Shining a Light on Dark Matter – Anne Green

4th November 2012

Today on The Science Show, whist most people will be looking to the heavens to see fireworks light up the sky, we are going all gothic and looking to the darkest corners of the universe.

It turns out that in the visible universe, everything which we can observe makes up a paltry 5% of the mass. So either our laws of physics need rewriting, or there are mysterious forces at play. Cosmologists across the planet fall into either camp, and we Have Dr. Anne Green from the School of Physics and Astronomy to talk about one candidate, the mysterious dark matter.

Dark matter is different to the stuff we see around us as light doesn’t reflect of absorb it. This makes it a darn sight more difficult to detect. However, it turns out gravity still behaves as anticipated, so dark matter can bend light and be detected that way.

A map of Dark Matter

But what on earth could dark matter actually be made of? Some mysterious exotic particle we have no knowledge about? Or possibly the fondant center of a chocolate Krispy Kreme? Find out the answer to this and more from 6pm on The Science Show.

Also on tonights show some of the team investigate what happens when you put metal in a microwave… Do not try this one at home!


Dark Matter:

There are a number of Dark Matter candidates, including WIMPs and MACHOs.

MACHOs for be, for example, black holes or neutron stars. However, theoretical work (i.e. lots of difficult and boring maths!) has shown they are not a candidate anymore.

WIMPs (Weakly Interacting Massive Particles)are the number one candidate, this is for 2 main reasons.

Firstly, they are “cold”. This basically means that they are heavy and slow moving. Dark Matter needs to be cold, otherwise (for reasons that are pretty complicated…) there would be no galaxies in the universe. Considering we can see galaxies in the universe, we know that the Dark Matter must be cold!

Secondly, presuming they do exist, the theoretical amount of WIMPs that there would be in the universe has been worked out. This amount is the same as the amount of Dark Matter in the universe. This seems like a pretty good indication that Dark Matter is made up of WIMPs!

Dark Matter

How can we detect Dark Matter?
We can either detect it directly, therfore looking directly for Dark Matter particles. Alternatively we can try and detect them indirectly by looking for the products of the potential dark matter candidates.

There is an experiment that involved the drilling into the ice caps of the South Pole to plant detectors. This is in order to detect Neutrinos that escape from the sun during annihilation reactions of WIMPs actually at the centre of the sun.

Other WIMP annihilation reactions produce large amounts of Gamma radiation that can also be detected by telescopes in space as well as on the surface of the earth, looking for the shower of radiation that passes throguth the atmosphere.

Dark Matter Pie Chart

Question Time:

If Dark matter doesn’t actually exist, what other reasons could there be for the strange effect on Gravity?

Answer: If we assume that all the mass in the universe is visible mass, then we would have to modify gravity, because the bending of space time is much larger than expected for the mass observed. However modifying gravity is not a very elegant solution and proves very difficult to do, with no advances so far. This is why the theory of Dark Matter is favoured in some opinions.