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{{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''' or '''lithium discrepancy''' refers to the discrepancy between the primordial [[Abundance of the chemical elements|abundance]] of [[lithium]] as inferred from observations of metal-poor ([[Stellar population|Population II]]) [[Stellar halo|halo stars]] in our Galaxy and the amount that should theoretically exist due to [[Big Bang nucleosynthesis]]+[[Wilkinson Microwave Anisotropy Probe|WMAP]] cosmic baryon density predictions of the [[Cosmic microwave background|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]] (<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> The discrepancy is highlighted in a so-called "Schramm plot", named in honor of astrophysicist [[David Schramm (astrophysicist)|David Schramm]], which depicts these primordial abundances as a function of cosmic baryon content from standard [[Big Bang nucleosynthesis|BBN]] predictions.
[[File:Schramm plot BBN review 2019.png|thumb|400px|This "Schramm plot"<ref> M. Tanabashi et al. (Particle Data Group), Phys. Rev. D 98, 030001 (2018) and 2019 update.</ref> depicts primordial abundances of 4He, D, 3He, and 7Li as a function of cosmic baryon content from standard BBN predictions. CMB predictions of 7Li (narrow vertical bands, at 95% CL) and the BBN D+4He concordance range (wider vertical bands, at 95% CL) should overlap with the observed light element abundances (yellow boxes) to be in agreement. This occurs in 4He and is well constrained in D, but is not the case for 7Li, where the observed Li observations lie a factor 3 − 4 below the BBN+WMAP prediction.]]
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