Primordial ⁷Li: nuclear-reaction-rate uncertainties versus stellar depletion in the Spite plateau
Open for decades, but with visible nuclear (A = 7 rates) and stellar (depletion with small scatter) routes, so it sits below the old 47. Leverage 26 is close to the old 24: BBN parameter inference and halo-star physics depend on it, little else. Application value 4.5.
The problem
The Big Bang predicts three times more lithium than old stars show. Is it nuclear data, stars or new physics?
- 1Big Bang theory predicts lithium amount
- 2Old stars show about three times less
- 3Maybe a reaction rate is wrong
- 4Maybe stars destroyed some lithium
- 5Or maybe new physics is needed
The discrepancy. Standard Big Bang nucleosynthesis (BBN), with the baryon density fixed by the CMB, predicts a ⁷Li/H ratio of about 5×10⁻¹⁰. Metal-poor halo stars on the Spite plateau show about 1.6×10⁻¹⁰, a factor of about 3 lower. Deuterium and helium agree with the CMB-inferred baryon density, so only lithium is anomalous.
Path to understanding
foundations → problemThe research entries this problem rests on, in the order to read them. Each links to its explainer.
- Start with
- Step 2
- Step 3
- Step 4
- Thermonuclear reaction rates and nucleosynthesisTheory · host
- Then
- Primordial ⁷Li: nuclear-reaction-rate uncertainties versus stellar depletion in the Spite plateauThe problem
State of the art
The n_TOF measurement of ⁷Be(n,p)⁷Li (arXiv:1803.05701, PRL 121, 042701, 2018) found a higher low-energy cross-section than evaluated but, in the BBN energy range, agreed with inferences from the time-reversed reaction, so it eases the problem only modestly. ⁷Be(n,α) and ⁷Be(d,p) have been measured and also do not close the gap. Coc & Vangioni's review of primordial nucleosynthesis (arXiv:1707.01004) and the 'bitter pill' analysis (arXiv:0808.2818) summarise the factor of about 3 discrepancy; deuterium and helium agree with CMB-inferred baryon density, so only lithium is anomalous. Stellar-depletion models that reduce the primordial value by the needed factor of 3 struggle to produce the observed small scatter and the observed dispersion at the lowest metallicities. A January 2026 study of ⁷Be weak and radiative transition rates in the BBN plasma finds only percent-level corrections, leaving the factor of 3–4 overprediction intact (Taioli et al., arXiv:2601.12438). No 2024–2026 rate measurement or depletion model accepted as resolving the problem was found.
Realistic applications
The resolution would confirm the BBN framework for early-universe parameter inference and the nuclear inputs used in stellar models, or reveal new physics in the first minutes. The nuclear cross-sections also feed solar-neutrino calculations. No other application.
What counts as solved
Resolved when a complete measured set of destruction and production rates for A = 7 nuclei, with percent-level uncertainty, together with a stellar depletion model consistent with observed scatter, reproduces the Spite plateau, or when a physical mechanism beyond the standard model is demonstrated.
How it gets verified
ObservationSettled by new measurements or observations.
Needs new laboratory nuclear-reaction measurements for A = 7 nuclei with percent-level uncertainty, together with stellar depletion models consistent with the observed scatter, so that the Spite plateau is reproduced, or a demonstrated mechanism beyond the standard model.
Claims under verification
+ add a claimNo claimed results on the Verification board for this problem yet.
Where the work is
Measurements of nuclear cross-sections and stellar spectroscopy, with theory of depletion in halo stars.
Bottlenecks
ranked by votes- #1TheoryStellar depletion with small scatter
Atomic diffusion and turbulent mixing models must produce a factor-of-3 depletion without breaking the narrow plateau.
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Progress
- Jan 18, 2026⁷Be transition rates revisitedsource
Plasma corrections to ⁷Be weak and radiative rates are percent-level and do not resolve the problem.
- Mar 15, 2018n_TOF ⁷Be(n,p) measurementsource
n_TOF measures ⁷Be(n,p)⁷Li over a wide energy range; the result eases but does not remove the problem (PRL 121, 042701).
- Jul 4, 2017BBN reviewsource
Coc & Vangioni review the status of primordial nucleosynthesis.
- Aug 21, 2008Lithium problem quantifiedsource
Cyburt, Fields & Olive show a factor of about 3 discrepancy.