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", named in honor of astrophysicist David Schramm, which depicts these primordial abundances as a function of cosmic baryon content from standard BBN predictions.

Contents

This "Schramm plot" depicts primordial abundances of He, D, He, and Li as a function of cosmic baryon content from standard BBN predictions. CMB predictions of Li (narrow vertical bands, at 95% CL) and the BBN D + He 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 He and is well constrained in D, but is not the case for Li, where the observed Li observations lie a factor of 3-4 below the BBN+WMAP prediction. Schramm plot BBN review 2019.png
This "Schramm plot" depicts primordial abundances of He, D, He, and Li as a function of cosmic baryon content from standard BBN predictions. CMB predictions of Li (narrow vertical bands, at 95% CL) and the BBN D +  He 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 He and is well constrained in D, but is not the case for Li, where the observed Li observations lie a factor of 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 produced both lithium-7 and beryllium-7, and indeed the latter dominates the primordial synthesis of mass 7 nuclides. On the other hand, the Big Bang produced lithium-6 at levels more than 1000 times smaller. 7
4Be
 later decayed via electron capture (half-life 53.22 days) into 7
3Li
 , so that the observable primordial lithium abundance essentially sums primordial 7
3Li
 and radiogenic lithium from the decay of 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

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

Proton-proton II chain reaction Proton-Proton II chain reaction.svg
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 Stable nuclides H to B.png
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. 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. SolarSystemAbundances.svg
Abundances of the chemical elements in the Solar System. Hydrogen and helium are most common, residuals within the paradigm of the Big Bang. 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] 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. [9] Spectroscopic observations of stars in NGC 6397, a metal-poor globular cluster, are consistent with an inverse relation between lithium abundance and age, but a theoretical mechanism for diffusion has not been formalized. [10] 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. [11] [12]

Nova Centauri 2013 is the first in which evidence of lithium has been found. Nova Centauri 2013 ESO.jpg
Nova Centauri 2013 is the first in which evidence of lithium has been found.

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. [11] [12] [14]

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. [15] [16] The Sun's surface layers have less than 1% the lithium of the original formation protosolar gas clouds despite the surface convective zone not being quite hot enough to burn lithium. [16] 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. [15] [16] The absence of lithium could also be a way to find new planetary systems. [15] However, this claimed relationship has become a point of contention in the planetary astrophysics community, being frequently denied [17] [18] but also supported. [19] [20]

Higher than expected lithium in metal-poor stars

Certain orange stars can also contain a high concentration of lithium. [21] 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. [11]

Proposed solutions

Possible solutions fall into three broad classes.

Astrophysical solutions

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. [6]

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. [22]

Nuclear physics solutions

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. [6]

Firstly, incorrect or missing reactions could give rise to the lithium problem. For incorrect reactions, major thoughts lie within revision to cross section errors and standard thermonuclear rates according to recent studies. [23] [24]

Second, starting from Fred Hoyle's discovery of a 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. [25] [26]

BBC Science Focus wrote in 2023 that "recent research seems to completely discount" such theories; the magazine held that mainstream lithium nucleosynthesis calculations are probably correct. [27]

Solutions beyond the Standard Model

Under the assumptions of all correct calculation, solutions beyond the existing Standard Model or standard cosmology might be needed. [6]

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; [6] however, results from the ATLAS experiment in 2020 have excluded many supersymmetric models. [28] [29]

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. [6]

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. [30]

See also

Related Research Articles

In physical cosmology, Big Bang nucleosynthesis is the production of nuclei other than those of the lightest isotope of hydrogen during the early phases of the universe. This type of nucleosynthesis is thought by most cosmologists to have occurred from 10 seconds to 20 minutes after the Big Bang. It is thought to be responsible for the formation of most of the universe's helium, along with small fractions of the hydrogen isotope deuterium, the helium isotope helium-3 (3He), and a very small fraction of the lithium isotope lithium-7 (7Li). In addition to these stable nuclei, two unstable or radioactive isotopes were produced: the heavy hydrogen isotope tritium and the beryllium isotope beryllium-7 (7Be). These unstable isotopes later decayed into 3He and 7Li, respectively, as above.

Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in a process called Big Bang nucleosynthesis. After about 20 minutes, the universe had expanded and cooled to a point at which these high-energy collisions among nucleons ended, so only the fastest and simplest reactions occurred, leaving our universe containing hydrogen and helium. The rest is traces of other elements such as lithium and the hydrogen isotope deuterium. Nucleosynthesis in stars and their explosions later produced the variety of elements and isotopes that we have today, in a process called cosmic chemical evolution. The amounts of total mass in elements heavier than hydrogen and helium remains small, so that the universe still has approximately the same composition.

<span class="mw-page-title-main">Stellar population</span> Grouping of stars by similar metallicity

In 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations. In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926.

<span class="mw-page-title-main">Reionization</span> Process that caused matter to reionize early in the history of the Universe

In the fields of Big Bang theory and cosmology, reionization is the process that caused electrically neutral atoms in the universe to reionize after the lapse of the "dark ages".

The Lambda-CDM, Lambda cold dark matter or ΛCDM model is a mathematical model of the Big Bang theory with three major components:

  1. a cosmological constant denoted by lambda (Λ) associated with dark energy,
  2. the postulated cold dark matter, and
  3. ordinary matter.
<span class="mw-page-title-main">Helium-4</span> Isotope of helium

Helium-4 is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consists of two protons and two neutrons.

<span class="mw-page-title-main">Metallicity</span> Relative abundance of heavy elements in a star or other astronomical object

In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable matter in the universe is either hydrogen or helium, and astronomers use the word "metals" as a convenient short term for "all elements except hydrogen and helium". This word-use is distinct from the conventional chemical or physical definition of a metal as an electrically conducting solid. Stars and nebulae with relatively high abundances of heavier elements are called "metal-rich" in astrophysical terms, even though many of those elements are nonmetals in chemistry.

16 Cygni or 16 Cyg is the Flamsteed designation of a triple star system approximately 69 light-years away from Earth in the constellation of Cygnus. It consists of two Sun-like yellow dwarf stars, 16 Cygni A and 16 Cygni B, together with a red dwarf, 16 Cygni C. In 1996 an extrasolar planet was discovered in an eccentric orbit around 16 Cygni B.

HD 20782 is the primary of a wide binary system located in the southern constellation Fornax. It has an apparent magnitude of 7.38, making it readily visible in binoculars but not to the naked eye. The system is located relatively close at a distance of 117 light-years based on Gaia DR3 parallax measurements, but it is receding with a heliocentric radial velocity of 40.7 km/s. At its current distance, HD 20782's brightness is diminished by 0.12 magnitudes due to interstellar extinction and it has an absolute magnitude of +4.61.

HD 86264 is a single star with an exoplanetary companion in the equatorial constellation of Hydra. It is too faint to be readily visible to the naked eye with an apparent visual magnitude of 7.41. The distance to this star, as determined by parallax measurements, is 219 light-years, and it is drifting further away with a radial velocity of +7.4 km/s. A 2015 survey ruled out the existence of any stellar companions at projected distances above 30 astronomical units.

<span class="mw-page-title-main">Steady-state model</span> Model of the universe – alternative to the Big Bang model

In cosmology, the steady-state model or steady state theory is an alternative to the Big Bang theory of creation of the universe. In the steady-state model, the density of matter in the expanding universe remains unchanged due to a continuous creation of matter, thus adhering to the perfect cosmological principle, a principle that says that the observable universe is always the same at any time and any place.

Deuterium fusion, also called deuterium burning, is a nuclear fusion reaction that occurs in stars and some substellar objects, in which a deuterium nucleus and a proton combine to form a helium-3 nucleus. It occurs as the second stage of the proton–proton chain reaction, in which a deuterium nucleus formed from two protons fuses with another proton, but can also proceed from primordial deuterium.

HD 79498 is a double star in the northern constellation of Cancer. The primary component of this pair has an orbiting exoplanet companion. This star is too faint to be viewed with the naked eye, having an apparent visual magnitude of 8.05. The system is located at a distance of 159 light years based on parallax measurements, and is drifting further away with a heliocentric radial velocity of 20 km/s. It has a relatively high proper motion, traversing the celestial sphere at an angular rate of 0.2″·yr−1.

HD 162826 is a star in the constellation Hercules. It is about 110 light-years away from Earth. With an apparent magnitude of 6.55, the star can be found with binoculars or a low-power telescope by reference to nearby Vega in the constellation Lyra.

In cosmology, the missing baryon problem is an observed discrepancy between the amount of baryonic matter detected from shortly after the Big Bang and from more recent epochs. Observations of the cosmic microwave background and Big Bang nucleosynthesis studies have set constraints on the abundance of baryons in the early universe, finding that baryonic matter accounts for approximately 4.8% of the energy contents of the Universe. At the same time, a census of baryons in the recent observable universe has found that observed baryonic matter accounts for less than half of that amount. This discrepancy is commonly known as the missing baryon problem. The missing baryon problem is different from the dark matter problem, which is non-baryonic in nature.

HD 146389, is a star with a yellow-white hue in the northern constellation of Hercules. The star was given the formal name Irena by the International Astronomical Union in January 2020. It is invisible to the naked eye with an apparent visual magnitude of 9.4 The star is located at a distance of approximately 446 light years from the Sun based on parallax, but is drifting closer with a radial velocity of −9 km/s. The star is known to host one exoplanet, designated WASP-38b or formally named 'Iztok'.

HD 219617 is a binary star system some 220 light-years away from the Solar System in the constellation Aquarius. It is composed of two metal-poor F-type subdwarf stars orbiting each other in a 388-year orbit. Another theory suggests that the binary star is composed of subgiant stars. Unlike many halo stars, which exhibit an excess of alpha elements relative to iron, HD 219617 is depleted in iron peak and alpha elements, although alpha elements concentrations are poorly constrained. The stellar chemical composition is peculiar, being relatively oxygen-enriched and extremely depleted in neutron capture elements. The helium fraction of the binary star at present cannot be reliably determined, and appears to be near the primordial helium abundance.

HD 46588 is a star in the northern circumpolar constellation Camelopardalis. It has an apparent magnitude of 5.44, allowing it to be faintly seen with the naked eye. The object is relatively close at a distance of only 59 light years but is receding with a heliocentric radial velocity of 15 km/s.

<span class="mw-page-title-main">HD 117566</span> High proper motion star; Camelopardalis

HD 117566, also known as HR 5091, is a solitary yellow-hued star located in the northern circumpolar constellation Camelopardalis. It has an apparent magnitude of 5.74, making it faintly visible to the naked eye. This object is relatively close at a distance of 291 light years based on Gaia DR3 parallax measurements but is receding with a heliocentric radial velocity of 14 km/s. At its current distance, HD 117566's brightness is diminished by 0.12 magnitudes due to interstellar dust.

References

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  19. ↑ Figueira, P.; Faria, J. P.; et al. (2014). "Exoplanet hosts reveal lithium depletion". Astronomy & Astrophysics. 570: A21. arXiv: 1409.0890 . doi: 10.1051/0004-6361/201424218 . ISSN   0004-6361.
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  21. ↑ Li, H.; Aoki, W.; Matsuno, T.; Kumar, Y. Bharat; Shi, J.; Suda, T.; Zhao, G.; 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 . S2CID   54205417.
  22. ↑ Hou, S. Q.; He, J. J.; Parikh, A.; Kahl, D.; Bertulani, C. A.; Kajino, T.; Mathews, G. J.; Zhao, G. (2017-01-11). "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 . ISSN   1538-4357. S2CID   568182.
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  24. ↑ Boyd, Richard N.; Brune, Carl R.; Fuller, George M.; Smith, Christel J. (November 2010). "New nuclear physics for big bang nucleosynthesis". Physical Review D. 82 (10): 105005. arXiv: 1008.0848 . Bibcode:2010PhRvD..82j5005B. doi:10.1103/PhysRevD.82.105005. ISSN   1550-7998. S2CID   119265813.
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  28. ↑ Collaboration, Atlas (2021). "Search for squarks and gluinos in final states with jets and missing transverse momentum using 139 fb$^{-1}$ of $\sqrt{s}$ =13 TeV $pp$ collision data with the ATLAS detector". Jhep. 02: 143. arXiv: 2010.14293 . doi:10.1007/JHEP02(2021)143. S2CID   256039464.
  29. ↑ Sutter, Paul (2021-01-07). "From squarks to gluinos: It's not looking good for supersymmetry". Space.com. Retrieved 2021-10-29.
  30. ↑ Holder, Gilbert P.; Nollett, Kenneth M.; van Engelen, Alexander (June 2010). "On Possible Variation in the Cosmological Baryon Fraction". Astrophysical Journal. 716 (2): 907–913. arXiv: 0907.3919 . Bibcode:2010ApJ...716..907H. doi: 10.1088/0004-637X/716/2/907 . ISSN   0004-637X.
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