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Galaxies on the move

Scientists detect a mysterious flow of galactic clusters

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7:55pm, September 26, 2008
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A newly discovered “dark flow” appears to carry clusters of galaxies toward a point in the southern sky, a new study suggests.

As if dark matter and dark energy weren’t confusing enough, researchers detected what they have dubbed dark flow while surveying 700 galaxy clusters — each containing hundreds to thousands of galaxies — within a radius of approximately 1 billion light-years. On average, the clusters appeared to move in a uniform direction at about 1,000 kilometers per second.

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

“We expected to find something completely different,” says Alexander Kashlinsky, an astrophysicist at NASA’s GoddardSpaceFlightCenter in Greenbelt, Md. "It’s basically a slope across the universe,” in a direction somewhere between the constellations of Centaurus and Vela.

The result flies in the face of one of cosmologists’ most cherished assumptions — backed by a vast 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.

But the findings further complicate the picture of cosmology, comments cosmologist Glenn Starkman of CaseWestern ReserveUniversity in Cleveland. The new results 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.

Harald Ebeling, an astronomer at the University of Hawaii’s Institute for Astronomy in Honolulu and a coauthor of the studies, says he and his team checked and rechecked their results for more than a year before publishing them. “We didn’t believe it for the longest time,” he says.

The researchers’ work built upon a survey of the entire sky in the X-ray spectrum taken by the orbiting telescope ROSAT in the early 1990s. Galaxy clusters are usually suffused in a thin but very hot plasma, which emits X-rays. Back then, Ebeling 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’ distance from Earth.

In the new study, the researchers 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.

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. The Doppler shift explains why the pitch of an ambulance’s siren sounds different depending on whether the ambulance is approaching or moving away from the listener.

The researchers looked for the kinetic Sunyaev-Zeldovich effect in CMB data released two years ago by NASA’s Wilkinson Microwave Anisotropy Probe mission. The effect 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 much 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. “The velocity is not only high,” Kashlinsky says, “but it also remains the same velocity as far as you can see.”

“People will be inherently skeptical of any such results,” Starkman says, since they call into question the standard, homogeneous model of the universe. “Even those who have doubts about the model don’t have better alternatives.” But, he adds, researchers should still take the results seriously.

Kashlinsky says that random energy fluctuations in the earliest split second of the Big Bang — the epoch of stupendous expansion called inflation — could have created a large imbalance in the distribution of matter. While the denser regions of the universe would now be forever out of sight, the imbalance could have left its mark on the overall structure of spacetime. Like a dining room table tipped so that all the dinner plates slide off in the same direction, the imbalance may have put the local corner of the universe on a slope.

Such a large-scale imbalance is “absolutely possible,” says cosmologist Andrei Linde of StanfordUniversity. But it would require some rather contrived tweaks to the still-tentative models of how inflation works. “Inflation typically makes the universe completely uniform,” Linde says. “People do not want to go in this direction without a really seriously demonstrated need.”

The results are something that people will “scratch their heads over,” says Ethan Vishniac of McMasterUniversity in Hamilton in Canada. As the editor in chief of The Astrophysical Journal, Vishniac personally reviewed one of the papers. While the team’s methods were correct, he says, there are still large margins of error in the data, and only more research will help settle the matter of the lopsided universe.

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