Cosmological lithium problem

In astronomy, the lithium problem, lithium discrepancy or lithium gap refers to the discrepancy between the observed 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 (¹H and ²H) and helium (³He and ⁴He) that are consistent with predictions.^[1]

Origin of lithium

One day after the Big Bang, the universe was made almost entirely of hydrogen and helium, with only very small amounts of the next three elements—lithium (Li), beryllium (Be), and boron (B).^[2]

The P-P II branch

Lithium is made by a proton-proton chain reaction.

 

Proton–proton II chain reaction

See also: Lithium burning

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.

The amount of lithium generated in the Big Bang can be calculated.^[3]

Models of the collisions that would have occurred found that there would have been one helium atom for every ten hydrogen atoms, which agrees with the observations of young stars.^[2]

 

Stable nuclides H to B

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.^[4] 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.^[5] Li, Be and B are rare because they are poorly synthesized in the Big Bang and also in stars.

Older stars seem to have less lithium than they should, and some younger stars have much more.^[6] The lack of lithium in older stars is apparently caused by the "mixing" of lithium into the interior of stars, where it is destroyed,^[7] 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.^[8][9]

 

Nova Centauri 2013 is the first in which evidence of lithium has been found.^[10]

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.^[8][9][11]

Less lithium in stars with planets

Stars without planets have 10 times the lithium as stars with planets in a sample of 500 stars.^[12] 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.^[12] The absence of lithium could also be a way to find new planetary systems.^[12]

Higher than expected lithium in metal-poor stars

Certain orange stars can also contain a high concentration of lithium.^[13] 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.^[8]

Proposed solutions

Numerous studies have been conducted in search of an explanation for this deficiency of lithium, all inconclusive.^[14] 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.^[15][6] 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.^[16]

See also

• Big Bang
• Isotopes of lithium
• List of unsolved problems in physics

References

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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 {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
7. Cain, Fraser (16 August 2006). "Why Old Stars Seem to Lack Lithium". Archived from the original on 4 June 2016. {{cite news}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
8. Emsley, John (2001). Nature's Building Blocks. Oxford: Oxford University Press. ISBN 978-0-19-850341-5.
9. Cain, Fraser. "Brown Dwarf". Universe Today. Archived from the original on 25 February 2011. Retrieved 17 November 2009.
10. "First Detection of Lithium from an Exploding Star". Archived from the original on 1 August 2015. Retrieved 29 July 2015. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
11. Reid, Neill (10 March 2002). "L Dwarf Classification". Archived from the original on 21 May 2013. Retrieved 6 March 2013. {{cite web}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)
12. Plait, Phil (Nov 11, 2009). "Want a planet? You might want to avoid lithium". Discover.
13. 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.
14. 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.
15. 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.
16. Xu, R. (2019). "Trinity of strangeon matter". arXiv:1904.11153. A bot will complete this citation soon. Click here to jump the queue