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Galaxy clusters slide to the south: mysterious flow could call universe's uniformity into question.

Clusters of galaxies are flowing through space, seemingly under the influence of a mysterious attractive force outside the visible universe, a new study suggests.

Researchers detected what they have dubbed "dark flow" while surveying 700 galaxy clusters. Each contained hundreds to thousands of galaxies all lying no farther than approximately 1 billion light-years from Earth. On average, the clusters appeared to move at about 1,000 kilometers per second in a uniform direction toward a point in the southern sky.

While no one knows the cause of the motion, the scientists suggest whatever the cause is, it may no longer lie in the visible universe. The work appears online in two separate papers, one in the Oct. 20 Astrophysical Journal Letters and one at

"We expected to find something completely different," says astrophysicist Alexander Kashlinsky of NASA's Goddard Space Flight Center in Greenbelt, Md. "It's basically a slope across the universe," in a direction somewhere between the constellations Centaurus and Vela.


This flow flies in the face of one of cosmologists' most cherished assumptions--backed by a wealth of data--that the universe is uniform. That is, its structure and the density of matter in it are about the same in all regions of the sky.

Glenn Starkman of Case Western Reserve University in Cleveland says the findings add to anomalies discovered in recent years in the cosmic microwave background, or CMB, the ubiquitous bath of cold radiation left over from the Big Bang. "It's yet another piece of evidence that, on the largest scales, either we're misunderstanding something or discovering something about the universe," Starkman says.

The researchers' work built on an X-ray spectrum survey of the entire sky taken by the orbiting telescope ROSAT in the early 1990s. Galaxy clusters are suffused in a thin but hot plasma, which emits X-rays. Back then, Harald Ebeling of the University of Hawaii's Institute for Astronomy in Honolulu and others used the ROSAT data to identify hundreds of large galaxy clusters by their X-ray halos, and matched that with optical-telescope data to estimate the clusters' distances from Earth.

In the new study, the team estimated the motion of each cluster with respect to the CMB radiation, which is believed to be "the ultimate reference" of movement on a cosmological scale, says Ebeling, a coauthor of the new papers.

As CMB radiation crosses a galaxy cluster, it gets scattered by electrons in the intergalactic plasma, Ebeling says. The scattering affects the radiation's frequency. The frequency goes up if the cluster is moving toward Earth and down if it's moving away. This is called the kinetic Sunyaev-Zeldovich effect, analogous to the familiar Doppler shift of sound waves, which explains why an ambulance's siren sounds different depending on whether the ambulance is approaching or moving away.

Using data released two years ago by NASA's Wilkinson Microwave Anisotropy Probe, the team looked for the effect and found it was extremely small--comparable to a temperature change of millionths of a kelvin, Ebeling says.

For a single cluster, a variation this small easily drowns in the larger experimental errors. Moreover, each cluster tends to move in its own direction, tugged by clusters nearby. But on average, the velocities showed a clear trend.

"People will be inherently skeptical of any such results," Starkman says, since the results question the standard, homogeneous model of the universe. "Even those who have doubts about the model don't have better alternatives." But, he adds, the study should still be taken seriously.

Kashlinsky says that random energy fluctuations in the earliest split second after the Big Bang--the epoch of expansion called inflation--could have created a large imbalance in the distribution of matter. This imbalance could have left its mark on the structure of spacetime.

Such a large-scale imbalance is "absolutely possible," says cosmologist Andrei Linde of Stanford University. But it would require some rather contrived tweaks to the still-tentative models of how inflation works.
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Title Annotation:Atom & Cosmos
Author:Castelvecchi, Davide
Publication:Science News
Geographic Code:1USA
Date:Oct 25, 2008
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