Dark flow transports galaxies across Universe

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All the galaxy clusters in the visible Universe are being pulled in a common direction by an unseen mass far beyond the horizon of our visible Universe. We asked Alexander Kashlinsky to describe the research that lead him and his colleagues to this conclusion.

How was the motion of the galaxies extracted from the motion of the Universe's expansion?
Our measurements used the so-called Sunyaev-Zeldovich (SZ) effect produced on the cosmic microwave background (CMB) photons by hot X-ray emitting gas in clusters of galaxies. If clusters move with respect to the CMB, this should leave an imprint in the SZ term, generally called the kinematic SZ effect. For each cluster this term is very difficult to isolate because of the presence of other (noise) components. However, if many clusters are assembled, the noise can be reduced while the KSZ term remains - in other words the component due to motion would then stand out from the noise. We have assembled a catalog of over 700 clusters extending to a distance of about 4.5 billion light years. We find that the entire cluster sample moves coherently (within the measurement errors) in the direction of the sky between Centaurus and Vela constellations. (The patch width of 20-degrees represents the error in the direction determination).

Why would the galaxies all head towards the same patch of sky or is this a function of your sample of clusters?
Subsamples of clusters at smaller distances move in more-or-less the same direction and at roughly the same velocity, so this motion seems to be independent of distance out to at least the depth of our catalog. Because the motion is measured to extend already that far, we conjecture that it likely extends across the entire visible Universe (horizon). This can be explained by the pull from far-out inhomogeneities, well outside the current horizon of about 14 billion light years. Such structures are expected to be there from pre-inflationary epochs, if indeed our observable homogeneous Universe formed as a result of inflationary expansion in the first moments of the Big Bang.

Why do you use the term 'dark flow'? Is it related to dark matter?
The flow created by the dark matter in our Universe, would be much smaller than what we measure and it should also decrease with increasing distance, contrary to our measurements. So in this sense, the flow is unlikely to be explained by the observed dark matter. Rather, we think it is caused by the parts of space time well outside of our horizon and which are very different from the homogeneous and isotropic space-time we observe "locally". We call it "dark flow" because the matter in the observable Universe cannot account for this motion.

What implications do your results have on theories of the Universe's evolution and inflation?
Imagine being in the middle of quiet ocean. As far as you can see to the horizon, the ocean seems smooth and isotropic (the same in every direction) and you may conclude that the entire cosmos is like that you see locally. But then you find a small flow in some direction extending across the entire field of view. The flow would then indicate the existence of other very different structures (say ravines to sink to, or mountains to flow from) from your local part of space-time (ocean). In other words, the ocean (locally observed space-time) is just a part of the larger and very different world (cosmos, say). So in this sense, our finding is very much in accord with inflation's most basic paradigm that the Universe began very inhomogeneous and that these parts of space-time have been pushed well outside the present horizon (in fact, much farther) by the inflationary expansion.

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