{{short description|Discrepancy between the observed abundance of lithium produced in Big Bang nucleosynthesis and the amount that should theoretically exist.}}
In [[astronomy]], the '''lithium problem''', '''lithium discrepancy''' or '''lithium gap''' refers to the discrepancy between the observed [[Abundance of the chemical elements|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]] (<sup>1</sup>H and [[deuterium|<sup>2</sup>H]]) and [[helium]] ([[helium-3|<sup>3</sup>He]] and [[helium-4|<sup>4</sup>He]]) that are consistent with predictions.<ref name=HouStats>{{cite journal |last=Hou |first=S.Q. |last2=He |first2=J.J. |last3=Parikh |first3=A. |last4=Kahl |first4=D. |last5=Bertulani |first5=C.A. |last6=Kajino |first6=T. |last7=Mathews |first7=G.J. |last8=Zhao |first8=G. |date=2017 |title=Non-extensive statistics to the cosmological lithium problem |journal=The Astrophysical Journal |volume=834 |issue=2 |pages= 165|doi=10.3847/1538-4357/834/2/165 |bibcode=2017ApJ...834..165H |arxiv=1701.04149 }}</ref>
