Cosmology is truly a remarkable science. Okay, all science is remarkable, but cosmology deals with something so neat and simple—the beginning of the Universe, where all of our reality was governed by fundamental physics. That simplicity is seen through the blurred vision of time, though. The remarkable part is how much detail we can extract from the fuzzy forms that are visible of the past.

One of those details is nucleosynthesis. The Big Bang theory predicts the elemental make up of the early Universe with amazing accuracy. Except for lithium. Lithium is either hiding, or there is an eater-of-lithium that shares an apartment with its better known cousin, the eater-of-socks. In lieu of evidence for an eater-of-lithium, scientists have been trying to figure out what might have prevented lithium from forming in the first place. One solution: a new particle that seems promising.

In the early Universe, there were no atoms or molecules as we know them today. The Universe was made up of protons and electrons that had too much energy to stick together, so they formed a kind of fluid, mixing and flowing around each other. But, as the Universe expanded, the fluid cooled, some of the protons began to stick together, grab a neutron or two, and form the first heavier elements.

To assemble your universe, unpack the parts on a clean flat surface

So, along with hydrogen (just a proton), occasionally a deuterium—a hydrogen with a neutron attached to the proton—would form. How occasionally? For every million hydrogen nuclei, 25 deuterium nuclei were made. And, here is an incredible thing: taking only the standard model of physics and the ratio of ordinary matter (baryons) to photons (light), cosmologists can predict this ratio. Observations show that this prediction is correct to within 1.6 percent.

Cosmologists can also predict the ratio for other elements as well. But observations here are a bit fraught, because heavier elements are also produced and consumed in stars, so it is difficult to determine how much was originally produced in the Big Bang. For example, Big Bang theory predicts that about five lithium atoms were made for every ten billion hydrogen atoms (I am only talking about one isotope of lithium, Li7, which has three protons and four neutrons and is the most abundant form). However, the best observations show under two lithium atoms for every ten billion hydrogen atoms.

The only reason that cosmologists are not crying into their cups about this particular failure is that they could blame observations. It was, until recently, possible that maybe, just maybe, the lithium was consumed at a greater than expected rate in stars or that the ratio of deuterium to hydrogen was a bit different to what was predicted. This wiggle room could kinda-sorta accommodate the missing lithium.

Unfortunately, the hydrogen to deuterium ratio has now been pinned down pretty accurately, so it is really difficult to come up with a consistent set of physical laws and constants that account for both the ratio of hydrogen to deuterium and the ratio of hydrogen to lithium.

Maybe super-symmetric models?

Which is kind of exciting. You can hear the whispers circulating: could this be evidence for new physics? Yes, maybe, according to a trio of researchers thats suggests not just new physics but a new particle.

The basic idea is this. This isotope of lithium originated in an unstable isotope of beryllium, the next heaviest element, formed after the Big Bang. So, if we have a particle that suppresses the formation of beryllium, then the lithium problem may well go away.

The researchers propose a pair of possible particles that participate in pinching protons, preventing the production of beryllium (try saying that five times fast). More seriously, the heart of the issue lies in how beryllium is formed. Theory predicts that it is the product of fusion between two isotopes of helium, which occurs very slowly.

So one possibility is that there's a particle that reacts with beryllium, causing it to decay back into two helium isotopes. This will reduce the abundance of lithium while not altering the ratios of hydrogen to deuterium (or the ratio between the helium isotopes). Even better, this sort of particle would increase the abundance of helium by only a minuscule amount.

A second solution is a bit more complicated: a particle that thumps deuterium, knocking a neutron out. The extra neutrons floating about suppress the formation of beryllium—how this works is not entirely clear to me, but I'd guess that the additional neutrons change the balance between the two helium isotopes, which slows beryllium production. Normally, adding neutrons would make things worse, because the neutrons would also create more deuterium. However, these neutrons come from destroying deuterium, and most of them go back into making deuterium again. So, again, the balance is maintained.

Before assembling nuclei, chose your beryllium inhibiting particle

The solution presented by the researchers appears to work as long as the new particle doesn't live too long. For lifetimes on the order of one hour, everything seems to work out pretty well. By which I mean that the other relevant numbers all turn out to be reasonable, and the ratios of different elements turn out within the bounds of observations.

Okay, that's the theory, but where is my new particle? Still missing. In fact, many of the "beyond" the Standard Model models don't admit a particle with the properties the researchers need. Except for axions. Axions are being searched for extensively by many different groups. The researchers calculated that one old and no longer operational experiment, called the Liquid Scintillator Neutrino Detector, might have been sensitive to the new particle.

Oddly, they didn't perform a corresponding calculation for currently operating experiments. However, it rather raises the question of where this particle might be? Is it lurking in old data waiting to be discovered?

Physical Review Letters, 2016, DOI: 10.1103/PhysRevLett.116.211303