Cosmological lithium problem

In astronomy, the lithium problem or lithium discrepancy refers to the discrepancy between the primordial abundance of lithium as inferred from observations of metal-poor (Population II) halo stars in our galaxy and the amount that should theoretically exist due to Big Bang nucleosynthesis+WMAP cosmic baryon density predictions of the CMB. 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. The discrepancy is highlighted in a so-called "Schramm plot", named in honor of astrophysicist David Schramm, which depicts these primordial abundances as a function of cosmic baryon content from standard BBN predictions. Minutes after the Big Bang, the universe was made almost entirely of hydrogen and helium, with trace amounts of lithium and beryllium, and negligibly small abundances of all heavier elements. Big Bang nucleosynthesis produced both lithium-7 and beryllium-7, and indeed the latter dominates the primordial synthesis of mass 7 nuclides. On the other hand, the Big Bang produced lithium-6 at levels more than 1000 times smaller. later decayed via electron capture (half-life 53.22 days) into , so that the observable primordial lithium abundance essentially sums primordial and radiogenic lithium from the decay of . These isotopes are produced by the reactions {| border="0" |- style="height:2em;" | ||+ || ||→ || ||+ ||_Gamma |- style="height:2em;" | ||+ || ||→ || ||+ ||_Gamma |- style="height:2em;" |} and destroyed by {| border="0" |- style="height:2em;" | ||+ ||_neutron ||→ || ||+ ||_proton |- style="height:2em;" | ||+ ||_proton ||→ || ||+ || |- style="height:2em;" |} 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%.