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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''', '''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 |
==Origin of lithium==
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The amount of lithium generated in [[Big Bang nucleosynthesis]] can be calculated.<ref>{{cite journal | bibcode= 1985ARA&A..23..319B | title= Big bang nucleosynthesis – Theories and observations | last1= Boesgaard | first1=A. M. | last2= Steigman | first2= G. | volume= 23 |date= 1985 | pages= 319–378 | journal= Annual Review of Astronomy and Astrophysics |id=A86-14507 04–90 |___location=Palo Alto, CA | doi= 10.1146/annurev.aa.23.090185.001535}}</ref>
Models of the collisions that would have occurred found that there would have been one helium atom for every ten hydrogen atoms which fits with the observations of young stars.<ref name="habitable">{{cite book |isbn=978-0691140063|title=How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind|last1=Langmuir|first1=Charles Herbert|last2=Broecker|first2=Wallace S.|year=2012}}</ref>
[[File:Stable nuclides H to B.png|thumb|center|800px|Stable nuclides H to B]]
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[[Image:SolarSystemAbundances.svg|thumb|center|800px|Abundances of the chemical elements in the Solar System. Hydrogen and helium are most common, residuals within the paradigm of the Big Bang.<ref>{{cite book |last1=Stiavelli |first1=Massimo |year=2009 |title=From First Light to Reionization the End of the Dark Ages |url=https://books.google.com/books?id=iCLNBElRTS4C&pg=PA8 |page=8 |publisher=[[Wiley-VCH]] |___location=Weinheim, Germany |isbn=9783527627370|bibcode=2009fflr.book.....S }}</ref> Li, Be and B are rare because they are poorly synthesized in the Big Bang and also in stars.]]
{{main|Lithium}}
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[[File:Nova Centauri 2013 ESO.jpg|thumb|[[Nova Centauri 2013]] is the first in which evidence of lithium has been found.<ref>{{cite web|title=First Detection of Lithium from an Exploding Star|url=http://www.eso.org/public/news/eso1531/|accessdate=29 July 2015|deadurl=no|archiveurl=https://www.webcitation.org/6aNmJZ6sh?url=http://www.eso.org/public/news/eso1531/|archivedate=29 July 2015|df=dmy-all}}</ref>]]
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.<ref name=emsley/><ref>{{cite web|url=http://www.universetoday.com/24593/brown-dwarf/|archiveurl=https://web.archive.org/web/20110225032434/http://www.universetoday.com/24593/brown-dwarf/|archivedate=25 February 2011|title=Brown Dwarf |accessdate=17 November 2009 |last=Cain |first=Fraser |publisher=Universe Today}}</ref><ref>{{cite web|url=http://www-int.stsci.edu/~inr/ldwarf3.html |archive-url=https://archive.is/20130521055905/http://www-int.stsci.edu/~inr/ldwarf3.html |dead-url=yes |archive-date=21 May 2013 |title=L Dwarf Classification|accessdate=6 March 2013 | first =Neill | last = Reid | date = 10 March 2002}}</ref> Certain orange stars can also contain a high concentration of lithium. Those orange stars found to have a higher than usual concentration of lithium (such as [[Centaurus X-4]]) 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.<ref name=emsley>{{Cite book|last=Emsley |first=John |title=Nature's Building Blocks |publisher=Oxford University Press |___location=Oxford|date=2001 |isbn=978-0-19-850341-5}}</ref>
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==Proposed solutions==
This deficiency of lithium may in part be due to faster destruction than synthesis of <sup>7</sup>Li and its progenitor [[beryllium-7|<sup>7</sup>Be]] in [[nuclear reaction]]s, 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.<ref name=Bertulani>{{cite journal |last=Bertulani |first=C.A. |last2=Shubhchintak |last3=Mukhamedzhanov |first3=A.M. |title=Cosmological lithium problems |date=2018 |journal=The European Physical Journal Conferences |volume=184 |pages=01002 |doi=10.1051/epjconf/201818401002 |arxiv=1802.03469|bibcode=2018EPJWC.18401002B }}</ref><ref name=MWoo>{{cite web |url=http://www.bbc.com/earth/story/20170220-the-cosmic-explosions-that-made-the-universe |title=The Cosmic Explosions That Made the Universe |last=Woo |first=Marcus |date=21 Feb 2017 |website=earth |publisher=BBC |access-date=21 Feb 2017 |quote=A mysterious cosmic factory is producing lithium. Scientists are now getting closer at finding out where it comes from |deadurl=no |archiveurl=https://web.archive.org/web/20170221214442/http://www.bbc.com/earth/story/20170220-the-cosmic-explosions-that-made-the-universe |archivedate=21 February 2017 |df=dmy-all }}</ref>
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