Cosmological lithium problem
In astronomy, the lithium problem, lithium discrepancy or lithium gap refers to the discrepancy between the observed abundance of lithium produced in Big Bang nucleosynthesis and the amount that should theoretically exist. Namely, the most widely accepted models of the Big Bang suggest that three times as much primordial lithium, in particular lithium-7, should exist. This contrasts with the observed abundance of isotopes of hydrogen (1H and 2H) and helium (3He and 4He) that are consistent with predictions.
Origin of lithium
One day after the Big Bang, the universe was made almost entirely of hydrogen and helium, with only very small amounts of lithium and beryllium.
The P-P II branch
Lithium-7 is made by a proton-proton chain reaction.
The P-P II branch is dominant at temperatures of 14 to 23 MK.
The amount of lithium generated in the Big Bang can be calculated. Hydrogen-1 is the most abundant nuclide, comprising roughly 92% of the atoms in the Universe, with helium-4 second at 8%. Other isotopes including 2H, 3H, 3He, 6Li, 7Li, and 7Be are much rarer; the estimated abundance of primordial lithium is 10−10 relative to hydrogen. The calculated abundance and ratio of 1H and 4He is in agreement with data from observations of young stars.
Observed abundance of lithium
Despite the low theoretical abundance of lithium, the actual observable amount is less than the calculated amount by a factor of 3-4. This contrasts with the observed abundance of isotopes of hydrogen (1H and 2H) and helium (3He and 4He) that are consistent with predictions.
Older stars seem to have less lithium than they should, and some younger stars have much more. The lack of lithium in older stars is apparently caused by the "mixing" of lithium into the interior of stars, where it is destroyed, while lithium is produced in younger stars. Though it transmutes into two atoms of helium due to collision with a proton at temperatures above 2.4 million degrees Celsius (most stars easily attain this temperature in their interiors), lithium is more abundant than current computations would predict in later-generation stars.
Lithium is also found in brown dwarf substellar objects and certain anomalous orange stars. Because lithium is present in cooler, less-massive brown dwarfs, but is destroyed in hotter red dwarf stars, its presence in the stars' spectra can be used in the "lithium test" to differentiate the two, as both are smaller than the Sun.
Less lithium in stars with planets
Stars without planets have 10 times the lithium as stars with planets in a sample of 500 stars. The sun has 1% of the amount of lithium in gas clouds. It is suspected that the gravitational pull of planets might churn up a star's surface, driving the lithium to hotter cores where lithium burning occurs. The absence of lithium could also be a way to find new planetary systems.
Higher than expected lithium in metal-poor stars
Certain orange stars can also contain a high concentration of lithium. Those orange stars found to have a higher than usual concentration of lithium orbit massive objects—neutron stars or black holes—whose gravity evidently pulls heavier lithium to the surface of a hydrogen-helium star, causing more lithium to be observed.
Numerous studies have been conducted in search of an explanation for this deficiency of lithium, all inconclusive. One theory suggests that the lithium problem may be partially caused by faster destruction than synthesis of 7Li and its progenitor 7Be in nuclear reactions, though no conclusive results on the reaction flow in Big Bang nucleosynthesis have been obtained. Newer theories involving physics beyond the standard model, involving not well understood dark matter, have also been proposed to explain the possible destruction of lithium, also inconclusively. However, one new theory posits that strangeon dark matter (a hypothetical mix of strange and dark matter) may be responsible for the destruction of 7Be before it decays to 7Li, as the low nuclear binding energy of 7Be renders it susceptible to destruction upon collision with strangeons.
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