On Wednesday 13th June, Dr Chamkaur Ghag – physics lecturer at UCL and current Chair of the Dark Matter UK Consortium which brings together the UK’s Dark Matter researchers – took us on a journey through time and space and asked the difficult questions: what is Dark Matter and how do we find it?

Dark Matter has eluded scientists for many years, and unlike regular visible matter (ie, stars, planets and all the stuff in the universe that we can see) it is yet to be completely defined. Visible matter accounts for 5% of the universe, while Dark Matter makes up an incredible 27%. The remaining 68% is Dark Energy, and although this represents the majority of the universe, it is very complex and leads to even more unanswered questions.

The theory of Dark Matter really took shape in the 1930s when Swiss physicist Fritz Zwicky found conclusive evidence that something we could not see was acting as a glue holding together galaxies. Enter Dark Matter. If Dark Matter were not there, we would expect to see stars pinging out of galaxies because of how fast they are moving, but this is not the case! To keep galaxies together you need a lot of gravity, and to have this much gravity, you need an awful lot of mass – 10 times the amount of mass that is visible. It is through this observation that the inference of Dark Matter was made.

In the 1970s, the American astronomer Vera Rubin provided further evidence of the existence of Dark Matter in huge quantities in every galaxy. The search then began to find out what Dark Matter was really made of.

What We Know About Dark Matter

Although there is a lot of evidence that demonstrates the existence of Dark Matter, it has never been directly detected. Of the four fundamental forces, gravitation, electromagnetism, the weak interaction and the strong interaction, it is known that Dark Matter exhibits gravitation. It is also known to have no electromagnetic properties – you cannot shine a light on it to see it as it does not interact with light, neither does it exhibit strong interactions. However, Dark Matter is stable; it was present at the start of the Universe and it is still around now. The prevailing theory is therefore that Dark Matter is made up of WIMPs – weakly interacting massive particles.

How to Measure Dark Matter

Dr Ghag explained that there are three ways of potentially detecting Dark Matter:

  1. Indirect Detection – trying to observe two particles of Dark Matter colliding by measuring the particles released when they annihilate one another and decay.
  2. Particle Collider – using a powerful particle collider like the Large Hadron Collider to smash two Standard Matter particles together to see if Dark Matter is produced.
  3. Direct Detection – trying the observe the collision of a Dark Matter particle with a Standard Matter particle and measuring the effects.

It is this last method that Dr Ghag is testing, using the Large Underground Xenon (LUX) experiment in South Dakota. This takes place in an old gold mine which shields the detector from as much environmental radiation as possible.

As with most things in life we strive for bigger and better and Dr Ghag and his colleagues are no different. They are currently building an even larger detector called the LUX Zeplin Detector which they hope will provide definitive evidence of Dark Matter particles.

Thank you to Dr Ghag for being our guide into the darkness and we hope that all who attended the talk enjoyed it as much as we did.

Our next Sci-Bar event will be on Tuesday 17th July and will introduce us into the layered world of 3D Printing – Life in plastic, it’s fantastic!

Reporter: Jeanne Kroeger

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