Cosmological lithium problem: Difference between revisions

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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]] (¹H and [[deuterium|²H]]) and [[helium]] ([[helium-3|³He]] and [[helium-4|⁴He]]) 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. 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% [[confidence level|CL]]) and the BBN D&nbsp;+&nbsp;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 of 3−4 below the BBN+WMAP prediction.]]

==Origin of lithium==

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The amount of lithium generated in the Big Bang 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> [[Hydrogen-1]] is the most abundant [[nuclide]], comprising roughly 92% of the atoms in the Universe, with [[helium-4]] second at 8%. Other isotopes including ²H, ³H, ³He, ⁶Li, ⁷Li, and ⁷Be are much rarer; the estimated abundance of primordial lithium is 10⁻¹⁰ relative to hydrogen.<ref name=23bbn>{{cite book |last1=Tanabashi |first1=M. |display-authors=et al. |editor-last1=Fields |editor-first1=B. D. |editor-last2=Molaro |editor-first2=P. |editor-last3=Sarkar |editor-first3=S. |title=The Review |date=2018 |chapter=Big-bang nucleosynthesis |journal=Physical Review D |volume=98 |pages=377–382 |doi=10.1103/PhysRevD.98.030001 |url=https://pdg.lbl.gov/2019/reviews/rpp2018-rev-bbang-nucleosynthesis.pdf

}}</ref> The calculated abundance and ratio of ¹H and ⁴He is in agreement with data from 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=CharlesC. HerbertH.|last2=Broecker|first2=WallaceW. S.|year=2012}}</ref>

===The P-P II branch===

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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 |pages=47–68 |doi=10.1146/annurev-nucl-102010-130445 |arxiv=1203.3551}}</ref> This contrasts with the observed abundance of isotopes of [[hydrogen]] (¹H and [[deuterium|²H]]) and [[helium]] ([[helium-3|³He]] and [[helium-4|⁴He]]) 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=MassimoM. |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"/> The lack of lithium in older stars is apparently caused by the "mixing" of lithium into the interior of stars, where it is destroyed,<ref name=cld>{{Cite news |url=http://www.universetoday.com/476/why-old-stars-seem-to-lack-lithium/ |title=Why Old Stars Seem to Lack Lithium |date=16 August 2006 |authorlast=Cain, Fraser|first=F. |url-status=live |archiveurl=https://web.archive.org/web/20160604044857/http://www.universetoday.com/476/why-old-stars-seem-to-lack-lithium/ |archivedate=4 June 2016 |df=dmy-all }}</ref> while lithium is produced in younger stars. Though it [[lithium burning|transmutes]] into two atoms of [[helium]] due to collision with a [[proton]] at temperatures above 2.4 million degrees Celsius (most stars easily attain this temperature in their interiors), lithium is more abundant than current computations would predict in later-generation stars.<ref name=emsley/><ref name="Cain">{{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=FraserF. |publisher=Universe Today}}</ref>

[[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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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 name="Cain"/><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 |url-status=dead |archive-date=21 May 2013 |title=L Dwarf Classification|accessdate=6 March 2013 | first =NeillN. | last = Reid | date = 10 March 2002}}</ref>

===Less lithium in stars with planets===

Stars without planets have 10 times the lithium as stars with planets in a sample of 500 stars.<ref name="Discover">{{cite journal |last1=Plait |first1=PhilP. |authorlink1=Phil Plait |journal= Discover| title=Want a planet? You might want to avoid lithium |date=Nov 11, November 2009}}</ref> The sun has 1% of the amount of lithium in gas clouds. It is suspected that the gravitational pull of planets might churn up a star's surface, driving the lithium to hotter cores where [[lithium burning]] occurs.<ref name="Discover"/> The absence of lithium could also be a way to find new planetary systems.<ref name="Discover"/>

===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=HainingH. |last2=Aoki|first2=WakoW. |last3=Matsuno|first3=TadafumiT. |last4=Kumar|first4=YerraY. Bharat|last5=Shi|first5=JianrongJ. |last6=Suda|first6=TakumaT. |last7=Zhao|first7=GangG. |last8=Zhao|first8=G.|bibcode=2018ApJ...852L..31L|arxiv=1801.00090}}</ref> Those orange stars found to have a higher than usual concentration of lithium 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=JohnJ. |title=Nature's Building Blocks |publisher=Oxford University Press |___location=Oxford|date=2001 |isbn=978-0-19-850341-5}}</ref>

==Proposed solutions==

Numerous studies have been conducted in search of an explanation for this deficiency of lithium, all inconclusive.<ref name=coc>{{cite journal |last=Coc |first=A. |last2=Uzan |first2=J.-P. |last3=Vangioni |first3=E. |title=Standard big bang nucleosynthesis and primordial CNO abundances after Planck |date=2014 |journal=Journal of Cosmology and Astroparticle Physics |volume=2014 |doi=10.1088/1475-7516/2014/10/050 |arxiv=1403.6694}}</ref> One theory suggests that the lithium problem may be partially caused by faster destruction than synthesis of ⁷Li and its progenitor [[beryllium-7|⁷Be]] 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=EPJ Web of 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=MarcusM. |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 |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 |df=dmy-all }}</ref> However, one new theory posits that strangeon dark matter (a hypothetical mix of [[strange matter|strange]] and dark matter) may be responsible for the destruction of ⁷Be before it decays to ⁷Li, as the low nuclear binding energy of ⁷Be renders it susceptible to destruction upon collision with strangeons.<ref name=Xu19>{{cite arxiv |last=Xu |first=R. |title=Trinity of strangeon matter |date=2019 |arxiv=1904.11153}}</ref>

==See also==