Is any consensus forming on the solution to the "Lithium Problem"?

Question

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.

Answer 1

There is no absolute consensus and nothing proven beyond doubt, but there are favourite explanations.

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 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.

EDIT: Update 18/11/19 from the "Lithium in the Universe" conference in Frascati, Rome.

Having sat through some review talks on the problem, I can summarise progress as:

Explanation 2 (Nuclear Physics) is dead. All avenues appar to be explored and the likely remaining uncertainties are at the level of 10%.

However, the following possibilities were also discussed.

4. Perhaps the fundamental constants have changed with time resulting in somewhat different binding energy differences between different nuclei? Scherrer & Scherrer (2017) for example discuss a scenario where the mass difference between 2 He nuclei and a 8Be nucleus changes with time in such a way that lots of 8Be is produced during big bang nucleosynthesis, but decays back into He later on. This does not alter the primordial He abundance inferred from present-day observations, but the removal of He at early epochs results in a lower production of 7Li.

5. Inhomogeneous magnetic fields, contributing significant energy density in the early universe may have led to temperature fluctuations at the epoch of nucleosynthesis. Such fluctuations could reduce the predicted Li abundances a bit, but they also change the predicted deuterium abundance and this wouldn't fit with the now quite precise measurements of the primordial D abundance and the Planck value of $\Omega_b h^2$.

6. It has been hypothesised that magnetic fields can play a role in filtering how much primordial Li makes it into the structures that form galaxies. i.e. There is a sort of chemical differentiation caused when there is a gradient of the magnetic field strength at right angles to the magnetic field. This accelerates ions down the gradient and possibly away from forming structures. Since Li is easily ionised in the early universe (compared with H and He), this might lead to the gas that formed the first stars being Li-poor.

At the end of the meeting, participants were asked (for fun) to vote on what they thought to most likely solution was. #1 Was first, with #3 a distant second.