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Why is there a discrepancy between the amount of lithium-7 predicted to be produced in Big Bang nucleosynthesis and the amount observed in very old stars?

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  • $\begingroup$ This is The Question, isn't it? $\endgroup$ – jaromrax Apr 20 '15 at 11:03
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    $\begingroup$ Why indeed? Could you clarify your question - if anyone knew the answer, it wouldn't be the topic of so much research. Are you just wanting to know what the alternatives are? $\endgroup$ – Rob Jeffries Apr 20 '15 at 12:16
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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 that 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 ("halo stars") in our Galaxy suggest that the primordial abundance was actually 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 that the equilibrium reactions in big bang nucleosynthesis were upset by the decay of massive dark matter particles.

  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 $^8$Be nucleus changes with time in such a way that lots of $^8$Be 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 $^7$Li.

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.

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