The Bullet Cluster of galaxies, which lies 3.8 billion light-years away, is one of hundreds that appear to be carried along by a mysterious cosmic flow (Image: NASA/STScI/Magellan/U.Arizona/D.Clowe et al.)

Hundreds of clusters of galaxies are streaming en masse towards a region at the edge of the visible universe. The discovery has taken astronomers completely by surprise because the movement is independent of the universe’s expansion.

A team led by Alexander Kashlinsky of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, stumbled upon the flow while studying how galaxy clusters affect the photons of the cosmic microwave background (CMB), the radiation left over from the big bang.

Kashlinsky has dubbed the stream “dark flow” to align it with the other as yet-unexplained characteristics of the universe, dark matter and dark energy.


The discovery arose because the movement of the clusters changed the frequency of CMB photons, creating a Doppler effect, the characteristic signature of movement. “To our great surprise, we found a very weak but identifiable [Doppler] signal, significantly above the noise,” Kashlinsky told New Scientist.

According to the observations, the clusters are all moving towards a patch of sky that lies between the constellations Vela and Centaurus. Some are speeding away at about 1000 kilometres per second, and it seems likely the flow continues to the edge of the visible universe.

“This motion extends all the way to at least a billion light years, and perhaps longer,” says Kashlinsky. “If it already goes so far, it would be very puzzling if it just stopped.”

Mystery mass

No one knows what might be causing the flow. The best guess so far is the gravitational pull of something with a very large mass that lies outside our observable universe. “I don’t know if I would call it matter. It could be some gigantic singularity,” Kashlinsky says.

If there is something so massive beyond the edge of the visible universe, it could provide support for inflation theory, which suggests that our universe underwent a period of exponential expansion just after it began. Inflation suggests that massive structures, though not visible to us, could exist beyond our horizon.

Dark flow is not the first CMB anomaly to hint at large-scale structures that cannot be explained. Hans Kristian Eriksen of the University of Oslo in Norway, who has done similar work, says the new study “is very interesting”.

“It suggests that there are structures on larger scales than people have believed so far,” he says.

Journal reference: Astrophysical Journal Letters

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