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TELESCOPES may soon be able to “see” the Universe’s dark matter—all
that stuff in space that doesn’t emit any light.

Astronomers have succeeded in locating and weighing a galaxy cluster solely
by the effect its gravity has on light from more distant objects. Within a
decade, their work could lead to a 3D map of the Universe’s dark matter, which
outweighs visible stars and galaxies by at least a factor of 10.

The astronomers exploited the phenomenon of “gravitational lensing”, in which
light from very distant galaxies is distorted by the gravity of massive objects
situated in a direct line between them and Earth. The shape and extent of the
distortion tell you about the location and mass of the intervening matter.

A team led by David Wittman and Anthony Tyson of Lucent Technologies’ Bell
Laboratories in Murray Hill, New Jersey, has now proved the technique works.
They found distorted images of distant galaxies and used a computer program to
figure out that a cluster of 15 galaxies must have caused the distortion. It
also allowed them to figure out the arrangement of the cluster in three
dimensions, its mass and its distance from Earth.

Crucially, the cluster is so faint that the astronomers couldn’t see it while
they were doing their analysis. But sure enough, when they pointed the 10-metre
Keck telescope in Hawaii in the right direction, they saw exactly what they had
predicted. “Now that we’ve demonstrated to sceptics that the technique works, we
can confidently locate matter that produces no light at all—dark matter,”
says Tyson.

The astronomers plan to use the technique to build up a 3D map of the
distribution of mass in small regions of the sky. Their ultimate goal is a 3D
map of mass for the entire Universe.

The map will require dedicated telescopes and one is being planned at the
moment, possibly to be sited in Chile. The 8.4-metre Large-aperture Synoptic
Survey Telescope could map much of the Universe’s matter within the next
decade.

Tyson admits that looking for dark matter is an odd task for an optical
telescope. “We are using a light-gathering instrument to image transparent
material that produces no light whatsoever,” he says. Nevertheless, “it’s a
technique with enormous promise”, says David Hough of Trinity University in San
Antonio, Texas.

Tyson says the lensing technique could also be used to investigate the claim
that mysterious “dark energy” is speeding up the Universe’s expansion.
Astronomers believe this is happening because of the way the recession
velocities of supernovae change with distance from the Earth.

“Dark energy, by opposing the gravity which causes matter to clump, slows the
development of structures in the Universe,” says Tyson. “By obtaining a 3D
picture of how such structures change with cosmic time, we should be able to
provide an independent check of the supernova results.”