After hydrogen and helium, lithium is the lightest and simplest element in the universe. It should have been everywhere right after the Big Bang...but the data shows a mysterious shortage. The explanation may point to an unlikely dark matter candidate.
The leading explanation for the creation of the lightest elements at the beginning of the universe is known as Big Bang Nucleosynthesis. As Physics World explains, the idea is that the primordial universe was once a thick soup of super-heated protons and neutrons. Those two particles began assembling into atomic nuclei, creating the hydrogen isotope deuterium as well as helium and lithium isotopes. Electrons entered the picture as temperatures dropped, and the afterglow of this process is what we now know as the Cosmic Microwave Background.
There's just one problem with this whole idea - our observations of the CMB suggest that there isn't enough lithium-7. In fact, there's only about a third the predicted amount, which is a fairly gaping hole in an otherwise satisfying hypothesis. To fill that gap, University of Florida physicists led by Pierre Sikivie point to the axion, a hypothetical particle that has long been discussed as a possible alternative candidate for dark matter. If the axion exists, it's a very light particle that barely interacts at all with matter. (For more on the axion, and other dark matter candidates, check out our primer.)
If the axion does exist - and that's still a pretty huge "if" - then some of its less recognized properties could actually be put to use explaining the lithium-7 discrepancy. Physics World has more:
Sikivie and colleagues point out that axions can form a Bose–Einstein condensate (BEC). Such condensates contain particles that have all fallen into their lowest energy state, and are best known to occur in low-density gases at temperatures close to absolute zero. But since the critical temperature for transition to a BEC depends on density, say the Florida researchers, particles can form BECs at higher temperatures as long as they are dense enough. Even in the primordial heat of the Big Bang, the researchers say, axions would easily be dense enough to form a BEC.
An axion condensate would have a marked effect on Big Bang nucleosynthesis. Passing photons would make waves in it, transferring heat and, ultimately, depleting in number. This means that the baryon-to-photon ratio would increase towards the time of recombination, giving cosmologists today a falsely high impression of the amount of lithium that should have been created.
It's an interesting possibility, and it certainly takes care of the lithium-7 problem - not to mention bolstering axion's candidacy for dark matter - but it has some substantial drawbacks. Basically, to solve the lithium-7 problem with axions, you're creating two completely new problems. The presence of axions would cause deuterium levels to be much higher than what we observe in the CMB, so you're essentially trading one discrepancy for another.
What's more, the number of neutrino types would have to increase from its current value of 3 or 4 to about 7 for this to work. According to Physics World, Sikivie acknowledges both of these problems, but he also points to one the fact that the European Space Agency's Planck Space Observatory is still measuring the number of neutrinos, and we can expect a new, more accurate measurement next year. If that number spikes up close to 7, then we might need to take a much closer look at this axion explanation.