The Lithium Experiment

In Physics by Brian Koberlein0 Comments

One of the big successes of the big bang model is its prediction of elemental abundances. The first elements were produced in the early moments of the universe through a process known as baryogenesis. This process is very complex, and it is highly dependent upon the temperature and density of the universe at that time. Change the temperature a bit one way or the other, and the initial ratio of primordial elements would be different. Knowing the temperature of the early universe, we can predict the amount of hydrogen vs. helium produced by the big bang, and this agrees fairly well with observation.

But the model also predicts that trace amounts of lithium should also be produced. According to the theory, for every 10 billion hydrogen atoms the big bang produced, it should also have produced a lithium atom. That may seem like a tiny amount, but it is much higher than we observe in the early universe. In other words, the model predicts much more lithium in the early universe than we observe, and this is known as the “lithium problem.”

There have been a couple of proposed solutions to this problem. One is that we simply aren’t seeing the amount of lithium that’s actually there. Lithium can be difficult to detect, and it is actually consumed in the fusion process of stars, so it is possible that the big bang prediction is right, and we simply need to determine where it went. Another idea is that somehow our model of baryogenesis is wrong. If, for example, there exists some complex nuclear interactions we don’t yet understand (perhaps due to physics beyond the standard model) then we could be over-predicting the amount of lithium in the early universe.

But now a new paper in Physical Review Letters shows the latter idea doesn’t work. The work was done at the Laboratory for Underground Nuclear Physics (LUNA) where they study nuclear fusion reactions. This new work presents the results of lithium-6 production at temperatures equal to the early universe. They found the rate of lithium production agrees with that predicted by big bang cosmology. So it seems the big bang model works yet again, and this means we still have a lithium problem.

Paper: M. Anders et al.  First Direct Measurement of the 2H(α,γ)6Li Cross Section at Big Bang Energies and the Primordial Lithium Problem. Physical Review Letters 113, 042501 (2014).

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