WIMPs may occupy the universe

As I type this post, the physics community is holding its collective breath waiting for news from the Large Hadron Collider, in the hope that it may give us a clue for the next step in understanding how the universe works. In the meantime, any unusual result like possible faster than light neutrinos is seized in the hope of finding something new.

Despite this, I can’t help wondering if we’re missing an obvious piece of the puzzle: dark matter and dark energy together account for about 96% of the total mass of the universe.

So if everything we can see and experiment on is a mere 4% of all that there is, is there any wonder it doesn’t make sense?

Okay then, do we know anything at all about what dark matter may be? Well, for a long time the leading candidates have been either MACHOs or WIMPs. Yes, those are the actual names.

MACHO stands for Massive Astrophysical Compact Halo Object, and it basically refers to clumps of normal matter that sit in the “haloes” around galaxies, but for some reason can’t be seen. They could be objects like planets, brown dwarfs (unignited stars) or even black holes.

Although MACHOs don’t emit enough light for us to see, they can be detected by gravitational microlensing, which is when their gravitational fields bend the light rays from distant stars when they pass in front of them.

Unfortunately, although surveys of gravitational microlensing have found MACHOs, there’s not enough of them to account for the universe’s missing mass.

So we turn to the other option: WIMPs, or Weakly Interacting Massive Particles. These are a postulated type of subatomic particle that only interacts via gravity and maybe the weak nuclear force.

This means they would have properties similar to neutrinos (i.e. very hard to detect), only heavier (hence the “massive”). And that’s why we haven’t yet been able to create them or detect them on Earth.

However we do have some indications they exist, from observing collisions between galaxies like in the famous Bullet Cluster (pictured).

The so-called Bullet Cluster, formed from the collision of two galaxies, as captured by NASA's Hubble Space Telescope and Chandra X-ray Observatory. Ordinary matter, shown in pink, has been distorted into a bullet shape by the collision, whereas the clusters of dark matter, shown in blue, appear to have passed straight through. This supports the theory that dark matter particles only interact by the force of gravity. (Image from NASA)

In these collisions, we see matter like hot gas piling up in the middle, but the centres of mass of the colliding galaxies – presumably made out of dark matter – pass straight through. This suggests that the dark matter is something that doesn’t interact much at all, like WIMPs.

Lately though, there have been some intriguing signals suggesting we’re getting closer to actually detecting these dark matter particles. Experiments like CRESST and DAMA in Gran Sasso (the same underground lab that detected those pesky neutrinos) have seen hints of WIMPs  colliding with nuclei.

And most recently, the balloon-borne experiment ARCADE has seen a radio signal coming from space that could be the result of WIMPs colliding with each other to produce pairs of electrons and positrons (Fornengo N, Lineros R, Regis M & Taoso M 2011, “A dark matter interpretation for the ARCADE excess?”, arXiv:1108.0569v1).

These are all very small, very early results, so we have to be careful not to jump to conclusions. But when we’re contemplating the universe and trying to come up with a “theory of everything”, we really shouldn’t ignore the other 96% we can’t see.


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