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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>Tanabashi, M., et al. (Particle Data Group), Phys. Rev. D 98, 030001 (2018) and 2019 update.</ref> depicts primordial abundances of <sup>4</sup>He, D, <sup>3</sup>He, and <sup>7</sup>Li as a function of cosmic baryon content from standard BBN predictions. CMB predictions of 7Li (narrow vertical bands, at 95% [[confidence level|CL]]) and the BBN D + <sup>4</sup>He 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 <sup>4</sup>He and is well constrained in D, but is not the case for <sup>7</sup>Li, where the observed Li observations lie a factor of 3−4 below the BBN+WMAP prediction.]]
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The P-P II branch is dominant at temperatures of 14 to {{val|23|u=MK}}.
[[File:Stable nuclides H to B.png|thumb|right|400px|Stable nuclides of the first few elements]]
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Possible solutions fall into three broad classes.
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Considering the possibility that BBN predictions are sound, the measured value of the primordial Lithium abundance should be in error and astrophysical solutions offer revision to it. For example, systematic errors, including ionization correction and inaccurate stellar temperatures determination could affect Li/H ratios in stars. Furthermore, more observations on Lithium depletion remain important since present Lithium level might not reflect the initial abundance in the star. In summary, accurate measurements of the primordial Lithium abundance is the current focus of progress, and it could be possible that the final answer does not lie in astrophysical solutions.<ref name="fields11" />
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Now consider the possibility that the measured primordial Lithium abundance is correct and based on the [[Standard Model]] of particle physics and the standard cosmology, then the Lithium problem implies errors in the BBN light element predictions. Although standard BBN rests on well-determined physics, the weak and strong interactions are complicated for BBN and therefore might be the weak point in standard BBN calculation.<ref name="fields11" />
First, reactions missing or incorrect could lead to Lithium problem. For incorrect reactions, major thoughts lie within revision to cross-section errors and standard thermonuclear rates according to recent studies.<ref>{{Cite journal|last=Angulo|first=C.|last2=Casarejos|first2=E.|last3=Couder|first3=M.|last4=Demaret|first4=P.|last5=Leleux|first5=P.|last6=Vanderbist|first6=F.|last7=Coc|first7=A.|last8=Kiener|first8=J.|last9=Tatischeff|first9=V.|last10=Davinson|first10=T.|last11=Murphy|first11=A. S.|date=September 2005
Second, starting from [[Fred Hoyle]]'s great discovery of [[carbon-12]] resonance, [[Resonance (particle physics)|resonance]] reactions, some of which might have evaded experimental detection or whose effects have been underestimated, become possible solutions to the Lithium problem and are studied by many recently.<ref>{{Cite journal|last=Hammache|first=F.|last2=Coc|first2=A.|last3=de Séréville|first3=N.|last4=Stefan|first4=I.|last5=Roussel|first5=P.|last6=Ancelin|first6=S.|last7=Assié|first7=M.|last8=Audouin|first8=L.|last9=Beaumel|first9=D.|last10=Franchoo|first10=S.|last11=Fernandez-Dominguez|first11=B.|date=December 2013
All these nuclear physics solutions are accessible in experimental validation, which make them highly possible resulted in a true answer.
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Under the assumptions of all correct calculation, solutions beyond the existing Standard Model or standard cosmology might be needed.<ref name="fields11" />
Dark Matter Decay and [[Supersymmetry]] provide one possibility, in which decaying dark matter scenarios introduce a rich array of novel processes that can alter light elements during and after BBN, and find the well-motivated origin in supersymmetric cosmologies. With fully operational [[Large Hadron Collider|LHC]], much of minimal supersymmetry lies within reach, which would revolutionize particle physics and cosmology if discovered.<ref name="fields11" />
Changing Fundamental Constants can be one possible solution, and it implies that first, atomic transitions in metals residing in high-redshift regions might behave differently from our own; second, Standard Model couplings and particle masses might vary; third, variation in nuclear physics parameters is needed.<ref name="fields11" />
Nonstandard Cosmologies indicate variation of baryon to photon ratio in different regions. One proposal is a result of large-scale inhomogeneities in cosmic density, different from homogeneity defined in the [[cosmological principle]]. However, this possibility requires a large amount of observations to test it.<ref>{{Cite journal|last=Holder|first=Gilbert P.|last2=Nollett|first2=Kenneth M.|last3=van Engelen|first3=Alexander|date=June 2010
==See also==
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