Posted: September 24, 2008
Using data from NASA's Wilkinson Microwave Anisotropy
The research team, lead by Alexander Kashlinsky of NASA's Goddard Space Flight Center, assembled a catalogue of over 700 galaxy clusters extending to a distance of about 4.5 billion light years, and found that the entire cluster sample is speeding towards a patch of sky between the Centaurus and Vela constellations at a rate of two million miles per hour.
"The clusters show a small but measurable velocity that is independent of the Universe's expansion and does not change as distances increase," says Kashlinsky. "We never expected to find anything like this."
Hot gas in moving galaxy clusters (white spots) shifts the temperature of cosmic microwaves. Hundreds of distant clusters seem to be moving toward one patch of sky (purple ellipse). Image: NASA/WMAP/A. Kashlinsky et al.
Of course, because of the nature of the expanding Universe, the clusters are not destined to collide, since the space into which they are apparently flowing is also expanding. Kashlinsky’s team has nicknamed this collective motion a "dark flow" in the vein of more familiar cosmological mysteries: dark energy and dark matter. "The distribution of matter in the observed Universe cannot account for this motion," he says. “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.”
The measurements were based on the so-called Sunyaev-Zeldovich (SZ) scattering effect produced on cosmic microwave background (CMB) photons by hot X-ray emitting gas in clusters of galaxies. Clusters don't precisely follow the expansion of space, so the wavelengths of scattered photons change in a way that reflects each cluster's individual motion, resulting in a minute shift of the microwave background's temperature in the cluster's direction.
“If clusters move with respect to the CMB, this should leave an imprint in the SZ term, generally called the kinematic SZ effect,” explains Kashlinsky. “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.”
The galaxy cluster 1E 0657-56 (known as the Bullet Cluster) lies 3.8 billion light-years away. It's one of hundreds that appear to be carried along by a mysterious cosmic flow. Image: NASA/STScI/Magellan/U.Arizona/D.Clowe et al.
The finding contradicts predictions from standard cosmological models which describe such motions as decreasing at ever greater distances. Cosmologists view the microwave background - a flash of light emitted 380,000 years after the big bang - as the Universe's ultimate reference frame. Relative to it, all large-scale motion should show no preferred direction. But big bang models that include a feature called inflation offer a possible explanation for the flow. Inflation describes a brief hyper-expansion early in the Universe's history, and if a correct portrayal of events then the Universe we can see is only a small portion of the whole cosmos.
Kashlinsky uses an analogy of sitting in the middle of a quiet ocean to explain: “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.”
Because the observed motion is measured to extend to the depth of the team’s catalogue, they conjecture that it likely extends across the entire visible Universe. “This can be explained by the pull from far-out inhomogeneities, well outside the current horizon of about 14 billion light years,” Kashlinsky tells Astronomy Now. “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.”
WMAP data released in 2006 support the idea that the Universe experienced inflation, and the new results could give astronomers a way to explore the state of the cosmos before inflation occurred. The team now plan to narrow down a few lingering uncertainties in their measurements, in particular surrounding the nature of how the million degree gas in the galaxy clusters is distributed. They plan to assemble an even larger and deeper catalogue of galaxy clusters to better measure the flow, using new WMAP data that is to be released in March 2009.