What explains the threefold lithium-7 deficit in old halo stars?
In plain words
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 statement
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).
What would settle it
A measurement of unprocessed lithium, or a validated stellar model reproducing the plateau from the BBN value.
Status in the literature
Unverified note
2024 to 2025 work favors stellar depletion and finds nuclear-rate solutions negligible, but no quantitative stellar model is established.
Related problems
- More general than Can stellar transport deplete lithium uniformly by a factor three?
- More general than What is the lithium abundance in gas never processed by stars?