The Quest for Dark Matter

The Astronomers and The Film Without a Protagonist

ESA“Galaxies that are closer have merged more. You could rewind them like a film, as it were.”

By Bart Braun

Leiden astronomers have determined the distribution of dark matter in a small piece of the universe, although it is still not exactly clear what dark matter is. “But you wouldn’t be here if it didn’t exist.”

If you study the universe carefully, you will see that something is wrong: Milky Ways twirl in spirals and physics can predict accurately how those spirals should look. Nevertheless, the outside of these spirals revolve much faster than they should according to the maths.
There are two obvious solutions for this: perhaps the maths are wrong. After all, the physics we apply to revolving galaxies is essentially the same we apply to pizza dough that is tossed in the air by pizza chefs, even though a small galaxy is still a hundred billion times a billion times larger than a pizza, and it could just be that other laws apply.
The second option is that the galaxy is heavier than it looks.
The velocity of the revolutions would actually be correct if we could assume that there is an enormous ball of mass in the centre of a galaxy. The only problem with this approach is that we cannot see the mass: it does not radiate any light or heat and we do not know any other way of measuring it directly.
Making up large amounts of indemonstrable ghost matter only to make your maths correct is an academic no-no. If biologists have not found any traces of an animal after fifty years, they regard it as extinct. And yet astronomers have clung to the notion of dark matter for much longer than half a century, and luckily for them, more and more evidence of its existence has been discovered. And not only that: the different approaches produced the same amount of dark matter. Everything we can see or measure, from neutrinos to people to stars, is made of something of which we know what it is. However, there seems to be about five to six times more dark matter than we supposed, and we do not know what it is made of.
“We know that dark matter has to be radically different to the matter we know”, explains Professor of Astronomy, Koen Kuijken. “It’s not part of ordinary particle physics but we do know that it is very important: you wouldn’t be here if it didn’t exist. Without dark matter, there would be too little mass in the universe to form galaxies.”
Kuijken and his colleague Henk Hoestra presented the results of their research at the American Astronomical Society’s Winter Meeting this month. Their research is based on one of the phenomena that force astronomers to believe that dark matter really exists. Galaxies are not only bigger than pizzas, they are also much heavier - so heavy, in fact, that they bend the light that travels around them: “gravitational lensing”, astrophysicists call it. Galaxies are much stronger lenses than you would expect by reason of their visible mass, and consequently, the phenomenon is an indication of invisible mass.
You can measure the lens effect on a well-defined, large photograph of the universe: the galaxies are not round but slightly elongated. “You have to determine the shape very precisely; it is not something you can see with the naked eye”, says Kuijken. The Leiden researchers used a series of photographs produced by the Canada-France-Hawaii-Telescope, to which a 340 Megapixel-camera had been attached. “We would not have been able to do this fifteen years ago, but technology has come a long way since then”.
The photograph shows how dark matter is distributed throughout the universe. Kuijken continues: “Of course, we want to test that idea that the matter causes codensation too, and we can see from the map we made that that is the case: the matter has merged into galaxies. And because, in astronomy, you are always looking back in time, you can also measure how the distribution has altered”. You see a galaxy that is six billion light years away as it was six billion years ago. The light from that moment has had to travel six billion years before it reached the Hawaiian telescope, so when you focus on a closer part of universe, you are looking at older stars. “Galaxies that are closer have merged more. You could rewind them like a film, as it were.”
However, it is still a film without a protagonist – to find that, we need particle physicists, according to Kuijken: “You can’t explain it all just with cosmology, although the numbers of particles and the mass will eventually have to match what astronomers can see. For instance, the whole thing about how condensation works only works with “cold” dark matter and wouldn’t work if the matter turned out to be super-fast”.
The astronomer is planning to work on a more extensive version of his present map: “Ten times the size, more definition and three dimensional. Do I think it’s boring doing the same thing, but more extensively? Not at all. You really do learn new stuff; it’s not just another decimal figure. And besides: I’m working with pictures of the night sky, and that is always very, very beautiful.”

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