Telescopes may soon be able to 鈥渟ee鈥� the Universe鈥檚 dark matter 鈥� all that stuff in space that does not 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鈥檚 dark matter, which outweighs visible stars and galaxies by at least a factor of 10.
The astronomers exploited the phenomenon of 鈥済ravitational lensing鈥�, 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.
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A team led by David Wittman and Anthony Tyson of Lucent Technologies鈥� Bell Laboratories in Murray Hill, New Jersey, has now proved the technique works.
Distorted image
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 could not 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.
鈥淣ow that we鈥檝e demonstrated to sceptics that the technique works, we can confidently locate matter that produces no light at all 鈥� dark matter,鈥� 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.
Dark energy
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鈥檚 matter within the next decade.
Tyson admits that looking for dark matter is an odd task for an optical telescope. 鈥淲e are using a light-gathering instrument to image transparent material that produces no light whatsoever,鈥� he says. Nevertheless, 鈥渋t鈥檚 a technique with enormous promise鈥�, 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 鈥渄ark energy鈥� is speeding up the Universe鈥檚 expansion. Astronomers believe this is happening because of the way the recession velocities of supernovae change with distance from the Earth.
鈥淒ark energy, by opposing the gravity which causes matter to clump, slows the development of structures in the Universe,鈥� says Tyson. 鈥淏y 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.鈥�