Dark flow: Proof of another universe?

  • #1
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I came across this article online recently and found it interesting. I thought I would share it since many members ask related questions about cosmology.

Kashlinsky, a senior staff scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, has been studying how rebellious clusters of galaxies move against the backdrop of expanding space. He and colleagues have clocked galaxy clusters racing at up to 1000 kilometres per second - far faster than our best understanding of cosmology allows. Stranger still, every cluster seems to be rushing toward a small patch of sky between the constellations of Centaurus and Vela.

Kashlinsky and his team claim that their observation represents the first clues to what lies beyond the cosmic horizon. Finding out could tell us how the universe looked immediately after the big bang or if our universe is one of many. Others aren't so sure. One rival interpretation is that it is nothing to do with alien universes but the result of a flaw in one of the cornerstones of cosmology, the idea that the universe should look the same in all directions. That is, if the observations withstand close scrutiny.
http://www.newscientist.com/article/mg20126921.900-dark-flow-proof-of-another-universe.html?page=1
 

Answers and Replies

  • #2
marcus
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Have to be cautious about anything that comes out in the New Sci.

We had some discussion of the Kashlinsky findings back in September-October (as I recall) when their two papers came out.

It's worth noting what Ned Wright had to say (brief comment in News of the Universe) back around then:
http://www.astro.ucla.edu/~wright/cosmolog.htm#News

He said there was a flaw in the Kashlinsky analysis and the "dark flow" conclusion could not be trusted. Conceivably that has been fixed and the finding can now be trusted. But I have not heard any word on that. As far as I know Wright's comment still stands.

The New Sci article with the quote from Laura Mersini-Houghton is a bad sign. My impression is she has hyped a variety of sensational stuff over the years that subsequently has just gone away. Again just my impression, but Kashlinsky is talking about fairly nearby and if there is some net motion there, relative to the Background---if there is some region of collective flow---that doesn't make it time for a Grand Hypothesis about branes and multiple universes and portals to mystery etc etc. If there is some collective drift and it is verified, then the usual thing would be to look for the simplest explanation, some minor modification of the picture that can take care of that without big leaps and additional speculative assumptions.

One thing you and I could do here is look and see who has cited the two Kashlinsky papers that came out in October. What caliber people have followed up in the intervening 6 or 7 months. I will get the links. (It could always surprise me.)

Here are the links to the papers (there has been no followup by Kashlinsky himself, these are still the latest on this from him!)
http://arxiv.org/abs/0809.3734
http://arxiv.org/abs/0809.3733
These are actually dated September, so it has been EIGHT months (and they hyped it immediately when it came out.) So lets look at the cites to those two papers:
http://arxiv.org/cits/0809.3734
http://arxiv.org/cits/0809.3733

3734 got 12 cits. I sampled one, namely
http://arxiv.org/abs/0811.1086
What this does is study how such a drift or flow kind of thing could result from a small fluctuation within the existing model, without invoking more fantastic stuff. In other words it refers to Kashlinsky (along with a dozen other papers of the same ilk) but deflates extraordinary conjecture.
You can sample some of the others. Here is what Sean Carroll had to say in 0811.1086. This is from the last paragraph in the introduction:
"There is another important motivation for studying deviations from pure statistical isotropy of cosmological perturbations: a number of analyses have found evidence that such deviations might exist in the real world [40]. These include the “axis of evil” alignment of low multipoles [41, 42, 43, 44, 45, 46, 47, 48], the existence of an anomalous cold spot in the CMB [49, 50, 51], an anomalous dipole power asymmetry [52, 53, 54, 55, 56], a claimed “dark flow” of galaxy clusters measured by the Sunyaev-Zeldovich effect [57], as well as a possible detection of a quadrupole power asymmetry of the type predicted by ACW in the WMAP five-year data [30]. In none of these cases is it beyond a reasonable doubt that the effect is more than a statistical fluctuation, or an unknown systematic effect; nevertheless, the combination of all of them is suggestive."

3733 got 9 cites. No one as prominent as Sean Carroll, so I would focus on the response to 3734.
 
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  • #3
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Have to be cautious about anything that comes out in the New Sci.
Okay. It was the first article I read from their online archive and I never heard of such a finding in cosmology

We had some discussion of the Kashlinsky findings back in September-October (as I recall) when their two papers came out.
I completely missed this.

It's worth noting what Ned Wright had to say (brief comment in News of the Universe) back around then:
http://www.astro.ucla.edu/~wright/cosmolog.htm#News

He said there was a flaw in the Kashlinsky analysis and the "dark flow" conclusion could not be trusted. Conceivably that has been fixed and the finding can now be trusted. But I have not heard any word on that. As far as I know Wright's comment still stands.
His arguments for criticism are beyond me at the moment, but it appears that they pose problems for the validity Kashlinsky's findings. The New Scientist article reports that two other teams found similar results to Kashlinsky, but on scales less than 200 million light years.
 
  • #4
marcus
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The New Scientist article reports that two other teams found similar results to Kashlinsky, but on scales less than 200 million light years.
That is a good detail to notice! I don't want to go back over the NewSci article, but you might get their authors' names and do an arxiv search. It is easy. You just go
http://arxiv.org/find
and type in the author's last name
 
  • #5
532
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That is a good detail to notice! I don't want to go back over the NewSci article, but you might get their authors' names and do an arxiv search.
Yes, you would think that the author of the article might mention these other author's who are in agreement, but this is not the case.

I believe that I found one paper using Paperscope 1.0 (if your interested, http://paperscope.sourceforge.net/index.htm). It's a neat little program, and it offered more citations (21) to Kashlinsky's October 2008 paper than arxiv.org apparently does from your previous post.

Consistently Large Cosmic Flows on Scales of 100 Mpc/h: a Challenge for the Standard LCDM Cosmology
Authors: Richard Watkins (Willamette), Hume A. Feldman (Kansas), Michael J. Hudson (Waterloo)
http://arxiv.org/abs/0809.4041

It states within,

Recently, Kashlinsky et al. (2008a,b), have claimed a detection of the bulk flow from the dipole of the CMB observed behind clusters of galaxies, with amplitude (2.8 ± 0.7µK) and direction towards l = 283 ± 14◦, b = 12 ± 14◦. They interpret this as a dipole in the kinetic Sunyaev-Zel’dovich e ect. The conversion from µK to km/s has some systematic uncertainty, but the authors interpret the bulk flow to be between 600 km s−1 and 1000 km s−1.

Our bulk flow result is in excellent directional agreement (6◦) with that found by Kashlinsky et al. (2008b). The amplitude of their flow (1000 km s−1) is considerably higher, but would be compatible if systematic and random errors reduced the Kashlinksy et al. result to ∼ 400 − 500 km s−1. However, we note that their sample is very much deeper than ours. Whereas our signal arises from within a volume of radius ∼ 100h−1 Mpc (z < 0.03), their signal is detected on much larger physical scales, with most of the signal arising from the shell in the range 0.04 < z < 0.2 (120h−1 Mpc < r < 600h−1 Mpc).
 

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