Dark matter's telltale sign
We can't see dark matter, but we know it exists by how it affects galaxies. In the last part of this series, Dr Karl gets in a spin about rotation.
Over the last two episodes, I've talked a lot about our universe, and more specifically, how on average there's about five times as much of this mysterious dark matter as there is of regular matter.
The astronomers have suspected the existence of dark matter for some 80 years. The clue that alerted astronomers was two kinds of "rotation". First, there was the rotation of galaxies around each other, and second, there was the rotation of stars within a galaxy.
So with regard to rotation of galaxies around each other, way back in 1933, the astronomer Fritz Zwicky took a very close look at a cluster (or group) of galaxies called the Coma Cluster. He measured the speeds of the individual galaxies inside this cluster and found, to his vast surprise, that they were going so fast that they should easily break away from the gravity of this cluster of galaxies. But they weren't breaking away. He worked out that to keep the Coma Cluster intact and stable, that there had to be some 10-20 times more matter in the Coma Cluster, than he could see with his telescope. This was one of the first hints of the existence of dark matter.
So what about the second clue, the stars rotating inside a single galaxy? Well, it was the astronomer Vera Rubin who discovered that anomaly in 1973. She would pick a galaxy (typically with about 100 billion stars in it), and using a new technology of the time, measure how fast the stars were orbiting around the centre. She got a real surprise.
It was nothing like our solar system.
In our solar system, the planets close to the Sun go around really rapidly, while the planets further out from the Sun travel much more slowly. So the innermost planet, Mercury, orbits at around 48 kilometers per second, our Earth travels a little slower at 30 kilometers per second, while Neptune way out on the edge crawls along at around 5.4 kilometres per second.
But that was not the case in Vera Rubin's galaxies. The stars near the bulging core of a galaxy travelled around at the same speed as the stars right out on the edge. In our Milky Way, most of the stars, whether they're close to the central black hole, or 50,000 light years away (way out on the edge), all travel at roughly the same speed, about 210-250 kilometres per second.
There was only one way that the theoretical astronomers could explain how a galaxy could spin like this. The maths told them that the galaxy had to be surrounded by a halo, or spherical ball, of some type of matter. And, because it didn't shine, they called it "dark matter".
So what do we know about this dark matter?
First, it's dark, because it doesn't interact with light, or indeed any electromagnetic radiation.
Second, it's definitely not regular matter, like the stuff you and I are made of. The astronomers tell us this from their observations of various galaxies, and star clusters, colliding.
Third, dark matter is not antimatter. If it were, we would see very specific gamma rays being produced whenever dark matter collided with regular matter.
Fourth, we know that it's not black holes. Black holes have a lot of mass crammed into a small volume, so they have a very strong local gravitational field. We would expect to see lots of so-called gravitational lenses, where gravity bends the light of a distant object. We're simply not seeing these gravitation lenses.
Fifth, astronomers have mapped dark matter on a huge scale, analysing the light from 10 million galaxies. They looked at this incoming light to see how it was curved or bent — presumably by intervening dark matter. After five years of hard work, they mapped an intricate cosmic web of intermingled visible galaxies and invisible dark matter. This web covers over a billion light-years. The dark matter seems to be a mesh, or in three dimensions, like a giant sponge with dense and empty regions. The regions of dense dark matter seem to cover a portion of sky about the size of several full moons.
There are three main contenders for the title of dark matter at the moment, each of them some kind of exotic and strange matter. The main categories are: hot dark matter, warm dark matter, and cold dark matter. At the moment, cold dark matter is winning, but that could change.
But if I've learned anything from Goldilocks and the Three Bears, I'll bet that cold dark matter will be just right…
Published 19 March 2013
© 2013 Karl S. Kruszelnicki Pty Ltd