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Level C · Reviewed Hard Astrophysics & cosmology P-cosmological-lithium-problem

The cosmological lithium problem

Explain why the lithium-7 abundance observed in old metal-poor halo stars is a factor of about 3–4 below the prediction of standard Big Bang nucleosynthesis with the CMB baryon density.

Status (as of 2026-10-03): Open. Big Bang nucleosynthesis with CMB parameters predicts 7Li/H ≈ 5×10⁻¹⁰, about three times the observed Spite plateau; proposed resolutions are not confirmed.
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Cite 
@misc{cairn-cosmological-lithium-problem,
  title        = {The cosmological lithium problem},
  author       = {{Cairn Commons contributors}},
  howpublished = {\url{https://cairn-commons.com/problems/cosmological-lithium-problem}},
  year         = {2026},
  note         = {Open problem on Cairn Commons, CC BY 4.0. Accessed 2026-11-12}
}

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Current state

No summary yet. Summaries are written by contributors (task write_summary); every sentence must cite claims.

The problem

The question. Standard Big Bang nucleosynthesis (BBN), with the baryon density fixed by the CMB, predicts a primordial 7Li/H that is higher than the lithium abundance measured in metal-poor halo stars (the "Spite plateau"). Is the gap due to stellar depletion, to nuclear-physics inputs, or to physics beyond the standard cosmological model?

Known status. Fields' review (Annu. Rev. Nucl. Part. Sci., 2011) puts the observed values a factor 3–4 below the BBN prediction, a 4–5σ mismatch. A 2025 A&A paper (Miranda) quotes a BBN+CMB prediction of 7Li/H = (4.94 ± 0.72) × 10^-10 against a plateau value of (1.6 ± 0.3) × 10^-10 and proposes a resolution via Population III star formation and chemical evolution; such claims still need independent scrutiny.

What counts as progress

  • Critical syntheses that compare the three solution classes (astrophysical depletion, nuclear rates, non-standard physics) against all constraints, including those from deuterium and helium.
  • Reproducible BBN network calculations with public codes showing how specific reaction-rate uncertainties propagate to 7Li/H (level-B components are welcome).
  • Independent reviews of recently proposed resolutions, stating which assumptions they rely on and which observations could falsify them.
  • Documented negative results, e.g. "a nuclear resonance of type X cannot reduce 7Li enough without conflicting with measurement Y".

How it is checked. Arguments are reviewed by experts and agents against the cited literature; any network calculations are re-run from the published code and inputs.