Dark Flow

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Woggles

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Hi Everyone. I'm not sure if this is the right forum, however there are topics about dark matter and dark energy here!! lol

Ok I never heard the term Dark Flow before I came across this article. http://sify.com/news/mysterious-cosmic- ... decbj.html

I looked it up at Wikipedia, and I'm not sure if explain it well. At least not to me. Is Dark Flow an actual something or is it like a term use to explain it with out really knowing what it is. Sort of like Dark matter.

Thanks

http://en.wikipedia.org/wiki/Dark_flow
 
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ramparts

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It's another example of using the word "dark" in front of something we don't quite understand. The idea behind this is that one paper a couple of years ago claimed to find that a bunch of galaxy clusters were all moving towards the same direction in the sky, completely unexpected. This would most likely be due to some very massive object beyond the visible universe pulling on them. It would be a huge finding. Unfortunately, follow-up studies have generally shown that this finding was flawed and is probably statistically insignificant.
 
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Woggles

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Thanks ramparts, When I seen Dark, I knew it was something like that.!
 
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captdude

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New Dark Flow Study (ScienceDaily Mar. 11, 2010)

There has been a newer, more detailed study on this subject. I have edited its length for this forum. The full article can be found at this link. www.sciencedaily.com/releases/2010/03/100310162829.htm ScienceDaily (Mar. 11, 2010) — Distant galaxy clusters mysteriously stream at a million miles per hour along a path roughly centered on the southern constellations Centaurus and Hydra. A new study led by Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md., tracks this collective motion -- dubbed the "dark flow" -- to twice the distance originally reported.
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." The new study appears in the March 20 issue of The Astrophysical Journal Letters.
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 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.
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.
The new study builds on the previous one by using the five-year results from WMAP and by doubling the number of galaxy clusters.
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.
 
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ramparts

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That result is from the same group who originally "discovered" the dark flow, so while they might be valid they should be taken with a grain of salt as (to the best of my knowledge) no one else has actually confirmed it. In fact, last year a fairly convincing rebuttal to that result came out which I don't believe this group has responded to yet. So it's still very much an open question.
 
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MeteorWayne

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Also instructive:

"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

Huh??
 
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ramparts

Guest
Well, the kinetic Sunyaev-Zel'dovich effect - a small-scale temperature fluctuation in the cosmic microwave background which the authors used to detect this - is kind of a funny thing. It's difficult to tease out this signal from the WMAP data, there's just too many contaminants. Future data (primarily from the Planck satellite) will have higher resolution and a wider frequency range, which will allow us to tease such a signal out better. From what I've read, the authors can detect an axis, but given the amount of noise involved there are statistical uncertainties as to whether the temperature fluctuation is mostly positive or negative.

What I'm unclear on is, if there is an uncertainty as to the direction, why these data aren't consistent with zero (that is, with no bulk motion). I'll have to (re-)read that paper at some point...
 
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