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The Dark Matter of Gravitational Lensing

Tom DKitching27 March 2015

Hello, I’m Tom Kitching, a cosmologist at UCL’s Mullard Space Science Laboratory. My research involves using the distorting effect of curved spacetime on the images of galaxies to map dark matter in the Universe. In some new research, my collaborators and I have pinned down the nature of dark matter to an unprecedented level of accuracy.

Dark matter is a substance that fills the Universe, and accounts for nearly 95% of all matter that exists – but we have no idea what it is. The name dark matter is however a bit of a misnomer. Dark matter is in fact transparent and doesn’t emit or absorb ordinary light at all. However it does interact with light in a very special way: its gravitational pull bends the light rays around and through it, in a similar way to how a magnifying glass does.

Our new research out now uses that bending effect, known as gravitational lensing, to make maps of dark matter around galaxy clusters.

Maps of dark matter locations around galaxy clusters. Credit: NASA, ESA, D. Harvey, R. Massey

Maps of dark matter locations around galaxy clusters from the research. Credit: NASA, ESA, D. Harvey, R. Massey

Galaxy clusters are particularly interesting places because dark matter, galaxies, and hot X-ray gas are all being smashed together as the Universe evolves. What we observe is a snapshot of these collisions, which are the biggest most energetic collisions in the Universe.

By mapping the different components of these collisions we can determine the physical properties of the dark matter. To date this approach had only ever been applied to only a few clusters of galaxies. In our research we applied to this to 72.

With this very large sample we determined that dark matter has to exist in galaxy clusters with a probability of 100,000,000,000,000:1. This is the most definitive detection of dark matter ever.

Using this very information rich data set we managed to measure how dark matter interacts with its self: a property known as its cross-section.

The cross-section measures what happens when two particles bump into each other. From the Earth we can test whether dark matter bumps into ordinary matter and what happens, which gives us clues to what dark matter is. But it’s only in space, around galaxy clusters that we can test what happens when dark matter bumps into itself, a crucial piece in the puzzle. By using this data we measured the cross-section to be smaller than previous experiments.

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