Light dark matter?

As I’m sure you all know, the best model we have of the universe says that about 80% of its matter content is in some unknown form we call ‘dark matter’. (Most of the total mass-energy content is in some unknown form we call ‘dark energy’, but that’s another story.) Perhaps the best suggestion regarding the nature of dark matter is that it consists of WIMPs – weakly interacting massive particles. But what those WIMPs are, and precisely how heavy they are, remains unclear.

Pie chart showing amounts of dark energy, dark matter, normal matter

Normal matter forms only a small part of the mass-energy inventory of the universe
Credit: NASA

There’s no general acceptance amongst the scientific community that dark matter particles have been directly detected, but there have been tantalising hints of WIMP detection in recent years. The Gran Sasso lab in Italy (which is home to the OPERA experiment, which recently observed the famous superluminal neutrino anomaly) is also home to the CRESST and DAMA experiments. Both experiments have made observations that are consistent with the detection of dark matter particles (it’s a strong claim in the case of DAMA). The Soudan mine in America is home to the COGENT experiment, which also saw events that are consistent with dark matter detection. Furthermore, the PAMELA cosmic-ray mission, which has been in orbit since 2006, has seen an abundance of positrons that some scientists have argued could be the product of dark matter annihilation.

In all the above cases the dark matter particles that are observed would be “light” particles – in other words, of relatively small mass.

A recent paper has added to the list of possible sightings of dark matter. The ARCADE ballon-borne experiment has been observing the sky in the radio spectrum, between 3-90 GHz, and has seen an excess of isotropic radiation. The paper suggests that this excess could be the result of WIMP annihilation: when WIMPs annihilate then many theoretical models suggest that they will generate pairs of electrons and positrons, which in turn will emit synchrotron radiation when they travel through magnetic fi elds. For this mechanism to explain the ARCADE results the WIMPs would need to have a mass in the range 10-20 GeV. This is not particularly massive; the WIMPs would be quite light.

So throwing all this evidence together can we conclude that dark matter particles are light? Well, no. The CRESST, DAMA and COGENT observations can all have other explanations, and in any case it’s difficult to reconcile all the data; whether the PAMELA excess can be attributed to annihilation of light dark matter particles has recently been called into question; and the ARCADE data could be the result of messy, poorly understood galactic astrophysics.

So maybe dark matter is light. But we’re far from knowing for sure.