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Cosmic push: x-ray study confirms universe's dark side.

Culling culling

removal of inferior animals from a group of breeding stock. The removal is premature, i.e. before completion of its life span, disposal of an animal from a herd or other group.
 clues from x rays emitted by distant clusters of galaxies, astronomers report new evidence that some mysterious force overcame gravity's tug about 6 billion years ago and ever since has been pushing galaxies apart at an accelerating rate. The results add to previous evidence for one of the strangest properties of the cosmos: Cosmic expansion is speeding up, rather than slowing down, in response to the mutual gravity of all material in the universe.

This bizarre state of affairs came to light in 1998, when scientists found that some distant exploded stars are dimmer dim·mer  
1. A rheostat or other device used to vary the intensity of an electric light.

a. A parking light on a motor vehicle.

b. A low beam.
, and therefore farther way, than expected. That was an indication that some cosmic push, referred to as dark energy, has been revving up the expansion of the universe (SN: 5/22/04, p. 230).

The new X-ray study of galactic clusters provides an independent method of detecting that cosmic push, say Andy Fabian and Steve Allen of the University of Cambridge in England and their colleagues. At a press briefing last week in Washington, D.C., they announced their findings, which are based on observations by NASA's orbiting Chandra X-ray Observatory Chandra X-ray Observatory

U.S. X-ray space telescope. It was named after astrophysicist Subrahmanyan Chandrasekhar and was launched into orbit in 1999. Its mirror, with an aperture of 1.2 m (4 ft) and a focal length of 10 m (33 ft), produces unprecedented resolution.
. The researchers also describe their work in an upcoming Monthly Notices of the Royal Astronomical Society Monthly Notices of the Royal Astronomical Society (MNRAS) is one of the world's leading scientific journals in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes peer-reviewed letters and papers reporting original research in relevant .

"This is the first time that clusters have been [successfully] used to measure . . . dark energy," comments X-ray astronomer J. Patrick Henry of the University of Hawaii (body, education) University of Hawaii - A University spread over 10 campuses on 4 islands throughout the state.

See also Aloha, Aloha Net.
 in Honolulu.

X rays emanate from hot gas that bathes each galaxy cluster. To look for signs of dark energy, the Cambridge astronomers and their colleagues examined the brightness and energy spectra of X-ray emissions from 26 clusters to calculate the exact distances to the clusters, known to lie between 1 billion and 8 billion light-years from Earth.

The team traced a history of cosmic expansion by combining the new distance measurements with other astronomers' observations of how fast the clusters are receding from Earth.

The results support the standard view that gravity put the brake on expansion of the universe in the first several billion years of cosmic history. They're also in agreement with recent supernova supernova, a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold.  studies showing that some 6 billion years ago, an unknown entity stepped on the cosmic accelerator. Furthermore, both the supernova and the new galaxy-cluster studies suggest that the density of dark energy doesn't vary over time.

"It is always encouraging when independent experiments yield similar results," says Louis-Gregory Strolger of the Space Telescope Science Institute The Space Telescope Science Institute (STScI) is the science operations center for the Hubble Space Telescope (HST; in orbit since 1990) and for the James Webb Space Telescope (JWST; scheduled to be launched in 2013).  in Baltimore, an astronomer who participated in the recent supernova studies.

If the density of dark energy is indeed constant, the universe will continue to expand faster and faster. Eventually, every galaxy will become so distant that it will be out of sight of all the others.

Allen notes that the findings don't entirely rule out a dark-energy density that increases with time. In that ease, cosmic expansion would ultimately speed up so rapidly that every galaxy and atom in the universe would be torn asunder a·sun·der  
1. Into separate parts or pieces: broken asunder.

2. Apart from each other either in position or in direction: The curtains had been drawn asunder.
 in a Big Rip This article is about a physics concept. For the Massively Multiplayer Online (MMO) Game Network, see The Big Rip.

The Big Rip is a cosmological hypothesis about the ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and
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Title Annotation:This Week
Author:Cowen, R.
Publication:Science News
Date:May 29, 2004
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