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Chandra opens new line of investigation on dark energy

NASA NEWS RELEASE
Posted: May 18, 2004

Astronomers have detected and probed dark energy by applying a powerful, new method that uses images of galaxy clusters made by NASA's Chandra X-ray Observatory. The results trace the transition of the expansion of the universe from a decelerating to an accelerating phase several billion years ago.


Chandra images of multimillion degree Celsius gas in galaxy clusters have provided astronomers with a powerful new method to probe the mass and energy content of the universe. A recent study of 26 clusters of galaxies confirms that the expansion of the universe stopped slowing down about 6 billion years ago, and began to accelerate. Credit: NASA/CXC/IoA/S.Allen et al.

"Dark energy is perhaps the biggest mystery in physics," said study leader Steve Allen of the Institute of Astronomy (IoA) University of Cambridge, England. "As such, it is extremely important to make an independent test of its existence and properties," he said.

Allen and his colleagues used Chandra to study 26 clusters of galaxies at distances between one and eight billion light years. These data span the time when the universe slowed from its original expansion, before speeding up again, because of the repulsive effect of dark energy.

"We're directly seeing the expansion of the universe is accelerating by measuring the distances to these galaxy clusters," said IoA scientist and study co-author Andy Fabian. "The new Chandra results suggest the dark energy density does not change quickly with time and may even be constant, consistent with the "cosmological constant" concept first introduced by Albert Einstein," he said.

If the dark energy is unchanging, the universe is expected to continue expanding forever, and more dramatic fates for the universe would be ruled out. These include the "Big Rip," where dark energy increases until galaxies, stars, planets and, finally, even atoms are torn apart, and the "Big Crunch," where the universe eventually collapses on itself.

Chandra's probe of dark energy uses X-ray observations to detect and study the hot gas in galaxy clusters. From these data, the ratio of the mass of the hot gas to the mass of the dark matter in a cluster can be determined. Since galaxy clusters are so large, the relative amounts of hot gas and dark matter should be the same for every cluster. Using this assumption, Allen and colleagues derive distances that show the expansion of the universe was first decelerating, and it began to accelerate about six billion years ago.

Chandra's observations agree with observations of distant supernovae, including those from NASA's Hubble Space Telescope (HST), that first showed dark energy's effect on the acceleration of the universe. Chandra's results are completely independent of the supernova technique.

"Our Chandra method has nothing to do with other techniques, so they're definitely not comparing notes, so to speak," said Robert Schmidt of the University of Potsdam, Germany, another co-author of the study.

Better limits on the amount of dark energy, and how it varies with time, are obtained by combining the X-ray results with data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP). It used observations of the cosmic microwave background radiation to discover evidence for dark energy in the very early universe. Using the combined data, Allen and his colleagues found dark energy makes up about 75 per cent of the universe, dark matter about 21 per cent, and ordinary matter about 4 per cent.

More detailed studies with Chandra, HST, WMAP and future X- ray missions like Constellation-X, should provide much more precise constraints on dark energy.

"Until we better understand cosmic acceleration and the nature of the dark energy, we cannot hope to understand the destiny of the universe," said Michael Turner, assistant director for mathematical and physics sciences, National Science Foundation, Arlington, Va.

The research team also included Harald Ebeling of the University of Hawaii and the late Leon van Speybroeck of the Harvard-Smithsonian Center for Astrophysics. These results appear in an upcoming issue of the Monthly Notices of the Royal Astronomy Society.

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X-ray Scan of Cosmos Probes Dark Energy

By Robert Roy Britt
Senior Science Writer
posted: 04:00 pm ET
18 May 2004

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 agrees 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 that 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.

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.

The 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, U.K.

Allen and colleague Andy Fabian presented their findings today 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.

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.

Chandra Animations