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Mar29-10, 04:08 PM
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Quote Quote by rhody View Post
From National Geographic: Daily News, by John Roach, March 22, 2010

New Proof Unknown "Structures" Tug at Our Universe

Popular accounts like the one in National Geographic can be misleading, so for comparison here is the March 2010 Astrophysical Journal article which the NatGeo news item was based on:

Kashlinsky's initial announcement in 2008 was met with skepticism and one or more apparent errors were pointed out. So far I don't think we can say that Kashlinsky is wrong or that he's right.

If it's right, well, it would be a big deal. A lot of people would want to get on the bandwagon and make their own observations. This hasn't happened yet AFAICS, but it could happen.

Kashlinsky's claims have changed, since 2008. Since his analysis was challenged he has gone back and got more data and analyzed everything more carefully. And he still claims a flow but some of the numbers look different.

Here's what Kashlinsky himself says in the abstract:
"...approximately constant velocity out to at least 800 Mpc. The significance of the measured signal peaks around 500 Mpc, most likely because the contribution from more distant clusters becomes progressively more diluted by the WMAP beam. We can, however, at present not rule out that these more distant clusters simply contribute less to the overall motion."

500 Mpc = 1.6 billion lightyears
800 Mpc = 2.6 billion lightyears

This is in terms of proper distance, so this is the size now of the patch of universe which is alleged to be drifting---collectively moving in some direction---relative to the Background.

He can't rule out that the collective drift only involve the relatively small patch extending roughly out to 2 billion lightyears. He claims statistical significance out to 2.6, but significance peaks at 1.6, and it could be that the farther out stuff is simply not moving much with the flow.

So it's possible that Kashlinsky won't convince a significant number of his colleagues and the idea will die. And it's also possible that he will convince people that there is this localized drift, extending out roughly 2 billion lightyears in one or more directions.

By the same measure of distance, the observable universe extends out over 45 billion lightyears in all directions. So even if Kashlinsky can establish that the drift is real, he is so far only saying something about a SMALL BLOB that is only around 4% of the present radius of the observable, which means 0.0006 of the volume. Less than one thousandth of the volume of that part of the universe which we are currently observing.

Also the speed he claims is comparatively small, compared with estimated rates of expansion. He claims 500 km per second. But the speed of light is 300,000 km/s. So he is talking about a drift which is only about one thousandth of the speed of light.

But most of the galaxies which we observe are currently receding (according to Hubble law) at rates in excess of the speed of light. So 1/1000 of c is not automatically a big deal.
Our own galaxy, in fact, has already been show to have a motion relative to Background which is about that rapid. 500 km/s is not unusual for the disorganized random motion of galaxies and clusters of galaxies---it is kind of normal. What would be surprising, and interesting, if Kashlinsky can establish it convincingly, is a large scale collective motion all in approximately the same direction!

I've been watching this since Fall of 2008 and so far I'm taking a wait-and-see attitude.

Glad to see it got into NatGeo. But still have to wait and see.
"Dark Flow" is a bit on the jazzy side.
Have to see what Ned Wright says about Kashlinsky et al series of papers about this.