Galaxy Clusters Have a Mysterious Motion

Sep 24, 2008

( -- The stars are in motion, and on a much larger scale than can be explained with current theories, according to astronomers at NASA, the University of Hawaii and UC Davis. The finding could improve our understanding of events in the first moments after the birth of the universe.

The researchers found an unexpected motion, or "flow," of galaxy clusters across a wide patch of the sky up to at least a billion light years away.

Local flows of galaxies and galaxy clusters are well-known, said Dale Kocevski, a postdoctoral researcher at UC Davis who was part of the team. For instance, our own Milky Way galaxy is moving toward the Andromeda galaxy, and the galaxy group that includes both Andromeda and the Milky Way is moving at about 600 kilometers a second.

These motions happen because matter -- both stars, dust and gas and the more abundant but invisible "dark matter" -- is distributed in uneven clumps, and gravity pulls galaxy clusters toward those clumps. Astronomers call these clumps "anisotropies."

But the newly detected motion is over far too large an area to be explained in that way, say Kocevski and colleagues Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md.; Fernando Atrio-Barandela of the University of Salamanca, Spain; and Harald Ebeling of the University of Hawaii.

"It's too big an effect to be due to anisotropies in matter or dark matter," Kocevski said. Because the force of gravity decreases with the inverse square of distance, an effect over such a wide area would require far more mass than can actually exist there, he said.

The astronomers found the effect by using a catalog of 700 galaxy clusters observed with X-ray astronomy, and NASA's Wilkinson Microwave Anisotropy Probe, which shows tiny variations in the microwave background radiation of the universe left over from the Big Bang.

Hot X-ray-emitting gas in a galaxy cluster scatters photons from the cosmic microwave background. The wavelengths of these scattered photons change in a way that reflects each cluster's individual motion. This results in a minute shift of the microwave background's temperature in the cluster's direction. In 2000, Kashlinsky and Atrio-Barandela found a way to amplify this effect and employ it to measure the motion of distant galaxy clusters.

Big Bang models that include a feature called inflation offer a possible explanation for the flow. Inflation is the idea that shortly after the Big Bang, the universe expanded extremely rapidly. If inflation did occur, then the universe we can see is only a small portion of the whole cosmos.

There may have been a "tilt" to the universe, or motion already in progress, before inflation occurred, Kocevski said. Or, the clusters could be responding to the gravitational attraction of matter that was pushed far beyond the observable universe by inflation.

"This measurement may give us a way to explore the state of the cosmos before inflation occurred," Kashlinsky said.

A paper describing the results was published online in the journal Astrophysical Journal Letters on Sept. 24.

Provided by UC Davis

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User comments : 7

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3 / 5 (2) Sep 24, 2008
And what exactly rules out the rotating universe? As I pointed out in a previous article comment, if there is no frame of reference from which you can compare motion, then how can you disprove it?

3 / 5 (1) Sep 24, 2008
"...and the galaxy group that includes both Andromeda and the Milky Way is moving at about 600 kilometers a second"

600 km/s relative to what???
3 / 5 (2) Sep 25, 2008
relative to the cosmic microwave background, rtfa
5 / 5 (1) Sep 25, 2008
I assume rtfa means "read the f*&in article"... there's no need for that, I read the article, didn't understand the frame of reference so asked the question. With your response and re-reading the article I now understand. Thank you.
3 / 5 (1) Sep 25, 2008
We really need that damned quantum gravity theory, without it cosmology is lost in speculations.
5 / 5 (1) Sep 25, 2008
tigger, I hadn't meant any offence.
not rated yet Oct 14, 2008
Perhaps this is a signal of a "baby universe" inflating and being pinched off from our own universe, as hypothesized in Andre Linde's work with spontaneous inflation and multiverse theory. It also brings to mind a possible influence of the "Great Attractor" discovered behind the Milky Way in Centaurus some years ago. This also may seem to be a more plausible explanation of this "dark flow". But more research will be needed to confirm and explain this latest cosmological curveball thrown our way.After reading both papers on this finding, I still find no reference ruling in or out possible effects of the "Great Attractor" located in the constellations Centaurus and Hydra at a distance close to this new "dark flow" anomaly( see the Wiki page for the "Great Attractor"). It is located behind the Milky Way in the so-called Zone of Avoidance. Alternatively, might a more distant, undiscovered supercluster exist even further out behind the GA? Until these questions are answered, I see no reason to invoke new astrophysics to explain their result. Already the Local Group and Virgo Cluster show kinematic movement towards the GA along with several other galaxy clusters. Maybe astronomers need to search for infalling galaxy clusters located on the far side of this apparent anomaly. Wikipedia entry mentions "The first indications of a deviation from uniform expansion of the universe were reported in 1973 and again in 1978. The location of the Great Attractor was finally determined in 1986, and is situated at a distance of somewhere between 150 million and 250 million light years (the latter being the most recent estimate) from the Milky Way, in the direction of the Hydra and Centaurus constellations. That region of space is dominated by the Norma cluster (ACO 3627),[1][2] a massive cluster of galaxies, containing a preponderance of large, old galaxies, many of which are colliding with their neighbours, and/or radiating large amounts of radio waves.

Attempts to further study the Great Attractor and other phenomena are hampered due to line of sight obstruction by its location in the zone of avoidance (the part of the night sky obscured by the Milky Way galaxy).The two published papers referenced in the above article can by found at the site as arXiv:0809.3734v1 and arXiv:0809.3733v1.

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