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{{Short description|Problem in astronomy}}
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 |
[[File:Schramm plot BBN review 2019.png|thumb|400px|This "Schramm plot"<ref>{{cite journal |
==Origin of lithium==
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==Observed abundance of lithium==
Despite the low theoretical abundance of lithium, the actual observable amount is less than the calculated amount by a factor of 3–4.<ref name=fields11>{{cite journal |last=Fields |first=B. D. |date=2011 |title=The primordial lithium problem |journal=[[Annual Review of Nuclear and Particle Science]] |volume=61 |issue=1 |pages=47–68 |doi=10.1146/annurev-nucl-102010-130445| doi-access=free |arxiv=1203.3551 |bibcode=2011ARNPS..61...47F}}</ref> 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/>
[[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=M. |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; the main source of these elements is [[cosmic ray spallation]].]]
Older stars seem to have less lithium than they should, and some younger stars have much more.<ref name="MWoo">{{cite web|title=The Cosmic Explosions That Made the Universe|url=http://www.bbc.com/earth/story/20170220-the-cosmic-explosions-that-made-the-universe|last=Woo|first=M.|date=21 Feb 2017|website=earth|publisher=BBC|url-status=live|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|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|df=dmy-all}}</ref> One proposed model is that lithium produced during a star's youth sinks beneath the star's atmosphere (where it is obscured from direct observation) due to effects the authors describe as "turbulent mixing" and "diffusion," which are suggested to increase or accumulate as the star ages.<ref>{{Cite journal |
[[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|url-status=dead|archiveurl=https://web.archive.org/web/20150801001700/http://www.eso.org/public/news/eso1531/|archivedate=1 August 2015|df=dmy-all}}</ref>]]
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===Less lithium in Sun-like stars with planets===
Sun-like stars without planets have 10 times the lithium as Sun-like stars with planets in a sample of 500 stars.<ref name="Discover">{{cite
{{Cite journal
|last1=Israelian |first1=G.
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|bibcode=2009Natur.462..189I
|arxiv = 0911.4198
|s2cid=388656
|quote=<span style="font-family:LatinModern;"><small>... confirm the peculiar behaviour of Li in the effective temperature range 5600–5900 K ... We found that the immense majority of planet host stars have severely depleted lithium ... At higher and lower temperatures planet-host stars do not appear to show any peculiar behaviour in their Li abundance.</small></span>}}</ref> The Sun's surface layers have less than 1% the lithium of the original formation [[Formation and evolution of the Solar System#Formation|protosolar gas clouds]] despite the surface convective zone not being quite hot enough to burn lithium.<ref name = "Israelian"/> It is suspected that the gravitational pull of planets might enhance the churning up of the star's surface, driving the lithium to hotter cores where [[lithium burning]] occurs.<ref name="Discover"/><ref name = "Israelian"/> The absence of lithium could also be a way to find new planetary systems.<ref name="Discover"/> However, this claimed relationship has become a point of contention in the planetary astrophysics community, being frequently denied<ref name="BaumannRamírez2010">{{cite journal|last1=Baumann|first1=P.|last2=Ramírez|first2=I.|last3=Meléndez|first3=J.|last4=Asplund|first4=M.|last5=Lind|first5=K.|display-authors=2|title=Lithium depletion in solar-like stars: no planet connection|journal=Astronomy and Astrophysics|volume=519|year=2010|pages=A87|issn=0004-6361|doi=10.1051/0004-6361/201015137|doi-access=free}}</ref><ref name="RamírezFish2012">{{cite journal|last1=Ramírez|first1=I.|last2=Fish|first2=J. R.|last3=Lambert|first3=D. L.|last4=Allende Prieto|first4=C.|display-authors=2|title=Lithium abundances in nearby FGK dwarf and subgiant stars: internal destruction, galactic chemical evolution, and exoplanets|journal=The Astrophysical Journal|volume=756|issue=1|year=2012|pages=46|issn=0004-637X|doi=10.1088/0004-637X/756/1/46|hdl=2152/34872|s2cid=119199829 |hdl-access=free}}</ref> but also supported.<ref name="FigueiraFaria2014">{{cite journal|last1=Figueira|first1=P.|last2=Faria|first2=J. P.|last3=Delgado-Mena|first3=E.|last4=Adibekyan|first4=V. Zh.|last5=Sousa|first5=S. G.|last6=Santos|first6=N. C.|last7=Israelian|first7=G.|display-authors=2|title=Exoplanet hosts reveal lithium depletion|journal=Astronomy & Astrophysics|volume=570|year=2014|pages=A21|issn=0004-6361|doi=10.1051/0004-6361/201424218|doi-access=free}}</ref><ref name="Delgado MenaIsraelian2014">{{cite journal|last1=Delgado Mena|first1=E.|last2=Israelian|first2=G.|last3=González Hernández|first3=J. I.|last4=Sousa|first4=S. G.|last5=Mortier|first5=A.|last6=Santos|first6=N. C.|last7=Adibekyan|first7=V. Zh.|last8=Fernandes|first8=J.|last9=Rebolo|first9=R.|last10=Udry|first10=S.|last11=Mayor|first11=M.|display-authors=2|title=Li depletion in solar analogues with exoplanets|journal=Astronomy & Astrophysics|volume=562|year=2014|pages=A92|issn=0004-6361|doi=10.1051/0004-6361/201321493|doi-access=free}}</ref>
===Higher than expected lithium in metal-poor stars===
Certain orange stars can also contain a high concentration of lithium.<ref name="high">{{cite journal |doi=10.3847/2041-8213/aaa438|title=Enormous Li Enhancement Preceding Red Giant Phases in Low-mass Stars in the Milky Way Halo|journal=The Astrophysical Journal|volume=852|issue=2|pages=L31|year=2018|last1=Li|first1=H. |last2=Aoki|first2=W. |last3=Matsuno|first3=T. |last4=Kumar|first4=Y.
==Proposed solutions==
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When one considers the possibility that the measured primordial Lithium abundance is correct and based on the [[Standard Model]] of particle physics and the standard cosmology, 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" />
Firstly, incorrect or missing reactions could give rise to the lithium problem. For incorrect reactions, major thoughts lie within revision to [[cross section (physics)|cross section]] errors and standard thermonuclear rates according to recent studies.<ref>{{Cite journal|
Second, starting from [[Fred Hoyle]]'s discovery of a [[Resonance (particle physics)|resonance]] in [[carbon-12]], an important factor in the [[triple-alpha process]], resonance reactions, some of which might have evaded experimental detection or whose effects have been underestimated, become possible solutions to the lithium problem.<ref>{{Cite journal|
=== Solutions beyond the Standard Model ===
Under the assumptions of all correct calculation, solutions [[beyond the standard model|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 the 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" /> however, results from the ATLAS experiment in 2020 have excluded many supersymmetric models.<ref>{{Cite journal|last=Collaboration|first=Atlas|
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. Additionally, Standard Model couplings and particle masses might vary; third, variation in nuclear physics parameters is needed.<ref name="fields11" />
Nonstandard cosmologies indicate variation of the 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|
== See also ==
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