An excess of radio waves recorded by a balloon-borne experiment could be a signal of dark matter, a new study suggests. Data from the ARCADE mission seem to fit in with other direct, recently reported evidence for dark matter, although some believe they may have a mundane explanation.

Dark matter is an elusive substance thought to make up more than 80% of the matter in the universe. While dark matter is invoked to explain the anomalous rotational velocity of galaxies and other mysterious astronomical phenomena, no-one has yet detected it conclusively. In recent years, however, hints of detections have been reported by several experimental collaborations, namely CRESST and DAMA at the underground lab in Gran Sasso, Italy, and CoGeNT at the Soudan mine in Minnesota, US. Other secondary evidence for dark matter has been reported by the satellite-borne experiment PAMELA and possibly the balloon-borne experiment ATIC – both of which have looked for excesses of electrons and positrons generated by dark-matter collisions.

Surfeit of radio waves

Now, it seems there may be secondary evidence from another balloon-borne experiment to add to the mix. In 2009 the ARCADE 2 collaboration reported that their experiment, which had taken radio measurements of the sky at frequencies between 3–90 GHz, had recorded a directionless or "isotropic" component of radio waves that was five to six times higher than contributions recorded in other surveys. Since then, studies have attempted to explain the excess as the remnant of, for example, supernovae or quasars, but none of these explanations has worked out.

Now, Nicolao Fornengo of the University of Turin in Italy and colleagues believe that the most likely explanation of the ARCADE excess is the existence of lots and lots of very faint sources, rather like the make up of "haloes" of dark matter outside galaxies. When dark-matter particles – known as weakly interacting massive particles, or WIMPs – collide and annihilate one another, they are thought to generate electrons and positrons, which subsequently generate radio waves via synchrotron emission as they travel through magnetic fields.

If such WIMPs were the root of the ARCADE radio excess, say Fornengo and colleagues, they would probably be quite light, with a mass between 10–20 GeV. That's "in the right ballpark of DAMA, CoGeNT and CRESST," says Fornengo. The results are due to be published in Physical Review Letters.

"Messy astrophysics"

Opinion is mixed among astrophysicists on how likely the researchers' dark-matter interpretation is. Douglas Scott at the University of British Columbia thinks a mundane interpretation is more likely. "If we're missing something, then the most likely candidate is that it's something to do with 'messy astrophysics' – i.e. details of the formation and evolution of galaxies," he says. "But it's worth keeping an open mind to the possibility that this is telling us something about particle physics and the nature of dark matter."

Alan Kogut, leader of the ARCADE collaboration at NASA's Goddard Space Flight Centre in Maryland, US, is "a little sceptical of any astrophysical result involving dark-matter annihilation", although he admits that the explanation fits. The Square Kilometre Array (SKA), a huge radio telescope in development in the southern hemisphere, should be able to provide corroboration, he adds.

Fornengo agrees that the SKA will shed more light on the ARCADE excess, and that an independent estimate of the universe's total isotropic radio background is needed. In the meantime, he likes the dark-matter interpretation because it "can reproduce the radio observations in a natural way, namely without introducing any further particularly optimistic assumption".

"I think it is an interesting exercise, and a suggestive one, but there a number of parameters that could entirely change the interpretation," says Stefano Profumo at the University of California, Santa Cruz, US. "So this is to be taken as a 'proof of principle' of a possible interpretation of the ARCADE excess, but not in any sense something that might point towards exotic origins such as dark-matter annihilation or decay."

A preprint of the paper is available at arXiv:1108.0569.