Apart from being a very nice review of Big Bang Nucleosynthesis (BBN), recent paper by Karsten Jedamzik and Maxim Pospelov discusses an important open problem in the physics of BBN.

In short, the lithium problem in physics of Big Bang Nucleosynthesis is seeming underproduction of . What do we mean by that?

Well, the lithium abundance as predicted by the standard Big Bang Nucleosynthesis theory is

. (1)

On the other hand, the observed abundance seems to be about

(2),

i.e., (1) and (2) differ at least by , i.e., the deviation is statistically significant.

Before turning to possible scenarios explaining this anomaly, one has to explain in more details how exactly one estimates primordial abundance of lithium from observations.

As authors of the review explain, the value for primordial abundance of lithium is derived from absorption lines in atmospheres of low-metallicity galactic halo stars. If metallicity of such a star is sufficiently low, the abundance of turns out to be independent of the value of metallicity and temperature (in a certain range of both parameters – matallicity should remain small). This shows that the origin of lithium is cosmological: heavy elements that contribute to metallicity are themselves produced in stars.

One may think that the effect (anomalously low value of lithium abundance) might be related to erroneous atmospheric temperature determination, but this turns out to be ruled out. Two possibilities to explain the effect remain: a) lithium might be destroyed due to nuclear burning in the stellar interior (so, the underabundance of lithium is really an astrophysical problem) – this possibility is not very deeply analysed and b) underabundance of lithium means that standard BBN picture should be modified (so the problem is cosmological really). The first possibility seems to be more down-to-earth, while the second is of course more interesting physically. For example, in many models of dark matter where dark matter is made of supersymmetric partners of SM particles, underabundance of lithium-7 is naturally achieved (its presence is related to NLSP->LSP decays realized in the early Universe).

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