New light on dark matter

Dark matter in a slice of the Universe a billion light years long

By BBC News Online science editor Dr David Whitehouse

Astronomers have obtained the first-ever glimpse of the distribution of the Universe's mysterious "dark matter" over a large region of the sky - a major advance in astronomy and cosmology.

The results give cosmologists their first clear window into the possible roles of dark matter in the evolution of the Universe.

The team used the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, to analyse the light from 200,000 distant galaxies. They looked for distortions in the passage of starlight caused by the gravitational effects of invisible dark matter.

Dark matter in red, distorted galaxies in blue

Dark matter is one of the greatest mysteries of modern science. Although it is invisible, its gravitational effects lead astronomers to believe it makes up at least 90% of the mass of the Universe.

But they do not know what it is or how it is distributed. Until they solve the dark matter puzzle, astronomers cannot claim to understand the Cosmos in any depth.

"To build computer models of the Universe, we need to have an idea of the total matter content of the Universe," said Dr Yannick Mellier, the team's leader.

"Since somewhere around 90% of this matter is invisible, it's hard for us to get a precise reading on this. Also, to test our models, to see if they accurately describe the Universe, we need to look at the results of our simulations against what is actually out there - what astronomers really see."

Faint galaxies

To determine the distribution of dark matter, Dr Mellier's team used a wide-field imaging camera to obtain high-resolution images of a two-square-degree section of sky (10 times the apparent size of full moon). Using state-of-the-art image analysis software, the team analysed the light from 200,000 very distant and faint galaxies.

The telescope scanned a two-degree square patch of sky

They were looking for the minute distortions that, in theory, should occur as the light passes through the gravitational fields of intervening dark matter. They found them but they were tiny - just 1% greater than what would be expected by chance alone.

Despite this marginal result, the researchers say they have confidence in their findings and report that two other groups may have seen the same thing.

The result of this work is the first map of dark matter in that area of sky. The result is not only a significant technological feat, but has been heralded as a major advance in astronomy and cosmology.

MegaCam soon

Dr Greg Fahlman, Director of the Canada-France-Hawaii Telescope said: "By 2002, we will have a new wide-field imaging camera on the telescope that will cover, with improved sensitivity, an area of sky three times greater than the current camera. This new instrument will greatly enhance our ability to map the cosmic distribution of dark matter."

MegaCam, as the camera is called, will provide astronomers with the data they need to develop significantly more accurate models of the Universe. "Our goal is to help create the first distribution maps of dark matter across the sky, similar to the distribution maps you currently see for galaxies," added Dr Fahlman.

This first observation of the gravitational distortion produced by dark matter is a significant achievement. The result immediately provides vital information about dark matter in the early Universe. More importantly, say astronomers, it demonstrates the feasibility of mapping the dark matter distribution across large areas of the sky.