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Cosmological lithium problem

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In astronomy, the lithium problem or lithium discrepancy refers to the discrepancy between the primordial abundance of lithium as inferred from observations of metal-poor (Population II) halo stars in our Galaxy and the amount that should theoretically exist due to Big Bang nucleosynthesis+WMAP cosmic baryon density predictions of the 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 2H) and helium (3He and 4He) that are consistent with predictions.[1] The discrepancy is highlighted in a so-called "Schramm plot"[2], which depicts these primordial abundances as a function of cosmic baryon content from standard BBN predictions.

This "Schramm plot" 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% CL) and the BBN D+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 3 − 4 below the BBN+WMAP prediction.

Origin of lithium

Minutes after the Big Bang, the universe was made almost entirely of hydrogen and helium, with trace amounts of lithium and beryllium, and negligibly small abundances of all heavier elements.[3]

Lithium Synthesis in the Big Bang

Big-bang nucleosynthesis produces both lithium-7 and beryllium-7, and indeed the latter dominates the primordial mass-7 production. On the other hand, the big bang produces lithium-6 at levels more than 1000 times smaller. 7
4Be later decays via electron capture into 7
3Li, so that the observable primordial lithium abundance essentially sums 7
3Li+7
4Be.

These isotopes are produced by the reactions

3
1H 		 +  4
2He 		 →  7
3Li 		 +  γ
3
2He 		 +  4
2He 		 →  7
4Be 		 +  γ

and destroyed by

7
4Be 		 +  n  →  7
3Li 		 +  p
7
3Li 		 +  p  →  4
2He 		 +  4
2He

The amount of lithium generated in the Big Bang can be calculated.[4] 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.[5] The calculated abundance and ratio of 1H and 4He is in agreement with data from observations of young stars.[3]

The P-P II branch

See also: Lithium burning

In stars, lithium-7 is made in a proton-proton chain reaction.

 
Proton–proton II chain reaction
3
2He 		 +  4
2He 		 →  7
4Be 		 +  γ
7
4Be 		 +  e−
 		 →  7
3Li-
 		 +  ν
e 		 +  0.861 MeV  /  0.383 MeV
7
3Li 		 +  1
1H 		 →  2 4
2He

The P-P II branch is dominant at temperatures of 14 to 23 MK.


 
Stable nuclides of the first few elements

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.[6] This contrasts with the observed abundance of isotopes of hydrogen (1H and 2H) and helium (3He and 4He) that are consistent with predictions.[1]

 
Abundances of the chemical elements in the Solar System. Hydrogen and helium are most common, residuals within the paradigm of the Big Bang.[7] 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.[8] The lack of lithium in older stars is apparently caused by the "mixing" of lithium into the interior of stars, where it is destroyed,[9] while lithium is produced in younger stars. Though it 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.[10][11]

 
Nova Centauri 2013 is the first in which evidence of lithium has been found.[12]

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.[10][11][13]

Less lithium in stars with planets

Stars without planets have 10 times the lithium as stars with planets in a sample of 500 stars.[14] 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.[14] The absence of lithium could also be a way to find new planetary systems.[14]

Higher than expected lithium in metal-poor stars

Certain orange stars can also contain a high concentration of lithium.[15] 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.[10]

Proposed solutions

Numerous studies have been conducted in search of an explanation for this deficiency of lithium, all inconclusive.[16] One theory suggests that the lithium problem may be partially caused by faster destruction than synthesis of 7Li and its progenitor 7Be in nuclear reactions, 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.[17][8] However, one new theory posits that strangeon dark matter (a hypothetical mix of 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.[18]

See also

References

  1. ^ a b Hou, S.Q.; He, J.J.; Parikh, A.; Kahl, D.; Bertulani, C.A.; Kajino, T.; Mathews, G.J.; Zhao, G. (2017). "Non-extensive statistics to the cosmological lithium problem". The Astrophysical Journal. 834 (2): 165. arXiv:1701.04149. Bibcode:2017ApJ...834..165H. doi:10.3847/1538-4357/834/2/165.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  2. ^ M. Tanabashi et al. (Particle Data Group), Phys. Rev. D 98, 030001 (2018) and 2019 update.
  3. ^ a b Langmuir, Charles Herbert; Broecker, Wallace S. (2012). How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind. ISBN 978-0691140063.
  4. ^ Boesgaard, A. M.; Steigman, G. (1985). "Big bang nucleosynthesis – Theories and observations". Annual Review of Astronomy and Astrophysics. 23. Palo Alto, CA: 319–378. Bibcode:1985ARA&A..23..319B. doi:10.1146/annurev.aa.23.090185.001535. A86-14507 04–90.
  5. ^ Tanabashi, M.; et al. (2018). "Big-bang nucleosynthesis". In Fields, B.D.; Molaro, P.; Sarkar, S. (eds.). The Review (PDF). Vol. 98. pp. 377–382. doi:10.1103/PhysRevD.98.030001. {{cite book}}: |journal= ignored (help)
  6. ^ Fields, B.D. (2011). "The primordial lithium problem". Annual Review of Nuclear and Particle Science. 61: 47–68. arXiv:1203.3551. doi:10.1146/annurev-nucl-102010-130445.
  7. ^ Stiavelli, Massimo (2009). From First Light to Reionization the End of the Dark Ages. Weinheim, Germany: Wiley-VCH. p. 8. Bibcode:2009fflr.book.....S. ISBN 9783527627370.
  8. ^ a b Woo, Marcus (21 February 2017). "The Cosmic Explosions That Made the Universe". earth. BBC. Archived from the original on 21 February 2017. Retrieved 21 February 2017. A mysterious cosmic factory is producing lithium. Scientists are now getting closer at finding out where it comes from
  9. ^ Cain, Fraser (16 August 2006). "Why Old Stars Seem to Lack Lithium". Archived from the original on 4 June 2016.
  10. ^ a b c Emsley, John (2001). Nature's Building Blocks. Oxford: Oxford University Press. ISBN 978-0-19-850341-5.
  11. ^ a b Cain, Fraser. "Brown Dwarf". Universe Today. Archived from the original on 25 February 2011. Retrieved 17 November 2009.
  12. ^ "First Detection of Lithium from an Exploding Star". Archived from the original on 1 August 2015. Retrieved 29 July 2015.
  13. ^ Reid, Neill (10 March 2002). "L Dwarf Classification". Archived from the original on 21 May 2013. Retrieved 6 March 2013.
  14. ^ a b c Plait, Phil (Nov 11, 2009). "Want a planet? You might want to avoid lithium". Discover.
  15. ^ Li, Haining; Aoki, Wako; Matsuno, Tadafumi; Kumar, Yerra Bharat; Shi, Jianrong; Suda, Takuma; Zhao, Gang; Zhao, G. (2018). "Enormous Li Enhancement Preceding Red Giant Phases in Low-mass Stars in the Milky Way Halo". The Astrophysical Journal. 852 (2): L31. arXiv:1801.00090. Bibcode:2018ApJ...852L..31L. doi:10.3847/2041-8213/aaa438.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  16. ^ Coc, A.; Uzan, J.-P.; Vangioni, E. (2014). "Standard big bang nucleosynthesis and primordial CNO abundances after Planck". Journal of Cosmology and Astroparticle Physics. 2014. arXiv:1403.6694. doi:10.1088/1475-7516/2014/10/050.
  17. ^ Bertulani, C.A.; Shubhchintak; Mukhamedzhanov, A.M. (2018). "Cosmological lithium problems". EPJ Web of Conferences. 184: 01002. arXiv:1802.03469. Bibcode:2018EPJWC.18401002B. doi:10.1051/epjconf/201818401002.
  18. ^ Xu, R. (2019). "Trinity of strangeon matter". arXiv:1904.11153.{{cite arXiv}}: CS1 maint: missing class (link) A bot will complete this citation soon. Click here to jump the queue
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