Primordial lithium problem

Old, metal-poor stars appear to contain less lithium than early-universe calculations predict. Their light reveals lithium in their atmospheres today. Did the stars destroy some of their original supply, or is something else wrong with the comparison?[1]

An old halo star’s light contains a lithium absorption feature, allowing an estimate of the lithium in its atmosphere. Why is this surviving amount lower than standard predictions for lithium-7 made in the early universe?

Surface material can mix into hotter layers, where nuclear reactions destroy lithium. A star can therefore preserve an ancient record while also altering it. Comparing that record with nucleosynthesis—the production of light nuclei in the universe’s first minutes—requires accounting for stellar processing and abundance-inference uncertainties.[1] [2]

Mixing material into a star’s hot interior can destroy lithium. Stellar depletion is a leading explanation, but it must explain the pattern across stars, not just lower the abundance in one model.[1]

The 2026 Particle Data Group review identifies stellar depletion as the leading candidate, not a completed reconciliation. A viable explanation must account for abundance patterns and the constraints from fragile lithium-6. Changes to early nucleosynthesis must also preserve deuterium, helium and cosmic-microwave-background agreement; the lithium mismatch alone does not establish new particle physics.[1]

Light elements let us test conditions in the universe’s first minutes. Understanding what stars changed helps us decide how faithfully their surviving lithium records that early history.[1]

A star’s present atmosphere links a much later observation to an otherwise inaccessible nuclear era. Working out how that record was altered tests the reliability of this reconstruction. Agreement among different light elements and cosmic matter-density measurements then provides a check that a proposed lithium explanation has not traded one mismatch for another.[1]