UT: Dark Statistics

[bystander]

Astronomy Without A Telescope – Dark Statistics
Universe Today | Steve Nerlich | 2011 Mar 26

[size=85]The dark flow hypothesis. A region of the observable universe is being influenced by a mysterious something outside the observable universe. [i](Source: universe-review.ca)[/i][/size]

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The hypothetical dark flow seen in the movement of galaxy clusters requires that we can reliably identify a clear statistical correlation in the motion of distant objects which are, in any case, flowing outwards with the expansion of the universe and may also have their own individual (or peculiar) motion arising from gravitational interactions.

For example, although galaxies have a general tendency to rush away from each other as space-time expands between them, the Milky Way and the Andromeda Galaxy are currently on a gravitationally bound collision course.

So, if you are interested in the motion of the universe at a large scale, it’s best to study bulk flow – where you step back from consideration of individual objects and instead look for general tendencies in the motion of large numbers of objects.

Very large scale observations of the motion of galaxy clusters were proposed by Kashlinsky et al in 2008⁽¹⁾ to indicate a region of aberrant flow, inconsistent with the general tendency in the motion and velocity expected by the expansion of the universe – and which cannot be accounted for by localized gravitational interactions.

On the basis of such findings, Kashlinsky has proposed that inhomogeneities in the early universe may have existed prior to cosmic inflation – which would represent a violation of the currently favored standard model for the evolution of the universe, known as the Lambda Cold Dark Matter (Lambda CDM) model.

The aberrant bulk flow might result from the existence of a large concentration of mass beyond the edge of the observable universe – or heck, maybe it is another adjacent universe. Since the cause is unknown – and perhaps unknowable, if the cause is beyond our observable horizon – the astronomical interrobang ‘dark’ is invoked – giving us the term ‘dark flow’.

To be fair, a lot of the more ‘out there’ suggestions to account for these data are made by commentators of Kashlinsky, rather than Kashlinsky and fellow researchers themselves – and that includes use of the term dark flow. Nonetheless, if the Kashlinsky data isn’t rock solid, all this wild speculation becomes a little redundant – and Occam’s razor suggests we should continue assuming that the universe is best explained by the current standard Lambda CDM model.

[size=85]The apparent 'dark flow' is alleged to show up in both close and distant galaxy clusters, suggesting it was there when the universe was very young. [i](Credit: Kashlinsky, NASA)[/i][/size]

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The Kashlinsky interpretation does have its critics. For example, Dai et al⁽³⁾ have provided a recent assessment of bulk flow based on the individual (peculiar) velocities of type 1A supernovae.

The Kashlinsky analysis is based on observations of the Sunyaev–Zel’dovich effect – which involves faint distortions in the cosmic microwave background (CMB) resulting from CMB photons interacting with energetic electrons – and these observations are only considered useful for identifying and observing the behavior of very large scale structures such as galaxy clusters. Dai et al instead use specific data points – being standard candle Type 1a supernovae observations – and look at the statistical fit of these data to the expected bulk flow of the universe.

So, while Kashlinsky et al say we should ignore the motion of individual units and just look at the bulk flow – Dai et al counter with saying we should look at the motion of individual units and determine how well those data fit an assumed bulk flow.

It turns out that Dai et al find the supernovae data can fit the general trend of bulk flow proposed by Kashlinsky – but only in closer (low red shift) regions. More significantly, they are unable to replicate any aberrant velocities. Kashlinsky measured an aberrant bulk flow of more than 600 kilometers a second, while Dai et al found velocities derived from Type 1a supernovae observations to best fit a bulk flow of only 188 kilometers a second. This is a close fit with the bulk flow expected from the Lambda CDM model of the expanding universe, which is around 170 kilometers a second.

Either way, it’s all down to a statistical analysis of general tendencies. More data would help here.

1. A Measurement of Large-Scale Peculiar Velocities of Clusters of Galaxies: Results and Cosmological Implications - A Kashlinsky et al
   • Astrophysical Journal Letters 686(2) L49 (2008 Oct 20) DOI: 10.1086/592947
     arXiv.org > astro-ph > arXiv:0809.3734 > 22 Sep 2008
2. A Measurement of Large-Scale Peculiar Velocities of Clusters of Galaxies: Technical Details - A Kashlinsky et al
   • Astrophysical Journal 691(2) 1479 (2009 Feb 01) DOI: 10.1088/0004-637X/691/2/1479
     arXiv.org > astro-ph > arXiv:0809.3733 > 22 Sep 2008 (v1), 04 Feb 2009 (v2)
3. Measuring the cosmological bulk flow using the peculiar velocities of supernovae - DC Dai, WH Kinney, D Stojkovic
   • arXiv.org > astro-ph > arXiv:1102.0800 > 03 Feb 2011
     Accepted for publication in Journal of Cosmology and Astroparticle Physics

http://asterisk.apod.com/viewtopic.php?p=117276#p117276

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[bystander]

Another Universe Tugging On Ours? Maybe Not
University of Buffalo | 2011 Apr 13

Data from exploding stars contradicts earlier study pointing to the possible existence of a sibling universe

In 2008, a research team led by a NASA scientist announced a startling discovery: Clusters of galaxies far apart from one another appeared to be traveling in the same direction.⁽¹⁾

The findings contradicted the standard model of the universe, which predicts that, as a whole, mass within our universe should flow randomly, in all directions, relative to the background radiation of the cosmos.

The one-way "dark flow" that the NASA-led group discovered created a mystery. What could account for the unexpected motion? Maybe another universe existed beyond the bounds of ours, dragging our stars ever closer through the pull of gravity.

Then again, maybe not.

A new study from the University at Buffalo contradicts the dark flow theory, showing that exploding stars in different parts of the universe do not appear to be moving in sync.⁽³⁾

Working with data on 557 such stars, called supernovae, UB scientists deduced that while the supernovae closest to Earth all shared a common motion in one direction, supernovae further out were heading somewhere else. The difference in motion became pronounced for stars 680 million or more light years away from Earth.

An article announcing the research results will appear in a forthcoming edition of the peer-reviewed Journal of Cosmology and Astroparticle Physics.

Though the findings disagree with the "dark flow" hypothesis, they coincide with the predictions of another model of the universe: Lambda-Cold Dark Matter, the standard model of cosmology.

"Our result is boring, in a way, because it matches your expectation for the standard cosmological model," said UB physicist William Kinney. "If it turns out that the NASA team led by Alexander Kashlinsky is right, it would be exciting because there would be some crazy thing going on that nobody understood. There would have to be something very radical, like a big mass outside of our universe that's pulling on stuff inside our universe. That would be big news."

"But our data do not match theirs," Kinney continued. "With our study, we're muddying the water. It's not yet clear who is right. We have to do more figuring to build up a more detailed and accurate picture of the universe."

Kinney, an associate professor, completed the study on supernovae with De-Chang Dai, a UB postdoctoral researcher who has since joined the University of Cape Town, and Dejan Stojkovic, an assistant professor of physics at UB.

The supernova data the team used to complete their study came from the Union2 data set, which the Supernova Cosmology Project at the Lawrence Berkeley National Laboratory released in 2010. Though Union2 incorporates astronomical observations from different telescopes and different times, the data set controls carefully for systematic bias and serves as a useful check for the possible presence of systematic errors in the work of Kashlinsky and others, Kinney said.

• [*] See previous post for article references[/i]