Galaxies found to be flowing together
POSTED: Saturday, October 04, 2008
|This story has been corrected. See below.|
Hundreds of galaxy clusters are flowing toward the same spot in the sky beyond the observable universe, a University of Hawaii astronomer and NASA team members have discovered.
The clusters, each with hundreds of millions of stars, are moving at more than 1 million mph toward the constellations Centaurus and Vela, said Harald Ebeling, co-author of a paper on the baffling finding in the Astrophysical Journal Letters.
"It's pretty bizarre," he said in an interview.
Ebeling used X-ray satellites and the largest optical telescopes on Mauna Kea to find galaxy clusters and measure their dimensions, even in regions of the sky largely obscured by the Milky Way. Working with him was Dale Kocevski, then a UH graduate student and now a University of California postdoctoral fellow.
Using new and existing data, they created the first all-sky catalog, identifying about 700 X-ray clusters, which made the cosmic flow study possible.
Ebeling said the "main culprit" behind the phenomenon seems to be the Shapley Concentration, a giant supercluster made up of about 25 galactic clusters about 700 million light-years away which pulls everything toward it.
But once the clusters reach the Shapley supercluster, he said, they do not subside or reverse—they defy current cosmic theories and keep on going.
"That was pretty stunning," Ebeling said. "Essentially, we don't know how far it goes."
In a release from NASA's Goddard Space Flight Center in Greenbelt, Md., team leader Alexander Kashlinsky said, "We never expected to find anything like this. ... The distribution of matter in the observed universe cannot account for this motion."
One explanation, Ebeling said, is a collection of superclusters even bigger and more distant than Shapley is attracting the galaxies. But he adds, "It's inconceivable to have a mass concentration just sitting out there beyond what we can see that would cause this."
He said another interesting idea proposed since the cosmic flow was detected is that instead of a gravitational pull by something beyond the observable universe, the galaxies are being pushed by an absence of mass in the local universe.
"It is getting kind of scary," Ebeling said.
Kashlinsky and Fernando Atrio-Barandela of the University of Salamanca, Spain, are experts in analyzing tiny variations in microwave radiation left over from the big bang 14 billion years ago when the universe was born.
They found a tiny shift in temperature in the cosmic microwave background that could be used with the Ebeling/Kocevski compilation of large samples of galaxy clusters to deduce the clusters' motion.
Kashlinksy said the clusters "show a small but measurable velocity that is independent of the universe's expansion and does not change as distances increase." He named the cosmic drift "dark flow," in line with other mysteries of the cosmos—dark energy and dark matter.
"We're trying to sort this all out," said Ebeling, who has proposed extending the study with a larger sample of galaxy clusters over the entire sky and pushing the measurements into farther distances using the latest cosmic microwave background data.
"If this flow is confirmed and found to continue to yet larger distances," he said, "it may offer us a fascinating opportunity to examine the state of the universe at a time that will never be accessible to direct observation."