Evidence record

What happened in the first three minutes?

Every sentence on the exploration belongs to at least one claim below. Each says what kind of statement it is, what it rests on, what the evidence does not establish, and how far it was checked. Nothing here is a score, and none of it is a verdict on a belief or a theory.

Last checked 24 September 2026.

Source checked: 53
A judgement, not a fact: 3
Presented as open: 1

If something here is wrong, misquoted, out of date or unfair, write to support@unknownoceans.com and give the claim number, which does not change.

What the question takes for granted

A question with a well-tested answer. Big bang nucleosynthesis is a calculation from laboratory physics with one input, checked against the light elements; the page reports where it succeeds (helium, deuterium) and the one place it does not (lithium).

The early universe was hot and dense enough for nuclear reactions
Established
This is the premise of the whole calculation, and its success with helium and deuterium is what the page offers as evidence for it.
Three minutes is when it happened
Established
The abundances are essentially fixed by about 180 seconds, which is where the title’s three minutes comes from. Reactions continue at a trickle for longer: Pitrou and colleagues put the whole window at about twenty minutes, and the page’s timeline says so.
Physics measured in laboratories today applies to the first minutes
Established
The calculation assumes it, and the agreement of its predictions with deuterium, helium and the microwave background is the test. The Particle Data Group describes the concordance as grounds for confidence in the extrapolation, with lithium as the exception.

How the question is labelled

Claim: The first cooking

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: First minutes

Kind: Framing. Signposting. It asserts nothing a source could check.

The question

Claim: The lightest elements were made before the universe was old enough to have a single star in it

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.1, Abstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Predictions of the abundances of the light elements, D,3He, 4He, and7Li, synthesized at the end of thefirst three minutes, are in good overall agreement with the primordial abundances inferred from observational data
Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: these abundances are inferred from observations made at much later epochs, after stellar nucleosynthesis has begun.

What this does not establish

Lithium is the exception: the amount made then is predicted, and the measured amount does not match it (claims 56 and 57).

Later work looked for

Crossref, 2020 on, "big bang nucleosynthesis": the 2025 Particle Data Group update keeps the same account.

Claim 3 · checked 24 September 2026 · AI-assisted audit

Claim: and the recipe can be checked.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: any abundance measurement determinesη, and additional measurements overconstrain the theory and thereby provide a consistency check.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 4 · checked 24 September 2026 · AI-assisted audit

Claim: For a few minutes the whole universe was hot and dense enough to fuse nuclei.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Precision big bang nucleosynthesis with improved helium-4 predictionssection I, Introduction
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: During the first ≈ 20 minutes of the Universe, when it was dense and hot enough for nuclear reactions to take place
Notation restored: Spacing and notation as the arXiv PDF of the review (1801.08023) renders them, checked with pdftotext -layout, 2026-09-24. The text as extracted reads: During the first≈20 minutes of the Universe, when it was dense and hot enough for nuclear reactions to take place
Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: which are essentially fixed by t ∼ 180 s.

What this does not establish

The yields are essentially fixed by about three minutes; reactions go on at a trickle to about twenty.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 5 · checked 24 September 2026 · AI-assisted audit

Claim: The theory predicts how much helium, deuterium and lithium should have come out, using one free number.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Reference work
The source reads: We first consider standard BBN, which is based on Standard Model physics alone, soNν= 3 and the only free parameter is the baryon-to-photon ratioη.

What this does not establish

The one free number assumes three neutrino species and the Standard Model. Once the microwave background fixes that number, the theory has no free parameter at all, as Pitrou and colleagues put it. It also predicts helium-3, which the page leaves out because it cannot be measured cleanly.

Claim 6 · checked 24 September 2026 · AI-assisted audit

Claim: The helium and the deuterium match observation to remarkable precision.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.5, closing paragraph
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: the remarkable concordance between the CMB and the D (as well as ⁴He) abundance, is a non-trivial success
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: the remarkable concordance between the CMB and the D (as well as 4 He) abundance, is a non-trivial success
Context
The LBT Yp Project. IV. A new value of the primordial helium abundanceabstract
Cited · Link answered, 19 October 2026 · Abstract only, 24 September 2026 · Astronomical observation · Measurement
The source reads: This result achieves an unprecedented precision of 0.5%, and it is in good agreement with the Big Bang nucleosynthesis (BBN) result
The newest helium measurement, read as an abstract only.
Qualifies
A new tension in the cosmological model from primordial deuterium?abstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model
The source reads: We point out a new ≃ 1.8σ-tension on the baryonic density, or equivalently on the D/H abundance

What this does not establish

How closely deuterium matches depends on which laboratory rates are used for the reactions that burn it; one team reports a mild tension (claim 40).

Later work looked for

Crossref, 2020 on, "primordial helium abundance" and "primordial deuterium": the LBT helium result (2026) agrees with the prediction at 0.5 per cent; EMPRESS (Matsumoto and others, 2022) finds helium about one sigma lower; the deuterium comparison is argued between teams, which the page now reports.

Claim 7 · checked 24 September 2026 · AI-assisted audit

Claim: The lithium does not.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.5
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: As shown in Fig. 24.1, stellar Li/H measurements are inconsistent with the D/H (and CMB) constraints, given the quoted error budgets.
Supports
Big bang nucleosynthesis: Present statusabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: In contrast with D/H and 4He, 7Li predictions continue to disagree with observations, perhaps pointing to new physics.

Later work looked for

Crossref, 2024 on, "lithium problem": proposals continue (Miranda 2025 argues from chemical evolution that there is no discrepancy), and the 2025 Particle Data Group update still calls the problem unresolved.

Claim 8 · checked 24 September 2026 · AI-assisted audit

Claim: Nuclear fusion needs a furnace.

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: For a short window very early on, the universe itself was one.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Precision big bang nucleosynthesis with improved helium-4 predictionssection I, Introduction
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: During the first ≈ 20 minutes of the Universe, when it was dense and hot enough for nuclear reactions to take place
Notation restored: Spacing and notation as the arXiv PDF of the review (1801.08023) renders them, checked with pdftotext -layout, 2026-09-24. The text as extracted reads: During the first≈20 minutes of the Universe, when it was dense and hot enough for nuclear reactions to take place

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 10 · checked 24 September 2026 · AI-assisted audit

Claim: That window closed after a few minutes, and it left almost nothing but hydrogen and helium.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Big bang nucleosynthesis: Present statussection II A
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: D and ³He are produced at the level of about 10⁻⁵ by number, and ⁷Li at the level of 10⁻¹⁰ by number. A cooling Universe, Coulomb barriers, and the mass gap at A = 8 prevents the production of other isotopes in any significant quantity.
Notation restored: Superscripts and subscripts restored as the arXiv PDF of the review renders them, checked with pdftotext -layout, 2026-09-24. The text as extracted reads: D and 3He are produced at the level of about 10 −5 by number, and 7Li at the level of 10 −10 by number. A cooling Universe, Coulomb barriers, and the mass gap at A = 8 prevents the production of other isotopes in any significant quantity.
Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2; section 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Nearly all neutrons end up bound in the most stable light element ⁴He. Heavier nuclei do not ... form in any significant quantity ... which are essentially fixed by t ∼ 180 s.
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: Nearly all neutrons end up bound in the most stable light element 4 He. Heavier nuclei do not ... form in any significant quantity ... which are essentially fixed by t ∼ 180 s.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Corrected

24 September 2026. Previously: “That window closed after a few minutes, and what it produced is still the bulk of ordinary matter.” None of the cited sources says what share of ordinary matter today is still primordial. What they do say is what the window left: hydrogen and helium, with traces of deuterium, helium-3 and lithium.

Claim 11 · checked 24 September 2026 · AI-assisted audit

Claim: Most of the hydrogen in your body has been hydrogen since then.

Kind: Historical claim. A statement about the past that historical evidence can test.
Review: Source checked. The cited passage was read, and it says this.

Supports
Where your elements came fromexplanation
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026
The source reads: The hydrogen in your body, present in every molecule of water, came from the Big Bang . There are no other appreciable sources of hydrogen in the universe.

What this does not establish

Carried by a NASA explainer written for the public rather than a paper. The hydrogen nuclei are protons left over from before fusion began, not something fusion made.

Claim 12 · checked 24 September 2026 · AI-assisted audit

What the furnace made

Claim: What the furnace made

Kind: Framing. Signposting. It asserts nothing a source could check.

A furnace that cools as it burns

Claim: A furnace that cools as it burns

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: In the first second the universe was hot enough that protons and neutrons converted into each other freely.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: at a temperature T ∼1 MeV , corresponding to an age t ∼1 s. At higher temperatures, weak interactions were in thermal equilibrium
Supports
Big bang nucleosynthesis: Present statussection II A
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: At these temperatures, weak interaction rates between neutrons and protons maintain equilibrium.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 15 · checked 24 September 2026 · AI-assisted audit

Claim: As it cooled, the conversions stopped and the ratio froze at roughly one neutron to six protons, set by the small mass difference between them and the temperature at which that happened.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: This resulted in departure from chemical equilibrium ... ≃ 1 MeV. The neutron fraction at this time ... ≃ 1/6
Qualifies
Big bang nucleosynthesis: Present statussection II A
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: At freeze-out, the neutron-to-proton ratio is given approximately by the Boltzmann factor, (n/p)_f ≃ e^(−∆m/T_f) ∼ 1/5, where ∆m = m_n − m_p is the neutron–proton mass difference.
Notation restored: Superscripts and subscripts restored as the arXiv PDF of the review renders them, checked with pdftotext -layout, 2026-09-24. The text as extracted reads: At freeze-out, the neutron-to-proton ratio is given approximately by the Boltzmann factor, ( n/p)f≃ e−∆m/Tf∼ 1/5, where ∆m = mn−mp is the neutron–proton mass difference.
The Cyburt review gives one to five; the page follows the Particle Data Group.

What this does not establish

One to six is approximate; the Cyburt review puts it at about one to five. The ratio is the mass difference over the freeze-out temperature, and that temperature depends on the weak interaction and on gravity.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Corrected

24 September 2026. Previously: “As it cooled, the conversions stopped and the ratio froze at roughly one neutron to six protons, set by the small mass difference between them.” The frozen ratio is a Boltzmann factor, the mass difference divided by the freeze-out temperature, and the Particle Data Group stresses that the temperature carries the weak and gravitational physics. The mass difference alone does not set it.

Claim 16 · checked 24 September 2026 · AI-assisted audit

Claim: Free neutrons then decayed for the minutes it took fusion to start, and by the time it did the ratio was about one to seven.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: After freeze-out, the neutrons were free to β-decay, so the neutron fraction dropped to n/p ≃ 1/7 by the time nuclear reactions began.
Supports
Precision big bang nucleosynthesis with improved helium-4 predictionssection I C, item 2, and figure 3 (time axis 0 to 300 s)
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: The abundance of neutrons is then about Xₙ ≃ 0.17 and it is eventually reduced to Xᴺᵘᶜₙ ≃ 0.125 by beta decay when nucleosynthesis starts
Notation restored: Spacing and notation as the arXiv PDF of the review (1801.08023) renders them, checked with pdftotext -layout, 2026-09-24. The text as extracted reads: The abundance of neutrons is then aboutXn≃ 0.17 and it is eventu-ally reduced toXNuc n ≃ 0.125 by beta decay when nucleosynthesis starts

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Corrected

24 September 2026. Previously: “by the time it did the ratio was about one in seven.” "One in seven" reads as a fraction of all nucleons, which is a different number; the sources give a ratio of neutrons to protons, as the sentence before does.

Claim 17 · checked 24 September 2026 · AI-assisted audit

Claim: Fusion could not begin immediately.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: photo-dissociation by the high number density of photons delays production of deuterium (and other complex nuclei)

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 18 · checked 24 September 2026 · AI-assisted audit

Claim: Any deuterium formed was broken apart by the ambient radiation almost at once, a delay known as the deuterium bottleneck.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Big bang nucleosynthesis: Present statussection II A
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: the large number of photons relative to nucleons, η−1∼ 109 causes the so-called deuterium bottleneck. BBN is delayed until η−1exp(−EB/T )∼ 1 when the deuterium destruction rate finally falls below its production rate.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 19 · checked 24 September 2026 · AI-assisted audit

Claim: Only when the temperature fell far enough did deuterium survive long enough to be built into helium.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: falls below unity at T ≃ 0.1 MeV; nuclei can then begin to form without being immediately photo-dissociated again.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 20 · checked 24 September 2026 · AI-assisted audit

Claim: By the time the universe was a few minutes old it was too cool and too thin to fuse anything else, and the composition was fixed until stars began to change it.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: which are essentially fixed by t ∼ 180 s. However, these abundances are inferred from observations made at much later epochs, after stellar nucleosynthesis has begun. Stars produce heavier elements such as C, N, O, and Fe
Supports
Big bang nucleosynthesis: Present statussection II A
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: A cooling Universe, Coulomb barriers, and the mass gap at A = 8 prevents the production of other isotopes in any significant quantity.
Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Only 2-body reactions, such as D(p, γ)³He and ³He(D, p)⁴He are important because the density by this time has become rather low – comparable to that of air!
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: Only 2-body reactions, such as D(p, γ)3 He and 3 He(D, p)4 He are important because the density by this time has become rather low – comparable to that of air!

What this does not establish

Beryllium-7 made in the window turns into lithium-7 much later, by capturing an electron; the Cyburt review counts both as mass-7, so the prediction already includes it.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Corrected

24 September 2026. Previously: “and the composition was fixed until the first stars formed many millions of years later.” None of the page’s sources dates the first stars. The Particle Data Group supports the point that matters here: stars are what altered the light elements afterwards.

Claim 21 · checked 24 September 2026 · AI-assisted audit

Claim: The window

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: 1 second

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: at a temperature T ∼1 MeV , corresponding to an age t ∼1 s.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 23 · checked 24 September 2026 · AI-assisted audit

Claim: Neutrons and protons stop converting into each other.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: This resulted in departure from chemical equilibrium ... ≃ 1 MeV.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 24 · checked 24 September 2026 · AI-assisted audit

Claim: The ratio freezes at roughly one to six, then drifts as free neutrons decay.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The neutron fraction at this time, n/p = e^(−Q/T_fr) ≃ 1/6, ... After freeze-out, the neutrons were free to β-decay, so the neutron fraction dropped to n/p ≃ 1/7 by the time nuclear reactions began.
Notation restored: Exponent and subscript restored as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: The neutron fraction at this time, n/p = e−Q/Tfr ≃ 1/6, ... After freeze-out, the neutrons were free to β-decay, so the neutron fraction dropped to n/p ≃ 1/7 by the time nuclear reactions began.

What this does not establish

The Cyburt review puts the frozen ratio nearer one to five.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 25 · checked 24 September 2026 · AI-assisted audit

Claim: The next few minutes

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

The bottleneck clears between freeze-out, at about one second, and the point where the yields are fixed, at about 180 seconds.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissections 24.2 and 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: falls below unity at T ≃ 0.1 MeV; nuclei can then begin to form ... which are essentially fixed by t ∼ 180 s.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Corrected

24 September 2026. Previously: “1 to 3 minutes” The sources place the bottleneck by temperature (about 0.1 MeV), not by clock time, and give only the brackets either side of it: about one second and about 180 seconds. The label now claims no more than that.

Claim 26 · checked 24 September 2026 · AI-assisted audit

Claim: The deuterium bottleneck clears.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Big bang nucleosynthesis: Present statussection II A
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: This occurs when the temperature is approximately T∼EB/ lnη−1∼ 0.1 MeV.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 27 · checked 24 September 2026 · AI-assisted audit

Claim: Deuterium survives, and is almost immediately built into helium.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Big bang nucleosynthesis: Present statussection II A
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: To a good approximation, almost all of the neutrons present when the deuterium bottle-neck breaks end up in 4He.
Supports
Precision big bang nucleosynthesis with improved helium-4 predictionssection I C, item 5
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: Then since the binding energy per nu-cleon of 4He is much larger than for deuterium, a network of reactions ends up in producing nearly only 4He and very tiny amounts of other light el-ements.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 28 · checked 24 September 2026 · AI-assisted audit

Claim: ~3 minutes

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: BBN theory predicts the universal abundances of D, ³He, ⁴He, and ⁷Li, which are essentially fixed by t ∼ 180 s.
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: BBN theory predicts the universal abundances of D, 3 He, 4 He, and 7 Li, which are essentially fixed by t ∼ 180 s.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 29 · checked 24 September 2026 · AI-assisted audit

Claim: Nearly every available neutron is locked into helium-4.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Nearly all neutrons end up bound in the most stable light element ⁴He.
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: Nearly all neutrons end up bound in the most stable light element 4 He.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 30 · checked 24 September 2026 · AI-assisted audit

Claim: Traces of deuterium, helium-3 and lithium remain.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: and ³He at the level of a few times 10⁻⁵ by number relative to H, and ⁷Li/H at the level of about 10⁻¹⁰
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: and 3 He at the level of a few times 10−5 by number relative to H, and 7 Li/H at the level of about 10−10

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 31 · checked 24 September 2026 · AI-assisted audit

Claim: ~20 minutes

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Precision big bang nucleosynthesis with improved helium-4 predictionssection I, Introduction
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: During the first ≈ 20 minutes of the Universe, when it was dense and hot enough for nuclear reactions to take place
Notation restored: Spacing and notation as the arXiv PDF of the review (1801.08023) renders them, checked with pdftotext -layout, 2026-09-24. The text as extracted reads: During the first≈20 minutes of the Universe, when it was dense and hot enough for nuclear reactions to take place

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 32 · checked 24 September 2026 · AI-assisted audit

Claim: Too cool and too sparse for fusion.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Precision big bang nucleosynthesis with improved helium-4 predictionssection I C, item 5
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: Nucleosynthesis is completely over for all elements whenT ≲ 0.01MeV

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 33 · checked 24 September 2026 · AI-assisted audit

Claim: The composition of ordinary matter is set, and nothing touches it until the first stars.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Stars produce heavier elements such as C, N, O, and Fe (collectively referred to as “metals”), and the ejecta from stellar processes can alter the light element abundances from their primordial values.

What this does not establish

Beryllium-7 from the window becomes lithium-7 long before the first stars, by capturing an electron, as the Cyburt review notes; the predictions count the two together.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 34 · checked 24 September 2026 · AI-assisted audit

One number, four predictions

Claim: One number, four predictions

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Big bang nucleosynthesis: Present statussection I
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Review of the field
The source reads: In SBBN, the abundances of the four light nuclei are usually parameterized by the baryon- to-photon ratioη≡nb/nγ

What this does not establish

The four are deuterium, helium-3, helium-4 and lithium-7. Only three can be tested: the Particle Data Group judges helium-3 unusable as a probe.

Claim 35 · checked 24 September 2026 · AI-assisted audit

Claim: The calculation has essentially one free parameter: how much ordinary matter there was, expressed as the ratio of baryons to photons.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Reference work
The source reads: so Nν = 3 and the only free parameter is the baryon-to-photon ratio η.

What this does not establish

Only in the Standard Model with three neutrino species; claim 65 is what happens when that is relaxed.

Claim 36 · checked 24 September 2026 · AI-assisted audit

Claim: Fix that and the yields of all the light nuclei follow from nuclear physics measured in laboratories.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Reference work
The source reads: The nuclear reaction cross sections important for BBN have all been measured at the relevant energies.
Qualifies
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The D(D, n)³He and D(D, p)³H reactions now not only dominate the uncertainty budget, but they can give significantly different D/H predictions depending on whether one uses just the empirical determination of the cross sections, or also uses theory to guide the functional form.
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: The D(D, n)3 He and D(D, p)3 H reactions now not only dominate the uncertainty budget, but they can give significantly different D/H predictions depending on whether one uses just the empirical determination of the cross sections, or also uses theory to guide the functional form.

What this does not establish

The measurements carry uncertainties, and for deuterium the prediction is less precise than the observation; the neutron lifetime is a laboratory input too.

Claim 37 · checked 24 September 2026 · AI-assisted audit

Claim: The same parameter is measured independently from the microwave background, which is a snapshot taken hundreds of thousands of years later by a completely different method.

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Reference work
The source reads: One can determine ηfrom the amplitudes of the acoustic peaks in the CMB angular power spectrum [82], makingitpossibletocomparetwomeasuresof ηusingverydifferentphysics, attwowidelyseparated epochs.
Supports
Planck 2018 results. VI. Cosmological parameterssection 8, Conclusions
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement
The source reads: from times prior to 380 000 years after the Big Bang, defining the last-scattering surface observed via the CMB
Supports
Planck 2018 results. VI. Cosmological parametersabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement
The source reads: baryon density Ωbh2 = 0.0224± 0.0001

What this does not establish

Nearly independent rather than wholly: the standard Planck analysis uses the nucleosynthesis calculation to fix helium, which the Particle Data Group says it depends on only weakly; without it the result barely moves.

Later work looked for

Crossref, 2020 on, "baryon density CMB": the 2025 Particle Data Group update still uses the Planck value; nothing that changes it.

Claim 38 · checked 24 September 2026 · AI-assisted audit

Claim: The two agree, within a margin that turns on laboratory rates for the reactions that burn deuterium.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Reference work
The source reads: this CMB estimate of the baryon density (narrow vertical band) is consistent with the BBN range
Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: they can give significantly different D/H predictions depending on whether one uses just the empirical determination of the cross sec-tions, or also uses theory to guide the functional form.
Supports
Planck 2018 results. VI. Cosmological parameterssection 7.6.1, Primordial deuterium
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Measurement
The source reads: Nuclear rate uncertainties are critically important in the dis-cussion of the compatibility between deuterium measurements and CMB data.

Later work looked for

Crossref, 2020 on, "primordial deuterium" and "d(p,γ)3He LUNA": after the LUNA measurement (2020) Yeh, Olive and Fields and Pisanti and colleagues find agreement and Pitrou and colleagues a 1.8 sigma tension; the page now reports both.

Corrected

24 September 2026. Previously: “The two agree.” The Particle Data Group and Planck both say the closeness of the match depends on which rates are used for the deuterium-burning reactions, and published teams disagree about it (claim 40). A flat "agree" hid a live argument.

Claim 39 · checked 24 September 2026 · AI-assisted audit

Claim: One team (Yeh, Olive and Fields) finds them consistent; another (Pitrou and colleagues) reports a mild tension of about 1.8 standard deviations.

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
The impact of new d(p,γ)3He rates on Big Bang Nucleosynthesisabstract; introduction
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model
The source reads: We thus find consistency among BBN theory, deuterium and ⁴He observations, and the CMB, when using reaction rates fit in our data-driven approach.
Notation restored: Superscript restored as the arXiv PDF renders it, checked with pdftotext, 2026-09-24. The text as extracted reads: We thus find consistency among BBN theory, deuterium and 4 He observations, and the CMB, when using reaction rates fit in our data-driven approach.
Supports
A new tension in the cosmological model from primordial deuterium?abstract; section 3; conclusion
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model
The source reads: We point out a new ≃ 1.8σ-tension on the baryonic density, or equivalently on the D/H abundance ... This is a 1.6σ tension with CMB (3) and 1.84σ tension with CMB+BAO (4). ... This is indeed a mild warning

What this does not establish

Pitrou and colleagues put it at 1.6 standard deviations against the CMB alone and 1.84 with baryon acoustic oscillations added, and call it a mild warning. Yeh, Olive and Fields reach consistency with rates fitted to data; a theoretical rate gives a lower deuterium prediction. Pisanti and colleagues (2021) also find agreement for deuterium; the 2025 Particle Data Group review calls the two measures consistent.

Later work looked for

Crossref, 2021 on, "primordial deuterium tension": nothing later settles it; the Particle Data Group (2025) still says the deuterium prediction depends on how the deuterium-burning rates are fitted.

Claim 40 · checked 24 September 2026 · AI-assisted audit

Claim: That agreement is one of the strongest pieces of evidence that the early universe was hot and dense in the way the model says, because there is no reason a nuclear calculation and a pattern of temperature ripples should return the same number by accident.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Follows the Particle Data Group’s reasoning that two measures made with very different physics at widely separated epochs are a key test. The ranking, "one of the strongest", is the page’s.

Context
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Agreement would endorse the standard picture, while disagreement could point to new physics during/between the BBN and CMB epochs.

Claim 41 · checked 24 September 2026 · AI-assisted audit

Claim: Prediction against measurement

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: About a quarter of the most pristine gas, by mass, is helium-4.

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2; section 24.3, equation 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Reference work
The source reads: the primordial mass fraction of ⁴He, Y_p ≡ ρ(⁴He)/ρ_b ... The primordial ⁴He abundance is best determined through recombination emission lines of He and H in the most metal-poor extragalactic HII (ionized) regions ... Y_p = 0.245 ± 0.003.
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: the primordial mass fraction of 4 He, Yp ≡ ρ(4 He)/ρb ... The primordial 4 He abundance is best determined through recombination emission lines of He and H in the most metal-poor extragalactic HII (ionized) regions ... Yp = 0.245 ± 0.003.
Context
The LBT Yp Project. IV. A new value of the primordial helium abundanceabstract
Cited · Link answered, 19 October 2026 · Abstract only, 24 September 2026 · Astronomical observation · Measurement
The source reads: By weighted average of our 15 low-metallicity targets, we determine y p = 0.08146 ± 0.00058, with the corresponding mass fraction Y p = 0.2458 ± 0.0013.
The newest measurement, read as an abstract only.

What this does not establish

The primordial value is an extrapolation of low-metallicity gas to zero metals, and systematic effects dominate its uncertainty.

Later work looked for

Crossref, 2020 on, "primordial helium abundance": the LBT project (Aver and others, 2026) gives 0.2458 ± 0.0013, in agreement; EMPRESS (Matsumoto and others, 2022) gives 0.2370, about one sigma lower by its own account. Still about a quarter.

Corrected

24 September 2026. Previously: “About a quarter of ordinary matter by mass is helium-4, everywhere anyone looks.” "Everywhere anyone looks" overstated the measurement: the primordial helium figure comes from the most metal-poor ionised gas in nearby dwarf galaxies, extrapolated to zero metals, and independently from the microwave background. Stars add helium, so it is not the same everywhere.

Claim 43 · checked 24 September 2026 · AI-assisted audit

Claim: The microwave background, read on its own, finds the same.

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Reference work
The source reads: The CMB damping tail is sensitive to the primordial4He abundance independently of both BBN and local4He measurements [90]. ThePlanck analysis using TT, TE, EE+lowE and lensing but not the BBNYp(η) relation gives a4He mass fraction0.239+0.024

What this does not establish

Much less precisely: about 0.24, with an uncertainty near 0.025 at 95 per cent confidence.

Later work looked for

Crossref, 2020 on, "CMB helium fraction": nothing that changes it.

Claim 44 · checked 24 September 2026 · AI-assisted audit

Claim: Made before the stars.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: These data confirm that the stellar contribution to the helium abundance is positively correlated with metal production, so extrapolation to zero metallicity gives the primordial4He abundanceYp.

Later work looked for

How the process works; a textbook account that later work refines in its numbers, not in its shape.

Claim 45 · checked 24 September 2026 · AI-assisted audit

Claim: The helium stars add rises with the metals they make, and traced back to gas with no metals at all, a quarter is still there.

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Reference work
The source reads: These data confirm that the stellar contribution to the helium abundance is positively correlated with metal production, so extrapolation to zero metallicity gives the primordial ⁴He abundance Y_p.
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: These data confirm that the stellar contribution to the helium abundance is positively correlated with metal production, so extrapolation to zero metallicity gives the primordial 4 He abundance Yp .

What this does not establish

An extrapolation, not a sighting of metal-free gas.

Later work looked for

Crossref, 2020 on, "primordial helium abundance": the newest determinations use the same method and agree.

Corrected

24 September 2026. Previously: “No population of stars could have enriched every galaxy to the same floor.” No source argues the old sentence. The Particle Data Group gives the actual reason the helium is primordial: stellar helium rises with metals, and the zero-metal intercept is still about a quarter.

Claim 46 · checked 24 September 2026 · AI-assisted audit

Claim: Deuterium abundance in gas clouds along the sight lines to distant quasars.

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Reference work
The source reads: High-resolution spectra reveal the presence of deuterium in high-redshift, low-metallicity quasar absorption systems through its isotope-shifted Lyman-αabsorption lines.

What this does not establish

Twelve high-precision determinations, as of the 2025 review.

Later work looked for

Crossref, 2020 on, "primordial deuterium abundance": the 2025 review adds a twelfth system; nothing that changes the method.

Claim 47 · checked 24 September 2026 · AI-assisted audit

Claim: Now measured more precisely than the theory predicts it,

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Even so, the nuclear uncertainties still leave D/H prediction errors larger than those of the observations
Supports
Big bang nucleosynthesis: Present statusabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: New D/H observations are now more precise than the corresponding theoretical predictions

Later work looked for

Crossref, 2020 on, "d(p,γ)3He LUNA": Yeh, Olive and Fields (2021) find the observation still more precise than the prediction after LUNA.

Claim 48 · checked 24 September 2026 · AI-assisted audit

Claim: which is an unusual position for a cosmological quantity.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s own remark on claim 48; no source ranks it.

Claim 49 · checked 24 September 2026 · AI-assisted audit

Claim: Lithium-7 in metal-poor stars sits well below the predicted value.

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.3, equation 24.4; section 24.5, equation 24.10
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Reference work
The source reads: The best-suited objects for determining the primordial abundance of ⁷Li are metal-poor stars in the Galactic halo ... Li/H|p = (1.45±0.25)×10⁻¹⁰. (24.4) ... stellar Li/H measurements are inconsistent with the D/H (and CMB) constraints ... Li/H|p = (4.72±0.7)×10⁻¹⁰ (24.10) exceeds the prediction of eq. 24.4 by a factor of 3.3
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: The best-suited objects for determining the primordial abundance of7Li are metal-poor stars in the Galactic halo ... Li/H|p = (1.45±0.25)×10−10. (24.4) ... stellar Li/H measurements are inconsistent with the D/H (and CMB) constraints ... Li/H|p = (4.72±0.7)×10−10 (24.10) exceeds the prediction of eq. 24.4 by a factor of 3.3
Equation 24.4 is the value derived from the stars and 24.10 the primordial value from the calculation, so the stars sit a factor of 3.3 below it; the chapter’s word “prediction” for 24.4 is its own.

Later work looked for

Crossref, 2024 on, "lithium problem" and "Spite plateau": nothing that removes the gap.

Corrected

24 September 2026. Previously: “Lithium-7 in the oldest stars sits well below the predicted value.” The measurement is made in metal-poor halo stars, selected by metallicity, not by age; the very lowest-metallicity stars are excluded because their lithium scatters.

24 September 2026. Previously: “Lithium-7 in old, metal-poor stars sits well below the predicted value.” The sources describe the stars as metal-poor halo stars and say nothing of their age; “old” went beyond them.

Claim 50 · checked 24 September 2026 · AI-assisted audit

Claim: Either the stars destroy it, or the measurements are off, or something happened that the model does not include.

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.5
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The question then becomes pressing as to whether this mismatch comes from systematic errors in the observed abundances, and/or uncertainties in stellar astrophysics or nuclear inputs, or whether there might be new physics at work

Later work looked for

Crossref, 2024 on, "lithium problem": the options are unchanged; the 2025 review leans to stellar depletion.

Corrected

24 September 2026. Previously: “Either the stars destroy it, or the nuclear inputs are off, or something happened that the model does not include.” The page itself says further down that the nuclear fix has been all but closed off, which both reviews support; offering it here as a live option contradicted that. Measurement error, which the Particle Data Group lists, replaces it.

Claim 51 · checked 24 September 2026 · AI-assisted audit

The one that does not fit

Claim: The one that does not fit

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Two predictions confirmed, one refused and one that cannot yet be tested

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Big bang nucleosynthesis: Present statussection VII, conclusion
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: Is two out of three okay? Despite the success of the BBN predictions for 4He and D/H, there remains a problem with 7Li
Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Consequently, we consider it inappropriate to use3He (and also D+3He) as a cosmological probe.

Later work looked for

Crossref, 2020 on, "primordial helium-3": the 2025 review still declines to use helium-3.

Corrected

24 September 2026. Previously: “Three predictions confirmed and one refused” Of the four predictions, helium-4 and deuterium are confirmed, lithium-7 is refused, and helium-3 has no reliable observational test. The Cyburt review’s own summary is "two out of three".

Claim 53 · checked 24 September 2026 · AI-assisted audit

Claim: is what a theory looks like when it is being used rather than admired.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s own reading of claim 53.

Claim 54 · checked 24 September 2026 · AI-assisted audit

The lithium problem

Claim: The lithium problem

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: The predicted amount of lithium-7 is about three times what is measured in the atmospheres of metal-poor stars in the halo of the galaxy.

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
Big bang nucleosynthesis: Present statussection VII, conclusion
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Review of the field
The source reads: The predicted primordial abundance is about a factor of three higher than the abundance determined from absorption lines seen in a population of low metallicity halo stars.

What this does not establish

The Particle Data Group treats the stellar value as a lower bound on the primordial one, since the stars may have destroyed some of it.

Later work looked for

Crossref, 2024 on, "lithium problem": the factor is unchanged in substance; see claim 57 for the updated figure.

Corrected

24 September 2026. Previously: “measured in the atmospheres of the oldest, least polluted stars in the galaxy.” The Particle Data Group derives the value from halo stars in a middle metallicity range and excludes the least polluted ones, whose lithium scatters for reasons nobody knows. "Least polluted" said the opposite.

24 September 2026. Previously: “measured in the atmospheres of old, metal-poor stars in the halo of the galaxy.” The sources describe the stars as low-metallicity halo stars and say nothing of their age; “old” went beyond them.

Claim 56 · checked 24 September 2026 · AI-assisted audit

Claim: The Particle Data Group puts the gap at a factor of 3.3, a 4.4 sigma discrepancy,

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.5, equation 24.10
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Reference work
The source reads: Li/H|p = (4.72±0.7)×10⁻¹⁰ (24.10) exceeds the prediction of eq. 24.4 by a factor of 3.3, corresponding to a 4.4σ discrepancy.
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: Li/H|p = (4.72±0.7)×10−10 (24.10) exceeds the prediction of eq. 24.4 by a factor of 3.3, corresponding to a4.4σdiscrepancy.

What this does not establish

The factor compares one prediction with one stellar estimate; both have uncertainties.

Later work looked for

Particle Data Group, 2025 update: factor 3.3 replaces 3.1; page updated.

Corrected

24 September 2026. Previously: “The Particle Data Group puts the gap at a factor of 3.1, a 4.4 sigma discrepancy,” The 2025 update revised the stellar lithium estimate to (1.45 ± 0.25) × 10^-10, which makes the factor 3.3; the discrepancy stays at 4.4 sigma. The citation now points at the 2025 edition.

Claim 57 · checked 24 September 2026 · AI-assisted audit

Claim: and it has survived decades of improved measurements on both sides.

Kind: Historical claim. A statement about the past that historical evidence can test.
Review: Source checked. The cited passage was read, and it says this.

A dozen years to 2015 in the Cyburt review, and still open in the 2025 update: more than two decades.

Supports
Big bang nucleosynthesis: Present statussection V, The lithium problem
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: This discrepancy constitutes the “Lithium Problem”, which was foreshadowed before CMB determinations of η, and has persisted over the dozen years since the first WMAP data release.
Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.5
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The cosmological lithium problemremains an unresolved issue in BBN.

Claim 58 · checked 24 September 2026 · AI-assisted audit

Claim: One ordinary explanation has been all but closed off.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.5
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: However, experimental searches have now closed off these possibilities [98–100], making a nuclear fix increasingly unlikely.
Supports
Big bang nucleosynthesis: Present statussection V
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: The upshot is that a “nuclear option” to the lithium problem is essentially excluded.

Later work looked for

Crossref, 2024 on, "lithium problem nuclear": nothing that reopens a nuclear fix.

Corrected

24 September 2026. Previously: “One ordinary explanation has been closed off.” The reviews say "essentially excluded" (2016) and "increasingly unlikely" (2025), not closed.

Claim 59 · checked 24 September 2026 · AI-assisted audit

Claim: The reviews work through the reactions in which an unmeasured resonance could have destroyed more of the beryllium-7 that later becomes lithium, and the experiments that looked have found none.

Kind: Experimental result. What an experiment or trial found, under its own conditions. A result in animals, cells or a simulation is not a result in people.
Review: Source checked. The cited passage was read, and it says this.

Supports
Big bang nucleosynthesis: Present statussection V
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Physical systems · Review of the field
The source reads: it is possible to systematically study all two-body reactions that enhance the destruction of ⁷Be. ... all rule out a ⁹B resonance. Similarly, ¹⁰C data rule out the needed resonance in ¹⁰C
Notation restored: Mass numbers set as superscripts, as the arXiv PDF of the review renders them, checked with pdftotext -layout, 2026-09-24. The text as extracted reads: it is possible to systematically study all two-body reactions that enhance the destruction of 7Be. ... all rule out a 9B resonance. Similarly, 10C data rule out the needed resonance in 10C
Supports
Big bang nucleosynthesis: Present statussection II A, on mass-7
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Review of the field
The source reads: Thus 7Be decays long after the ∼ 3 min timescale of BBN, yet after recombination, all mass-7 takes the form of 7Li.

Later work looked for

Crossref, 2024 on, "7Be destruction resonance": nothing that reverses the searches.

Corrected

24 September 2026. Previously: “The reviews work through the reactions that could have hidden the lithium behind an unknown resonance, and the experiments that looked have found none, which leaves either something those stars do to their own lithium, or physics the calculation does not contain.” A resonance would not hide lithium; it would destroy more beryllium-7, which is where most primordial lithium comes from. The sentence is split, and its last clause is corrected in claim 61.

Claim 60 · checked 24 September 2026 · AI-assisted audit

Claim: That leaves errors in the stellar measurements, something those stars do to their own lithium, or physics the calculation does not contain.

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.5
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: whether this mismatch comes from systematic errors in the observed abundances, and/or uncertainties in stellar astrophysics or nuclear inputs, or whether there might be new physics at work

Corrected

24 September 2026. Previously: “which leaves either something those stars do to their own lithium, or physics the calculation does not contain.” The Particle Data Group lists systematic error in the observed abundances as an option alongside the two the page named.

Claim 61 · checked 24 September 2026 · AI-assisted audit

Claim: The Particle Data Group now counts the stars as the leading explanation,

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.5
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: This strengthens the case for stellar depletion as the leading resolution to the lithium problem

What this does not establish

The same review says stellar depletion to the required level generally needs fine-tuning or ad hoc mechanisms.

Corrected

24 September 2026. Previously: “” Added: the 2025 review reads the recent lithium-6 limits as strengthening stellar depletion, which the page had not reported.

Claim 62 · checked 24 September 2026 · AI-assisted audit

Claim: and still calls the problem unresolved.

Kind: Open question. Something the page says is not known.
Review: Presented as open. The page says this is unresolved, and the review found that it is.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.5
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The cosmological lithium problemremains an unresolved issue in BBN.

Claim 63 · checked 24 September 2026 · AI-assisted audit

How much room is left for new physics

Claim: How much room is left for new physics

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Because the yields depend on how fast the universe expanded, they constrain anything that would have added to the energy density at the time, including additional species of neutrino.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Reference work
The source reads: The presence of additional neutrino flavors (or of any otherrelativisticspecies)atthistimeincreases g∗, hencetheexpansionrate, leadingtoalargervalue ofTfr,n/p, and thereforeYp

What this does not establish

The review states it for relativistic species; other additions act through the same expansion rate but are model-dependent.

Claim 65 · checked 24 September 2026 · AI-assisted audit

Claim: It is a constraint rather than a detection.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.4, after equation 24.9
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: which is consistent with the Standard Model.

Later work looked for

Particle Data Group 2025 update: ACT and SPT report effective neutrino numbers of 2.86 and 2.83, also consistent; no extra species detected.

Claim 66 · checked 24 September 2026 · AI-assisted audit

Claim: Counting neutrino families from the light elements returns close to the three the Standard Model has: 2.898 plus or minus 0.141 at 68 per cent confidence.

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
Review of Particle Physics, chapter 24: Big Bang Nucleosynthesissection 24.4, equation 24.9
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Reference work
The source reads: A maximum likelihood analysis on η and N_ν based on ⁴He and D abundances nearly identical to those above finds the (correlated) 68% CL ranges to be η₁₀ = 6.088 ± 0.054 and [45, 78, 86] N_ν = 2.898 ± 0.141
Notation restored: Mass numbers and powers of ten set as superscripts, as the Particle Data Group PDF renders them; checked against the PDF (1 December 2025) with pdftotext -layout, 2026-09-24. The text as extracted reads: A maximum likelihood analysis on η and Nν based on 4 He and D abundances nearly identical to those above finds the (correlated) 68% CL ranges to be η10 = 6.088 ± 0.054 and [45, 78, 86] Nν = 2.898 ± 0.141
Context
The impact of new d(p,γ)3He rates on Big Bang Nucleosynthesisabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model
The source reads: giving the effective number of light neutrino species N_ν = 2.880 ± 0.144 in good agreement with the Standard Model of particle physics.
Notation restored: Superscript restored as the arXiv PDF renders it, checked with pdftotext, 2026-09-24. The text as extracted reads: giving the effective number of light neutrino species Nν = 2.880±0.144 in good agreement with the Standard Model of particle physics.

What this does not establish

Strictly the effective number of light, nearly massless species, fitted jointly with the baryon density; it is not a count of particle families in the laboratory sense.

Later work looked for

Crossref, 2020 on, "effective number of neutrinos BBN": Yeh, Olive and Fields (2021) get 2.880 ± 0.144; the 2025 review keeps 2.898 ± 0.141.

Claim 67 · checked 24 September 2026 · AI-assisted audit

What was read

Big bang nucleosynthesis: Present statusCyburt, Fields, Olive and Yeh, Reviews of Modern Physics · arXiv:1505.01076v1 (5 May 2015), the accepted text of the Reviews of Modern Physics article; read abstract, sections I, II A, IV, V (the lithium problem) and VI, and the conclusionRead in full, 24 September 2026 · Copyright; quoted brieflySection numbers are the arXiv version’s.
Planck 2018 results. VI. Cosmological parametersPlanck Collaboration, Astronomy & Astrophysics · arXiv:1807.06209v4 (August 2021), the version of record’s text; read abstract, section 7.6.1 and section 8Read in full, 24 September 2026 · Copyright; quoted briefly
Review of Particle Physics, chapter 24: Big Bang NucleosynthesisFields, Molaro and Sarkar, Particle Data Group · Particle Data Group, Review of Particle Physics chapter 24, revised August 2025, PDF dated 1 December 2025 (2024 edition and 2025 update); read in full. The 2024 edition (revised August 2023) was also read, to see what changed.Read in full, 24 September 2026 · Copyright; quoted brieflyThe first extraction joined some words to the symbols beside them ("element4He"). The same PDF was extracted again with pdftotext in the integrity pass, which keeps the spacing but sets superscripts on the line ("4 He"); where an excerpt carries one, the reader is shown the notation restored.
Precision big bang nucleosynthesis with improved helium-4 predictionsPitrou, Coc, Uzan and Vangioni, Physics Reports · arXiv:1801.08023v3 (6 February 2025); read abstract, section I and section I C, Main eras of BBNRead in full, 24 September 2026 · Copyright; quoted briefly
The impact of new d(p,γ)3He rates on Big Bang NucleosynthesisYeh, Olive and Fields, Journal of Cosmology and Astroparticle Physics · arXiv:2011.13874, abstract and full text (PDF, pdftotext)Read in full, 24 September 2026 · arXiv non-exclusive distribution licence; quoted brieflyThe abstract was first read as served, with its LaTeX markup; the full text was read from the arXiv PDF in the integrity pass, and excerpts now come from it.
A new tension in the cosmological model from primordial deuterium?Pitrou, Coc, Uzan and Vangioni, Monthly Notices of the Royal Astronomical Society · arXiv:2011.11320v3, abstract and full text (PDF, pdftotext)Read in full, 24 September 2026 · CC BY 4.0The abstract was first read as served, with its LaTeX markup; the full text was read from the arXiv PDF in the integrity pass, and excerpts now come from it.
The LBT Yp Project. IV. A new value of the primordial helium abundanceAver, Skillman, Pogge and others, The Astrophysical Journal · Crossref record of the Astrophysical Journal article (published 9 September 2026), abstractAbstract only, 24 September 2026 · Copyright; quoted briefly
Where your elements came fromNASA, Astronomy Picture of the Day, 24 October 2017 · apod.nasa.gov/apod/ap171024.html, the full explanationRead in full, 24 September 2026 · Copyright; quoted brieflyAstronomy Picture of the Day is written by professional astronomers for a general audience; it is cited here for one plain statement, not for a measurement.