Blame dark matter underdog for mystery missing lithium

AN UNDERDOG dark-matter particle could explain why the universe seems strangely low on lithium. If the idea holds up, it will be a boon in the hunt for dark matter, the stuff needed to account for 80 per cent of the universe's matter.

In the universe's first few fiery minutes, nuclear reactions forged a host of light elements, including helium, deuterium and lithium, in a process called big bang nucleosynthesis. The amounts of these elements present in the early universe, gleaned from ancient stars and primordial gas clouds, match theory, except in one respect: they contain much less of the dominant form of lithium, lithium-7, than expected. There has never been a satisfactory explanation for this.

Now help comes in the shape of hypothetical dark-matter particles called axions. These light particles were dreamed up in the 1970s as part of a theory to explain why the strong nuclear force, unlike the other forces, does not change if a particle is swapped for the antimatter counterpart of its mirror image. Axions are not the dominant theory for dark matter. That accolade goes to weakly interacting massive particles, or WIMPs. But as neither WIMPs nor axions have ever been observed, the jury is still out.

In the latest research, the underdog axions score a point. The rates of nuclear reactions that produced lithium-7 depend partly on the amount of energy that was present in the form of light. As we cannot tell how much light was there directly, we infer it from the cosmic microwave background (CMB), the echo of the big bang that emerged 380,000 years later. This is used to estimate how much lithium should be present: more light skews reaction rates and lowers expected levels of lithium.

Ozgur Erken of the University of Florida in Gainesville and colleagues suggest that something cooled photons between the synthesis of lithium and the emergence of the CMB, causing the photon energy to be underestimated, and inflating the expected amounts of lithium.

Born with very little kinetic energy, axions are a prime suspect. When their cooling power is accounted for, the predicted lithium abundance drops by half, the team calculate (Physical Review Letters, DOI: 10.1103/PhysRevLett.108.061304). "We're excited that it gives about the right correction," says Pierre Sikivie, Erken's colleague.

Adding in axions also creates a problem, however. Without them, CMB measurements are consistent with about four types of neutrino, close to the three types glimpsed in experiments. But if axions are present, they would skew this measurement and imply about seven neutrino types, Erken's team calculate. This makes Gary Steigman of Ohio State University in Columbus, who was not involved in the study, sceptical of the axion explanation for the lithium-7 anomaly.

An answer should come in 2013 when much better measurements of the CMB are expected from the Planck satellite. Our best chance of glimpsing axions, meanwhile, lies in an upgraded version of an experiment called ADMX, due to start up towards the end of this year. It may also be possible to infer their existence from data from the Large Hadron Collider at CERN near Geneva in Switzerland, where they should boost the production of Higgs bosons.

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