The dark side of the universe

by Rebecca Priestley / 21 February, 2013
Scientists are gradually finding out more about a mysterious part of outer space.
A 3D map of dark matter in a region of the sky from the Hubble Cosmic Evolution Survey
First you don’t see it, now you do: a 3D map of dark matter in a region of the sky from the Hubble Cosmic Evolution Survey. Photo/Getty Images.

Reader Terry Jones points out that in the January 5 Science column (“It all started with
a Big Bang”), in which I spoke to Professor Richard Easther about the origin of the universe, we made no reference to dark energy or dark matter. I need little convincing to devote another column to weird physics and cosmology, so this time I asked Easther, head of the University of Auckland’s Department of Physics, what we know about these dark and mysterious parts of the universe.

“The first hint that the universe contains dark matter came in the 1930s,” says Easther. “Astronomers looking at a big cluster of galaxies noticed that the galaxies inside the clusters were travelling through space too fast for the cluster to hold together under its own gravity.

“Since the 1970s, astronomers have found other evidence that galaxies are more massive than you would think from just counting up their stars; their outer parts spin faster than they ‘should’, and their gravity bends the path of light coming from objects behind them more than the combined mass of their stars alone can account for.

“It is possible that we just don’t understand gravity very well, especially when it operates on objects millions of light years apart. But each of these anomalies needs a different correction to standard theories of gravity.

“Another explanation is that galaxies weigh more than you guess from counting up the stars inside them. We call the stuff providing this extra mass ‘dark matter’, because it is not emitting light. We can explain all these gravity anomalies with just one assumption – that galaxies are actually around 80% dark matter and 20% stars, gas and dust.”

The fact that we can’t see it – dark matter does not interact with light – tells us that although dark matter has mass, it has no electrical charge, because light scatters off charged particles. Dark matter is therefore not made of atoms, which contain positively charged protons and negatively charged electrons.

The evidence for dark energy came in 1998, when astronomers discovered that the expansion rate of the universe was accelerating. Dark energy makes this possible, says Easther.

“It seems that every point in space has a little packet of dark energy associated with it. As the universe expands, the universe makes more dark energy as it makes more space. It sounds like you’re violating energy conservation, but general relativity tells you that if you have a bit of energy at each point in space, the universe tries to dilute that by accelerating its expansion rate – and that’s just what we see when we look up at the sky.”

The big question now, says Easther, is exactly what dark energy and dark matter are. “There’s a lot of work going on into figuring out what the properties of dark energy are and whether the amount of dark energy at each point is constant in space and time, which we can measure from looking at distant exploding stars and huge surveys showing the positions of millions of galaxies in space.”

We can try to detect dark matter on Earth. “Our galaxy, the Milky Way, contains a huge cloud of dark matter and the Earth moves through it as the Sun orbits the centre of the galaxy.

“Because dark matter doesn’t interact electrically, almost all the dark matter particles that encounter the Earth pass right through it. But when a collision does happen, it will be quite violent – if a dark matter particle collides with an atom in a detector, that atom will look like it’s been hit by a baseball.

“A lot of big detectors are being built, so the next few years should tell us a lot about what dark matter is – or isn’t.”

You can read more from Richard Easther at his new blog, which features “excursions into cosmology, astrophysics, particle physics, science news and scientific perspectives on everyday life”.

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