# Is any consensus forming on the solution to the “Lithium Problem”?

The "Lithium Problem" relates to the fact very-low-metallicity stars appear to have a Li/H ratio approximately one third of what would be expected. The ratio should be the same as the prediction from Big Bang nucleosynthesis theory. A possibly related problem is that the ⁶Li/⁷Li ratio is several orders of magnitude too high.

• A cursory google search brings up this, which discusses a recent paper that purports to offer a potential resolution: a separate approximation that was in common use is alleged to be flawed, and it is suggested that the correct one may resolve in part or in whole the problem. I'm not informed enough to know how well-supported the referenced paper is, and if any sort of "consensus" has been formed, though. – zibadawa timmy Sep 16 '15 at 0:56
• The original academic paper ref'ed in the article @zibadawatimmy linked to can be read here: arxiv.org/pdf/1502.01250.pdf ~ Simply because that is a brand new solution, I doubt it has much consensus around it yet. There are a lot of competing solutions out there. This problem is over a decade old. – Aabaakawad Sep 16 '15 at 1:12

No. There is no consensus.

The discrepancy between the predicted big bang nucleosynthetic abundance of Lithium 7 and the measured value can be summarised as follows.

If we take what we know about the the baryonic mass density of the universe and the Hubble constant, we get a self-consistent picture between the cosmic microwave background, observations of galaxy recession etc. and the estimated primordial abundances of Helium and Deuterium.

The problem arises because these same cosmological parameters predict a primordial lithium abundance of $3\times10^{-10}$, when expressed as a ratio to the hydrogen abundance.

On the other hand, measurements of the Li abundance present in the photospheres of the oldest stars (a.k.a. "halo stars") in our Galaxy suggest that the primordial abundance was about $1.2\times10^{-10}$.

The factor of 2-3 difference between these numbers is about 4-5 times the measurement precision. This is the so-called "Lithium problem".

The potential solutions are reviewed by Fields (2012). They fall into the following categories.

1. Astrophysical solutions - that we don't understand our measurements of the Li abundances because of an imperfect understanding of the atmospheres of low metallicity stars; or that we don't understand interior mixing mechanisms that mean at the photosphere, we see material that has been mixed upwards from the interior where the Li has been depleted in nuclear reactions.

2. Nuclear physics - maybe the details of the reaction rates and cross-sections in the big bang model are awry? There are still some sizeable uncertainties here which remain to be nailed down, but are seen as rather unlikely solutions.

3. Additions to the standard big bang model. This includes things like inhomogeneous nucleosynthesis in the early universe - i.e. that it was clumpy even at this early stage. Other possibilities include the possibility that various "constants" were actually different in the early universe or that the equilibrium reactions in big bang nucleosynthesis were upset by the decay of massive dark matter particles.

Thus there are lots of ideas to solve this problem, and other ideas which suggest it is not so much a problem, but that we can't do the measurements properly.

• TYVM. I don't have enough rep to vote up yet. – Aabaakawad Sep 17 '15 at 6:18