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Gravity's lens: Finding a dim cluster

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12:22pm, September 25, 2001
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Relying solely on a gravitational mirage rather than visible images, astronomers have discovered a previously unknown cluster of galaxies and measured its distance from Earth.

This first-time feat, which uses the properties of mass to infer the location of unseen objects, bodes well for searches of dark matter–the mysterious, invisible material that presumably makes up more than 95 percent of the mass of the universe.

All matter, whether it's visible or not, exerts a gravitational tug. The tug of a massive object not only attracts other bodies but also bends the path of light from more distant objects that lie directly behind it. The bending can elongate the images of distant galaxies, as seen from Earth, or create multiple images of the same object. This phenomenon, known as gravitational lensing, is a well-documented prediction of Albert Einstein's theory of general relativity.

To search for lenses, J. Anthony Tyson of Lucent Technologies' Bell Laboratories in Murray Hill, N.J., and his colleagues scanned a relatively barren patch of sky using a 4-meter telescope at the Cerro Tololo Inter-American Observatory in La Serena, Chile. With the aid of a computer program, they then identified elongated images of background galaxies–a sign that the light had passed through an intervening galaxy cluster that had not been previously seen. The program also generated an estimate of the mass and distance of the cluster.

The distance estimate precisely matched follow-up observations of the redshift of the galaxy cluster, a direct indicator of its distance from Earth. The team reported its findings in the Aug. 20 Astrophysical Journal Letters.

"Most of the 'big action' in the universe is governed by the mass of objects, not by their visible light or other electromagnetic radiation," notes Tyson. "This fundamentally new approach lets us measure mass instead of light," providing a truer view of the distribution of matter in the universe. Ultimately, he says, "we will be able to do a blind search and effectively become intergalactic prospectors for extremely faint clumps of mass in any direction, out to about half the estimated age of the universe."

Tyson adds that the prevalence of gravitational lenses provides another test of whether or not the universe has revved up the rate at which it is expanding, as recent studies of distant supernovas have indicated (SN: 4/7/01, p. 218: A Dark Force in the Universe).

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