Detecting Bulk Motion of 700 Clusters with KSZ Effect: Results & Implications

[marcus]

some astrophysicists may have detected a slight average drift in a population of 700 clusters. I believe the clusters surveyed are relatively close to us, within distances on the order of a billion LY. (We see matter out to about 45 billion LY, so one billion is not anywhere near the horizon of observation.)

Two of the authors involved are Alexander Kashlinsky and Harald Ebeling.

The speed of the drift is not so great. I think it is on the order of 1000 km/s.

What is remarkable is the coherence. If this is confirmed it would be interesting.

The technique used to detect the bulk drift is clever. It uses the interaction of hot Xray-emitting gas with the cold CMB photons--the kinematic Sunyaev-Zeldovich (KSZ) effect. Inverse Compton scattering---a hot electron bumps a cold photon and gives it a kick. So the temperature of the CMB can be changed just on those pixels where clusters, with their hot gas, have been identified. A statistically significant measured change of CMB temperature amounting to detection of a dipole. If confirmed. The direction of the bulk motion is towards the constellation Hydra, in the southern hemisphere.http://arxiv.org/abs/0809.3734
A measurement of large-scale peculiar velocities of clusters of galaxies: results and cosmological implications
A. Kashlinsky (GSFC), F. Atrio-Barandela (U of Salamanca), D. Kocevski (UC Davis), H. Ebeling (U of Hawaii)
Ap.J. (Letters), in press. 20 Oct issue (Vol. 686)
(Submitted on 22 Sep 2008)

"Peculiar velocities of clusters of galaxies can be measured by studying the fluctuations in the cosmic microwave background (CMB) generated by the scattering of the microwave photons by the hot X-ray emitting gas inside clusters. While for individual clusters such measurements result in large errors, a large statistical sample of clusters allows one to study cumulative quantities dominated by the overall bulk flow of the sample with the statistical errors integrating down. We present results from such a measurement using the largest all-sky X-ray cluster catalog combined to date and the 3-year WMAP CMB data. We find a strong and coherent bulk flow on scales out to at least > 300 h^{-1} Mpc, the limit of our catalog. This flow is difficult to explain by gravitational evolution within the framework of the concordance LCDM model and may be indicative of the tilt exerted across the entire current horizon by far-away pre-inflationary inhomogeneities."

http://arxiv.org/abs/0809.3733
A measurement of large-scale peculiar velocities of clusters of galaxies: technical details
A. Kashlinsky (GSFC), F. Atrio-Barandela (U of Salamanca), D. Kocevski (UC Davis), H. Ebeling (U of Hawaii)
Astrophysical Journal, in press.
(Submitted on 22 Sep 2008)

"This paper presents detailed analysis of large-scale peculiar motions derived from a sample of ~ 700 X-ray clusters and cosmic microwave background (CMB) data obtained with WMAP. We use the kinematic Sunyaev-Zeldovich (KSZ) effect combining it into a cumulative statistic which preserves the bulk motion component with the noise integrated down. Such statistic is the dipole of CMB temperature fluctuations evaluated over the pixels of the cluster catalog (Kashlinsky & Atrio-Barandela 2000). To remove the cosmological CMB fluctuations the maps are Wiener-filtered in each of the eight WMAP channels (Q, V, W) which have negligible foreground component. Our findings are as follows: The thermal SZ (TSZ) component of the clusters is described well by the Navarro-Frenk-White profile expected if the hot gas traces the dark matter in the cluster potential wells. Such gas has X-ray temperature decreasing rapidly towards the cluster outskirts, which we demonstrate results in the decrease of the TSZ component as the aperture is increased to encompass the cluster outskirts. We then detect a statistically significant dipole in the CMB pixels at cluster positions. Arising exclusively at the cluster pixels this dipole cannot originate from the foreground or instrument noise emissions and must be produced by the CMB photons which interacted with the hot intracluster gas via the SZ effect. The dipole remains as the monopole component, due to the TSZ effect, vanishes within the small statistical noise out to the maximal aperture where we still detect the TSZ component. We demonstrate with simulations that the mask and cross-talk effects are small for our catalog and contribute negligibly to the measurements. The measured dipole thus arises from the KSZ effect produced by the coherent large scale bulk flow motion."

Thanks to Derekmcd, and also to Thenewmans, for alerting us to the Space.com article on this.
Thenewmans pointed out that the Space.com article contained factual errors.
As a rule one can't rely on Space.com journalism in the details but they call attention to new stuff, which is helpful.
In this case the journalist Clara Moskowitz said their population extended out to 6 billion LY, but all I find in the actual articles is repeated mention of an outer limit of a bit under 1.4 billion LY. What she says about the extent of the observable universe is quite misleading. But it's still a useful article, inaccuracy notwithstanding.

 

[derekmcd]

In this case the journalist Clara Moskowitz said their population extended out to 6 billion LY, but all I find in the actual articles is repeated mention of an outer limit of a bit under 1.4 billion LY. What she says about the extent of the observable universe is quite misleading. But it's still a useful article, inaccuracy notwithstanding.

Where do you get 1.4Gly from? I see the survey extended out to redshift of .3 or a bit over 3Gly. Still not close to the aforementioned 6Gly. I reread the paper to see if I missed something, but found nothing. They did mention in the 'Future prospect' section that they desire to extend the survey out to .7 which would coincide with 6Gly.

Anyhow, what ever is causing this phenomena is quite intriguing. The implications could be far reaching. Not really sure what it means to the Big Bang theory and the LambdaCDM model as they only address the observable universe. I suppose that if the source of this cause is from 'outside' the observable universe, then it is no longer outside and the theories and model will need to be tweaked... again. This could be one heck of a tweak that might be far reaching in its implications. Too soon to start speculating, though. Let them fine-tune their measurements and go from there.

 

[marcus]

Where do you get 1.4Gly from?

Is one of us making an error in unit conversion---it could be me. Why don't you take another look.

In their abstract, they say:
We find a strong and coherent bulk flow on scales out to at least > 300 h^-1 Mpc

and they repeat that same figure of 300 h^-1 Mpc
several times. It is the only place I can find where they give an outer limit of their catalog, or say how far out their statistical result extends.

That is where I get the figure of 1.4 billion LY. Actually 1.38, but I rounded off to 1.4
One megaparsec (Mpc) is 3.26 million LY.

I take the value of h to be 0.71

So we are talking about (300/0.71)*3.26 million LY.

How did you come up with your estimate of z = 0.3 ?
I didn't see that anywhere in the papers.

 

[John86]

Today i found this paper, the conclusions are similair although les spectaculair.

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)
(Submitted on 23 Sep 2008)
Abstract: Peculiar velocity surveys have non-uniform spatial distributions of tracers, so that the bulk flow estimated from them does not correspond to that of a simple volume such as a sphere. Thus bulk flow estimates are generally not strictly comparable between surveys, even those whose effective depths are similar. In addition, the sparseness of typical surveys can lead to aliasing of small scale power into what is meant to be a probe of the largest scales. Here we introduce a new method of calculating bulk flow moments where velocities are weighted to give an optimal estimate of the bulk flow of an idealized survey, with the variance of the difference between the estimate and the actual flow being minimized. These "minimum variance" estimates can be designed to estimate the bulk flow on a particular scale with minimal sensitivity to small scale power, and are comparable between surveys. We compile all major peculiar velocity surveys and apply this new method to them. We find that most surveys we studied are highly consistent with each other. Taken together the data suggest that the bulk flow within a Gaussian window of radius 50 Mpc/h is 407 km/s toward l=287 and b=8. The large-scale bulk motion is consistent with predictions from the local density field. This indicates that there are significant density fluctuations on very large scales. A flow of this amplitude on such a large scale is not expected in the WMAP5-normalized LCDM cosmology, for which the predicted one-dimensional r.m.s. velocity is ~110 km/s. The large amplitude of the observed bulk flow favors the upper values of the WMAP5 error-ellipse, but even the point at the top of the WMAP5 95% confidence ellipse predicts a bulk flow which is too low compared tot hat observed at >98% confidence level.

here the whole paper

http://arxiv.org/abs/0809.4041


Greetings
John

 

[marcus]

John,
thanks for finding this.
Let me urge you to give the link to the abstract page, next time you link to a paper
I find that a lot more helpful, because I may want to check out the authors' other papers or the citations to and by. I may just want to look at the abstract page and not download the whole PDF.
In this case, the abstract page to the paper you mention is:
http://arxiv.org/abs/0809.4041

 

[John86]

Ok Thanks Marcus.

This paper is a little different of the papers of the Nasa team. But it is in the same league.

 

[derekmcd]

Is one of us making an error in unit conversion---it could be me. Why don't you take another look.

In their abstract, they say:
We find a strong and coherent bulk flow on scales out to at least > 300 h^-1 Mpc

and they repeat that same figure of 300 h^-1 Mpc
several times. It is the only place I can find where they give an outer limit of their catalog, or say how far out their statistical result extends.

That is where I get the figure of 1.4 billion LY. Actually 1.38, but I rounded off to 1.4
One megaparsec (Mpc) is 3.26 million LY.

I take the value of h to be 0.71

So we are talking about (300/0.71)*3.26 million LY.

How did you come up with your estimate of z = 0.3 ?
I didn't see that anywhere in the papers.

I don't thinks it is a matter of unit conversion. In their more detailed (.3733v1), they show that their cluster catalog extends out to z $$\leq$$ .3.

In the paper (.3734v1) they state on page 6: (any typos are mine)

"Our results indicate a statistically significant bulk-flow component in the final filtered maps for cluster samples in the z-bins from z $$\leq$$0.05 to $$\leq$$0.3 corresponding to median depth to z $$\cong$$0.1, {snip}"

My interpretation could very well be wrong, but I'm assuming there are statistical uncertainties at ranges above and below 300 h^-1 Mpc, and therefor limit their results to that distance. However their catalog does extend to z = .3.

Essentially, their results are limited to 300 h^-1 Mpc, but the survey of the clusters cataloged extends out to z = .3.

 

[yuiop]

I thought it would be interesting to visually compare the drift anomaly with other anomalies that have been claimed to have been observed in the WMAP data.

The first diagram in the top left (I) shows the the region on the all sky WMAP image where the Kashlinksy drift is heading towards highlighted in purple. This diagram is from this website http://www.world-science.net/othernews/080923_wmap.htm which says "The astronomers detected bulk cluster motions of nearly two million miles per hour, toward a 20-degree patch of sky between the constellations of Centaurus and Vela. Their motion was found to be constant out to at least about one-tenth of the way to the edge of the visible universe."

The second diagram (II) at top right shows the "power asymmetry" where one half of the sky appears to have a greater fluctuation amplitude (of about 10 percent) than the other half. I have exagerated the effect by an order of magnitude in the image and I have added an ellipse to highlight the region of least contrast. The original unadulterated image from this blog by Sean Carroll http://cosmicvariance.com/2008/06/08/the-lopsided-universe/ is shown as diagram (IV) in the composite image I uploaded. If maps (I) and (II) are orientated the same way then the region of lower power fluctuation amplitude is in roughly the same direction as the Kashlinsky drift.

The diagram on the bottom left (III) shows the "quadropole pattern of statistical anisotropy" or "elongated universe" effect as described here http://cosmicvariance.com/2008/07/17/a-new-cmb-anomaly/. The two red dots on that image shows where the effect is strongest. I suspect the two dots are opposite poles as WMAP diagrams show the whole sky and any two opposing poles of the sky are always visible in the one map. The orientation of the axis joining the poles is very roughly aligned with the orientation of the ellipse in diagram (II).

At this time it not clear to me if all the maps shown are presented in the "standard orientation". Image (I) is obviously in the normal WMAP orientation, but the distinctive voids are not obvious in the other images and so it hard to be sure how they are aligned. Does anyone here know?

If the Carroll "lopsided universe" divides the sky into two hemispheres it would be interesting to know where the "equator of the universe" is located on the map relative to the ecliptic plane which is normally the long axix of the WMAP.

 

[Chronos]

It's definitely cutting edge, I just don't follow what edge is being cut. No new metric is proposed so it appears they are mainly about being argumentative. I'm not saying there is anything wrong with that, merely that it is uninteresting.