Cosmological lithium problem: Difference between revisions

Revision as of 23:51, 17 August 2023

Revision as of 01:15, 23 August 2023

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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]] (¹H and [[deuterium|²H]]) and [[helium]] ([[helium-3|³He]] and [[helium-4|⁴He]]) that are consistent with predictions.<ref name=HouStats>{{cite journal |last1=Hou |first1=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 |s2cid=568182 |doi-access=free }}</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>{{cite journal | last1=Tanabashi | first1=M. | last2=Hagiwara | first2=K. | last3=Hikasa | first3=K. | last4=Nakamura | first4=K. | last5=Sumino | first5=Y. | last6=Takahashi | first6=F. | last7=Tanaka | first7=J. | last8=Agashe | first8=K. | last9=Aielli | first9=G. | last10=Amsler | first10=C. | display-authors=5|collaboration=Particle Data Group| title=Review of Particle Physics | journal=Physical Review D | publisher=American Physical Society (APS) | volume=98 | issue=3 | date=2018-08-17 | issn=2470-0010 | doi=10.1103/physrevd.98.030001 | page=030001| bibcode=2018PhRvD..98c0001T |doi-access=free}} 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.]]

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===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. Bharat|last5=Shi|first5=J. |last6=Suda|first6=T. |last7=Zhao|first7=G. |last8=Zhao|first8=G.|bibcode=2018ApJ...852L..31L|arxiv=1801.00090|s2cid=54205417 |doi-access=free }}</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=J. |title=Nature's Building Blocks |publisher=Oxford University Press |___location=Oxford|date=2001 |isbn=978-0-19-850341-5}}</ref>

==Proposed solutions==

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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 levels 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" />

Some astronomers suggest that the velocities of nucleons do not follow Maxwell-Boltzmann distribution. They test the framework of Tsallis non-extensive statistics.Their result suggest that 1.069<q<1.082 is a possible new solution to the cosmological lithium problem.<ref>{{Cite journal |last1=Hou |first1=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. |title=Non-Extensive Statistics to the Cosmological Lithium Problem |date=2017-01-11 |url=http://dx.doi.org/10.3847/1538-4357/834/2/165 |journal=The Astrophysical Journal |volume=834 |issue=2 |pages=165 |doi=10.3847/1538-4357/834/2/165 |arxiv=1701.04149 |bibcode=2017ApJ...834..165H |s2cid=568182 |issn=1538-4357 |doi-access=free }}</ref>

=== Nuclear physics solutions ===