Image: Dark flow
Dr. T.H. Jarrett  /  IPAC/Caltech
This all-sky view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way and has been desaturated to serve as the background for the dark flow plots.
updated 3/18/2010 6:01:39 PM ET 2010-03-18T22:01:39

The puzzling migration of matter in deep space — dubbed "dark flow" — has been observed at farther distances than ever before, scientists have announced.

Distant galaxy clusters appear to be zooming through space at phenomenal speeds that surpass 1 million mph. The clusters were tracked to 2.5 billion light-years away — twice as far as earlier measurements.

This motion can't be explained by any known cosmic force, the researchers say. They suspect that whatever's tugging the matter may lie beyond our observable universe.

The notion is a controversial one because it has only been measured by one group of scientists in one set of data so far.

"We understand why this idea is so annoying at times," said study leader Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md. "In fact, part of the motivation for our ongoing project was precisely to rule it out. But it is in the data, we don't see it going away."

Kashlinsky and colleagues first reported their measurements of the dark flow in 2008. They measured signals of the movement in the leftover light from the Big Bang, thought to have created the universe 13.7 billion years ago. That leftover light is called the cosmic microwave background (CMB) radiation.

The dark flow appears to be moving in the direction of the southern constellations Centaurus and Hydra.

Over the universe's horizon
The new study is based on a larger data set of about 1,500 galaxy clusters and CMB measurements taken over five years by space and ground-based telescopes. The researchers say their new results strengthen indications that the dark flow is real.

"It looks indeed that the entire observable universe is moving with respect to the CMB radiation," Kashlinsky told "We can now say that more confidently than our initial supposition."

The researchers think dark flow may be caused by structures that lie beyond the horizon of our own universe. As odd as that may sound, some cosmologists think that our universe is actually only a bubble of space-time that was created during a period of rapid cosmic expansion, called inflation, after the Big Bang. Other bubbles may also have been created where inflation took place at a different rate, and perhaps something in one of the other bubbles is tugging at our universe.

The researchers hope to further test the dark flow using upcoming data expected from the Planck satellite, which launched last year.

Other evidence
Some potential support for the dark flow idea came from an independent study that found a similar motion, albeit in individual galaxies, and not clusters of galaxy, Kashlinsky said.

That team — which includes researchers Richard Watkins of Willamette University in Salem, Ore., Hume Feldman of the University of Kansas, and Michael Hudson of Canada's University of Waterloo — found a sampling of galaxies that also displayed a collective motion, which happened to be in the same direction as the dark flow measured by Kashlinsky and team.

"We see the flow in the same direction, no question about it," Feldman said. "That is very odd, it's not what you would expect."

But Feldman cautioned that his observations were not necessarily of the same dark flow, since they were on a completely different scale of relatively nearby objects.

"There's nothing in our flow that says that their flow does not exist," Feldman said. "On the other hand, there's nothing in our flow that says their flow does exist, except that it's in the same direction."

Kashlinsky and colleagues' new findings are detailed in the March 20 issue of The Astrophysical Journal Letters.

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