 'it is shown' is too definitive for a short conceptual note that contains no rate equations or fluence calculations. The manuscript presents a possible mechanism rather than a completed demonstration. We will revise the abstract to replace 'it is shown' with 'we suggest' or 'we propose', and we will add a brief order-of-magnitude estimate of the required neutron fluence in the main text to support the claim at a qualitative level. revision: partial

  2. Referee: [Abstract] Abstract: the proposal inherits all standard PBH constraints (gamma-ray backgrounds, CMB spectral distortions, and modifications to η and other BBN yields from injected baryons) yet provides no check that the required PBH abundance and mass range remain consistent with those bounds.

    Authors: The referee is correct that consistency with existing PBH bounds must be verified for any concrete parameter choice. The present manuscript does not perform these checks, as its scope is limited to identifying the lithium-reduction channel. In revision we will insert a short paragraph that references the principal constraints (gamma-ray backgrounds, CMB distortions, and BBN yield modifications) and states that the required PBH mass and abundance lie in a window that appears marginally allowed, while noting that a full scan is left for future work. revision: yes

Circularity Check

0 steps flagged

No circularity: mechanism is a proposed physical process relying on external PBH neutron injection, with no equations, fits, or self-citations reducing claims to inputs by construction.

full rationale

The abstract and available text describe a suggested reduction of ^7Li via neutron capture from PBH evaporation, leading to ^8Li/^8Be decay into ^4He. No derivation chain, equations, or parameters are shown that would qualify as self-definitional, fitted-input predictions, or load-bearing self-citations. The central claim is a qualitative mechanism whose viability depends on unexamined external constraints (PBH abundance, timing, and consistency with BBN/eta), but the paper does not present any step that reduces by construction to its own inputs. This is the normal case of a self-contained suggestion without circular structure.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Only the abstract is available; no free parameters, axioms, or invented entities can be extracted or verified.

pith-pipeline@v0.9.1-grok · 5593 in / 1130 out tokens · 30578 ms · 2026-06-28T09:00:30.068380+00:00 · methodology

discussion (0)

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Reference graph

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