Café Scientifique – “What is Dark Matter?”

Professor Justin Reed from University of Surrey gave a good entry-level presentation on dark matter to a pretty full Monroe’s.

Since its discovery by the Swiss astronomer Fritz Zwicky in the 1930′s, dark matter has continued to capture the public imagination. It raises the velocity of stars and gas in galaxies, bends light around massive galaxy clusters and promotes the growth of structure in the Universe. In this talk, Justin will explain the key evidences for dark matter, and our latest theories for what it is. He will show that the latest data points towards dark matter being some new particle that lies beyond the standard model of particle physics. If this is correct, then billions of these particles will flow through your head by the time you finish reading this paragraph (without effect thankfully). This is such a striking thought that it has already inspired many artists and writers, from Cornelia Parker’s “Cold Dark Matter” sculpture to Philip Pullman’s “His Dark Materials”. Justin will conclude with a look to the future and our prospects for detecting or creating such a particle in the next five years.

I was impressed by his smooth, clear delivery and the Q&A afterwards was well handled too.

The content is obviously something he teaches often and there’s a 39 minute video available online of a more in-depth version of his talk that Justin gave a few years ago in Switzerland.
If you only speak English, skip the first 5 minutes. Brownie points to Justin for starting off in German.


To quickly show how people like Justin propose the existence of dark matter, here’s a Deep Chandra X-ray image of the Bullet Cluster.

  • The colourful area is showing the energy released by the collision of two galaxy clusters (which are now moving apart to left and right). The blue area is over a megaparsec in width – in comparison, the Andromeda Galaxy is about 0.78 Mpc (2.5 million light-years) from the Earth. Making the Kessel Run in 12 parsecs doesn’t impress anyone at this scale.
  • The green contour lines overlaying the energy map show the mass density as calculated by gravitational lensing – that is, where mass must be to explain the distortion around it of light coming from objects way beyond the cluster.

So, as the clusters pass through, and away from, each other, the gases are being dragged behind through friction. The centres of mass density for the two clusters, though, are not being distorted in this way at all which would indicate the mass is almost entirely matter that doesn’t interact with anything it passes. If it did then the green contour lines would be arranged around the centre of the collision.


Takeaways from the talk

  • Dark matter should be called ‘invisible matter’. If it was just dark (like dust), we would see the stuff blocking light. As it doesn’t interact with light enough to be detectable, it’s invisible, not dark. Not such a catchy label, though.
  • The spread of dark matter in a galaxy like ours doesn’t follow the disk shape but is more of a fuzzy ball. As there is no interaction with ‘normal’ matter, and so no way to lose energy, there is no tendency for the dark matter to drop down into the plane of rotation.
  • I need to understand vacuum energy and Casimir plate experiments.
  • Finally, it is part of the scientist’s code that the effects of gravity must be demonstrated with a tennis ball and string.


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