“Dark Flow” of Billions of Stars Racing Towards Edge of Observable Universe (Best of 2010)

Cosmos_2 "Dark Flow" sounds like a new SciFi Channel series. It's not! The dark flow is controversial because the distribution of matter in the observed universe cannot account for it. Its existence suggests that some structure beyond the visible universe — outside our "horizon" — is pulling on matter in our vicinity.

Back in the Middle Ages, maps showed terrifying images of sea dragons at the boundaries of the known world. Today, scientists have observed strange new motion at the very limits of the known universe – kind of where you'd expect to find new things, but they still didn't expect this.  A huge swath of galactic clusters seem to be heading to a cosmic hotspot and nobody knows why.


Cosmologists regard the microwave background — a flash of light emitted 380,000 years after the universe formed — as the ultimate cosmic reference frame. Relative to it, all large-scale motion should show no preferred direction.

A new 2010 study has tracked the mysterious cosmic 'dark flow' to twice the distance originally reported. The study was led by Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

"This is not something we set out to find, but we cannot make it go away," Kashlinsky said. "Now we see that it persists to much greater distances – as far as 2.5 billion light-years away," he added.

The clusters appear to be moving along a line extending from our solar system toward Centaurus/Hydra, but the direction of this motion is less certain. Evidence indicates that the clusters are headed outward along this path, away from Earth, but the team cannot yet rule out the opposite flow.

"We detect motion along this axis, but right now our data cannot state as strongly as we'd like whether the clusters are coming or going," Kashlinsky said.

The unexplained motion has hundreds of millions of stars dashing towards a certain part of the sky at over eight hundred kilometers per second.  Not much speed in cosmic terms, but the preferred direction certainly is: most cosmological models have things moving in all directions equally at the extreme edges of the universe.  Something that could make things aim for a specific spot on such a massive scale hasn't been imagined before.  The scientists are keeping to the proven astrophysical strategy of calling anything they don't understand "dark", terming the odd motion a "dark flow".

A black hole can't explain the observations – objects would accelerate into the hole, while the NASA scientists see constant motion over a vast expanse of a billion light-years.  You have no idea how big that is.  This is giant on a scale where it's not just that we can't see what's doing it; it's that the entire makeup of the universe as we understand it can't be right if this is happening.

The hot X-ray-emitting gas within a galaxy cluster scatters photons from the cosmic microwave background (CMB). Because galaxy clusters don't precisely follow the expansion of space, the wavelengths of 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. The change, which astronomers call the kinematic Sunyaev-Zel'dovich (KSZ) effect, is so small that it has never been observed in a single galaxy cluster.

But in 2000, Kashlinsky, working with Fernando Atrio-Barandela at the University of Salamanca, Spain, demonstrated that it was possible to tease the subtle signal out of the measurement noise by studying large numbers of clusters.

In 2008, armed with a catalog of 700 clusters assembled by Harald Ebeling at the University of Hawaii and Dale Kocevski, now at the University of California, Santa Cruz, the researchers applied the technique to the three-year WMAP data release. That's when the mystery motion first came to light.

The new study builds on the previous one by using the five-year results from WMAP and by doubling the number of galaxy clusters.

"It takes, on average, about an hour of telescope time to measure the distance to each cluster we work with, not to mention the years required to find these systems in the first place," Ebeling said. "This is a project requiring considerable followthrough."

According to Atrio-Barandela, who has focused on understanding the possible errors in the team's analysis, the new study provides much stronger evidence that the dark flow is real. For example, the brightest clusters at X-ray wavelengths hold the greatest amount of hot gas to distort CMB photons. "When processed, these same clusters also display the strongest KSZ signature — unlikely if the dark flow were merely a statistical fluke," he said.

In addition, the team, which now also includes Alastair Edge at the University of Durham, England, sorted the cluster catalog into four "slices" representing different distance ranges. They then examined the preferred flow direction for the clusters within each slice. While the size and exact position of this direction display some variation, the overall trends among the slices exhibit remarkable agreement.

The researchers are currently working to expand their cluster catalog in order to track the dark flow to about twice the current distance. Improved modeling of hot gas within the galaxy clusters will help refine the speed, axis, and direction of motion.

Future plans call for testing the findings against newer data released from the WMAP project and the European Space Agency's Planck mission, which is also currently mapping the microwave background.

Which is fantastic!  Such discoveries force a whole new set of ideas onto the table which, even if they turn out to be wrong, are the greatest ways to advance science and our understanding of everything. One explanation that's already been offered is that our universe underwent a period of hyper-inflation early in its existence, and everything we think of as the vast and infinite universe is actually a small corner under the sofa of the real expanse of reality.  Which would be an amazing, if humbling, discovery.

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Luke McKinney with Casey Kazan.

Source:

http://www.nasa.gov/centers/goddard/news/releases/2010/10-023.html

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