About 18 months ago, we discussed a mysterious "dark flow" that was seen in early releases of the data from the Wilkinson Microwave Anisotropy Probe data. At that time, we remarked that it was little more than a cosmic curiosity and possibly a statistical blip. New research from the same group, using the more complete five-year data set from the WMAP cosmic microwave background imager and X-ray luminosity data, reveals that the dark flow is still there, and that it runs deeper than previously thought.
In a paper published in the March 20th issue of The Astrophysical Journal Letters, the authors report that they have followed the dark flow—which appears to involve matter streaming either into or out of the constellation Centaurus/Hydra—to a distance of 2.5 billion light years. So far, even with the full five-year data set available, the authors can detect motion, but not whether the matter it coming towards us or moving away from us. But they can tell that it is moving, and in a definite direction.
Relative motion of matter in the Universe is to be expected, but motion in a preferential direction is not. According to our best understanding of how the matter in the Universe was distributed, there's no way of accounting for this flow. The obvious alternate explanation is a little unnerving: something outside of our visible universe is pulling on the matter that we can see.
The researchers are currently adding more galaxies to their catalog in order to track the dark flow to twice its current distance. They hope that improved modeling of the motion of hot gas within galaxy clusters will lead to further refinement of the measured speed and direction of motion. Fortunately, we'll soon have even newer data from the WMAP project, as well as upcoming data from the ESA's Planck mission.
The Astrophysical Journal Letters, 2010. DOI: 10.1088/2041-8205/712/1/L81