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Vera Cooper Rubin and dark matter

On 25 December 2016, the American astronomer Vera Cooper Rubin passed away, aged 88. One of her most significant contributions to physics was the discovery of the discrepancy between the predicted and the observed angular motion of galaxies, by studying galactic rotation curves and analysing rotation rates. Although her findings were initially met with scepticism, Rubin’s results were confirmed over subsequent decades.

Though dark matter has little to do with our daily lives, it is a significant help in understanding the configuration of the universe, as more than a quarter of its content is dark matter. Dark matter is invisible: it doesn’t absorb light, it doesn’t emit light, it doesn’t reflect light, and it doesn’t interact with light, which is what we use to detect things. It is only because of Rubin’s research on galactic rotation curves that we know it is there.

The rotation curve of the Triangulum Galaxy

From diagram above, we can see that the measured velocity (upper curve) doesn’t fall off with distance from the galaxy as calculations using visible matter would suggest. This indicates that there is much more mass at the edges of galaxies than we can detect.

So where does the extra mass come from? Physicists think that there is something present, dark matter, that has mass but doesn’t reflect light, so can’t be detected using radio telescopes. They now agree to represent it as large “blobs” around ordinary substance.

The gravitational pull of a Luminous Red Galaxy warping light from a blue galaxy further away, taken with the Hubble Space Telescope

The arcs pictured above are additional observational evidence of dark matter, as they can be used to estimate the masses of galaxy clusters.

You can do an experiment at home to see similar arcs yourself without a telescope like Hubble. All you need is a piece of paper and a glass:

  1. Break the glass, and shave off its top so you can use the base as a lens
  2. Draw a small dot on the paper
  3. Put your “lens” above the dot and align it so the dot is in its centre.

You should see a ring, and if you move the lens a little to one side the ring will break into a few arcs!

Scientists are still trying to find out how to prove the whole theory of dark matter, and find a use for it. It is clear that this is another giant step for mankind in the exploration of our universe, and that is why I like physics. As you dig deeper, you become more and more aware of how innocent you were to all of these facts, and find the whole universe more and more beautiful!