Image: Galaxy clusters
NASA / CXC / IoA / S. Allen et al.
The Chandra X-Ray Observatory analyzed the hot gas surrounding galaxy clusters to determine their mass and distance. These three examples are Abell 2029, MS2137.3-2353 and MS1137.5+6624, seen as they looked 1 billion, 3.5 billion, and 6.7 billion years ago, respectively. Researchers found that the clusters were significantly farther away than they would have been if dark energy were not a factor.
By Senior Science Writer
updated 5/18/2004 5:27:25 PM ET 2004-05-18T21:27:25

A comprehensive X-ray screening of the cosmos confirms a popular notion of a stop-and-go universe that may expand forever.

New data from the Chandra X-ray Observatory agree with previous findings from the Hubble Space Telescope and other observatories, showing that the universe initially expanded rapidly, slowed down under its own weight, then began accelerating about 6 billion years ago. All galaxies except those bound in local groups are now racing away from each other at ever-faster speeds.

While scientists aren't sure yet if the current acceleration is constant, the new study points in that direction.

The work provides no new clues as to why the universe's expansion is accelerating, however. Astronomers blame dark energy, a mysterious force that they cannot explain but which appears to make up about 75 percent of the universe's mass and energy. The phenomenon was first noticed in 1998 through Hubble observations of distant exploding stars called supernovas.

Researchers are trying to figure out if dark energy is a constant force or if it changes over time.

If dark energy's repulsive force decreases in strength over the eons, the universe could reverse course and collapse in the distant future. If dark energy gets stronger, the acceleration could lead to a Big Rip, in which all matter is shredded. If it is constant, then the acceleration will merely continue, ultimately rendering ever-more-distant galaxies beyond the sight of any possible telescope.

Checking clusters
Chandra looked at 26 galaxy clusters, each surrounded by a cloud of hot gas and held together by dark matter, another unknown thing invoked to explain why the galaxies don't just fly apart, as they would if left to gravity from regular matter alone.

Chandra's bestThe clusters are about 15 percent visible matter. The rest of the regular matter is hot gas only visible in X-rays. Chandra allowed the researchers to determine the masses of the cluster and thereby learn how far away each one is.

"The distances to the clusters are all significantly larger than if there were no dark energy," said Steve Allen of the Institute of Astronomy in Cambridge, England.

Allen and colleague Andy Fabian presented their findings Tuesday at a NASA press conference.

Fabian explained that the clusters they studied are spread across time and space, throughout the decelerating phase and the acceleration phase.

New way to look at mystery
The work provides a "vital" new way to probe dark energy that validates the Hubble findings, said Kim Weaver, an astrophysicist at NASA's Goddard Space Flight Center.

"We can now be quite confident that the expansion of the universe is speeding up," said University of Chicago cosmologist Michael Turner, the assistant director for mathematical and physics sciences at the National Science Foundation. "This is not a fluke that is going to go away."

Turner, who like Weaver was not involved in the Chandra study, added that scientists are still thoroughly confused about what dark energy is, calling it the most profound question in science and one that probably won't be answered by the current generation of experts.

"Until we understand what this dark energy is, all possibilities are open for the future of the universe," Turner said.

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