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Title: What explains the threefold lithium-7 deficit in old halo stars?
Plain: Old metal-poor stars in the Milky Way halo show a constant lithium level, but it is about three times below the Big Bang prediction. Either the stars destroyed lithium, or the early universe made less of it.
Precise: Standard BBN with the Planck baryon density $\eta \sim 6.1e-10$ predicts $\mathrm{Li}/\mathrm{H} \sim 5e-10$, while the Spite plateau of metal-poor dwarfs ($[\mathrm{Fe}/\mathrm{H}] < -1.5$) gives $\mathrm{Li}/\mathrm{H} \sim 1.6e-10$, a factor 3 to 4 at more than $5\,\sigma$. Identify the cause: stellar depletion by atomic diffusion and mixing, nuclear-rate errors, or non-standard physics (decaying particles, varying constants).
Settled by: A measurement of unprocessed lithium, or a validated stellar model reproducing the plateau from the BBN value.
Kind: phenomenon
Answer type: identification
References: (empty)
Literature status: open
Status note: 2024 to 2025 work favors stellar depletion and finds nuclear-rate solutions negligible, but no quantitative stellar model is established.
Posed since: (empty)