Outside the visible universe

[Norman Albers]

Science News of Oct. 25 reports "Galaxy clusters slide to the south". I quote the article: "Clusters of galaxies are flowing through space, seemingly under the influence of a mysterious attractive force outside the visible universe, a new study suggests. Researchers have detected what they have dubbed 'dark flow' while surveying 700 galaxy clusters [within one billion LY of Earth]. On average the clusters appeared to move at about 1,000 km/sec in a uniform direction toward a point in the southern sky." My my, it seems there is sort a hole in our baloon. We work with the Robertson-Walker metric assumptions of uniformity, and this is clearly a simple model. Hang onto your hats, folks.

[jjuris]

wasn't there an idea that gravity is the the most predominant force in the universe? Stronger than the weak and strong nuclear. And that it somehow leaked to another brane?

[Norman Albers]

Yes, but did you think we see all that is to be accounted for in any Big Bang? Or Big Bounce or whatever?

[Martin]

... My my, it seems there is sort a hole in our baloon. We work with the Robertson-Walker metric assumptions of uniformity, and this is clearly a simple model. Hang onto your hats, folks.

 

Nice find! I remember discussing that when the study came out some weeks back, late September as I recall. The method they used involved a massive statistical analysis looking for very small (borderline observable) imbalances. As I recall their result has not been duplicated by any other group, and some people were taking a skeptical wait and see attitude.

 

But it is definitely radical. If confirmed it would make for big changes in cosmology.

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EDIT, I found some material I posted elsewhere around 25 September---apparently there was a Space.com article---referring to some stuff posted on arxiv.org around 22 September.

 

Two of the authors involved are Alexander Kashlinsky and Harald Ebeling. The speed of the drift they think they have detected (around 1000 km/s) doesn't seem so great. What seems remarkable to me is the coherence. If this is confirmed it would be interesting. The technique used to detect the bulk drift 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."

 

I've highlighted their essential message.

 

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."

 

 

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Edited October 30, 2008 by Martin

[npts2020]

Talk about expanding horizons!

[Norman Albers]

Thanks for fleshing out the study's details, Martin. I am tweaked by the idea that we may be observing the effects of something beyond the visible horizon.

[Realitycheck]

Let's do the timewarp again!

[scalbers]

To some extent this sort of thing should be expected, as the actual universe is larger than the visible universe, and the visible horizon is currently shrinking with respect to the number of galaxies we can see. I think the more significant aspect of this is talking about inhomogeneities from a time prior to inflation, if this is in fact demanded by the observations.

[Norman Albers]

Prior to or after inflation? I thought inflation was supposed to iron out all the wrinkles.