Dark matter comes out of the cold

Read More about Dark Matter

By Jonathan Amos
BBC News science reporter

This strange material that dominates the Universe but which is invisible to current telescope technology is one of the great enigmas of modern science.

That it exists is one of the few things on which researchers have been certain.

But now an Institute of Astronomy, Cambridge, team has at last been able to place limits on how it is packed in space and measure its "temperature".

"It's the first clue of what this stuff might be," said Professor Gerry Gilmore. "For the first time ever, we're actually dealing with its physics," he told the BBC News website.

Science understands a great deal about what it terms baryonic matter - the "normal" matter which makes up the stars, planets and people - but it has struggled to comprehend the main material from which the cosmos is constructed.

'Magic volume'

Astronomers cannot detect dark matter directly because it emits no light or radiation.

Its presence, though, can be inferred from the way galaxies rotate: their stars move so fast they would fly apart if they were not being held together by the gravitational attraction of some unseen material.

Such observations have established this dark material makes up about 80-85% of the Universe that is matter.

Now, the Cambridge team has provided new information with its detailed study of 12 dwarf galaxies that skirt the edge of our own Milky Way.

Using the biggest telescopes in the world, including the Very Large Telescope facility in Chile, the group has made detailed 3D maps of the galaxies, using the movement of their stars to "trace" the impression of the dark matter among them and weigh it very precisely.

With the aid of 7,000 separate measurements, the researchers have been able to establish that the galaxies contain about 400 times the amount of dark matter as they do normal matter.

"The distribution of dark matter bears no relationship to anything you will have read in the literature up to now," explained Professor Gilmore.

"It comes in a 'magic volume' which happens to correspond to an amount which is 30 million times the mass of the Sun.

"It looks like you cannot ever pack it smaller than about 300 parsecs - 1,000 light-years; this stuff will not let you. That tells you a speed actually - about 9km/s - at which the dark matter particles are moving because they are moving too fast to be compressed into a smaller scale.

"These are the first properties other than existence that we've been able determine."

Knowledge advance

The speed is a big surprise. Current theory had predicted dark matter particles would be extremely cold, moving at a few millimetres per second; but these observations prove the particles must actually be quite warm (in cosmic terms) at 10,000 degrees.

The most likely candidate for dark matter material is the so-called weakly interacting massive particle, or Wimp.

They are predicted by certain theoretical extensions to the accepted description of matter and forces, the Standard Model of Fundamental Particles and Interactions. But also their presence would go a long way to explaining the structure and geometry of the Universe we observe.

Professor Bob Nichol, from the Institute of Cosmology and Gravitation at the University of Portsmouth, described the Cambridge work as "awesome".

"If this temperature for the dark matter is correct, then it has huge implications for direct searches for these mysterious particles (it seems [science] may be looking in the wrong place for them) and for how we thought the galaxies and clusters of galaxies evolve in the Universe.

"Having 'hotter' dark matter makes it harder to form the smallest galaxies, but does help to make the largest structures. This result will generate a lot of new research."

Big neighbours

Experimental crystal detectors placed down the bottom of deep mines are hoping to record the passage through normal matter of these hard to grasp dark matter particles.

Researchers would hope also that future experiments in particle accelerators will give them greater insight into the physics of dark matter.

"It turns out the Milky Way is more massive than we thought," said Professor Gilmore.

"It now looks as though the Milky Way is the biggest galaxy in the local Universe, bigger even than Andromeda. It was thought until just a few months ago that it was the other way around."

The Cambridge University team expects to submit the first of its results to a leading astrophysics journal in the next few weeks.

Jonathan.Amos-INTERNET@bbc.co.uk
http://news.bbc.co.uk/2/hi/science/nature/4679220.stm