This article on lithium problem is a guest article by Anja Sjöström, an IB diploma student from Switzerland.
When comparing the theoretical abundance in H or He at early stages of the Universe with the experimental values observed in astronomical objects which reflect primordial abundances as faithfully as possible to our date, the measurements seem to match the theoretical predictions of the formation of light elements after the Big Bang, determined through a study of the Big Bang nucleosynthesis and precise calculations of baryonic ratios.
However, when the abundance of another light element, lithium, is studied, a discrepancy between astronomers’ observations and the theoretical amount of lithium arises. A problem identified as the cosmological lithium problem or the “Schramm plot”.
Big Bang Nucleosynthesis (BBN) theory
The BBN theory yields precise quantitative predictions of the primordial abundances of light elements or chemical compounds (notably deuterium, helium-3, helium-4, lithium-7) at the end of the Big Bang, from around 1 second to 3 minutes after it. To determine the abundance of primordial baryonic matter, including lithium, this theory determines the baryon-to-photon ratio on which it depends. This value is assumed to be constant or to yield a unique range, under the assumption that the universe is homogeneous. According to calculations, lithium-7 should account for about 10-9 of all primordial nuclides, and lithium-6 about 10-13.
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Incongruity With The Observations
In addition to BBN as a reliable and verified theory to explain the abundance of most light baryonic matter, the Wilkinson Microwave Anisotropy Probe (WMAP) had from 2001 to 2010 studied Cosmic Microwave Background as well as the Planck space probe from 2009-2013, in attempt to yield two independent values of the baryon-to-photon ratio.
Combined with these measurements, scientists had also attempted to study the composition of stellar bodies in which very little stellar nucleosynthesis (the synthesis of chemical elements through nuclear fusion within stars) is being or has been performed. Thus, the primordial abundances and ratios of baryonic matter would have been almost inconsequentially altered and the assumption of reflecting a similar lithium abundance would be correct.
Such phenomena were to be found in stars discovered by astronomers in the 1980s in old dwarf galaxies. These stars, being referred to as poor-metal or population II stars, are known to produce little elements other than hydrogen and helium through nucleosynthesis and thus, are said to have maintained constant amounts of lithium throughout their lifetime, reflecting primordial conditions.
Furthermore, other stellar bodies which were several thousands of light-years away are being used to study the early baryonic composition of the universe. Thus when observed from Earth are seen in their early stages, they accurately reflect the conditions shortly after the Big Bang.
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However, even though the theoretical abundance of lithium in the early stages of the universe seems to be well explained and additionally, backed up by the observed correspondence in abundances of hydrogen and helium, the observed amounts of lithium are thoroughly inconsistent, increasing by a factor of 3 to 4 times in poor-metal stars as well as decreasing considerably in stars hosting planetary systems.
Solutions To Lithium Problem
Two ways to explain the discrepancies in theoretical and observed lithium abundances are first of all to modify the speculations of BBN theory, and second of all to account for a faulty selection of stars and astronomical bodies which were said to accurately mirror the ratios of baryonic matter in the primordial stages of the universe. As such, these poor-metal stars might have hosted processes of lithium decay or destruction causing their abundances to have evolved over time to no longer accurately reflect primordial conditions. As such, the evolution of lithium in stars is yet poorly understood.
However, we now know that altering BBN is not yet necessary as evidence rises to assert that lithium in metal-poor stars is in fact not primordial. As such, the focus should be directed towards understanding processes of lithium depletion within stars for when studying metal-poor interstellar gas astronomers have come to realize that these areas remain unaffected by processes altering chemical composition over time. As such, a correlation was noticed in the lithium spectrum of the Small Magellanic Cloud, a dwarf galaxy near the milky way in which the abundance of lithium seems to closely match predictions of the theory of BBN.