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Cosmic dark flow
mystery deepens

Posted: 12 March 2010

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The motion of distant galaxy clusters streaming at a million miles per hour along a path centred on the southern constellations Centaurus and Hydra has been tracked to twice the distance originally recorded.

Nicknamed the 'dark flow', Alexander Kashlinsky of NASA's Goddard Spaceflight Centre and colleagues find that this mysterious stream persists to a distance of some 2.5 billion light years. The dark flow is controversial because the distribution of matter in our Universe cannot account for it, which suggests the presence of some structure beyond our visible Universe that is controlling the flow.

The Coma Galaxy Cluster, also known as Abell 1656, is more than 300 million light-years away and is named for its parent constellation, Coma Berenices. It appears to participate in the dark flow. Image: Jim Misti (Misti Mountain Observatory).

"The dark flow cannot arise from gravitational instability, which has been a standard paradigm for the origin of peculiar velocities," Kashlinsky tells Astronomy Now. "That component is undoubtedly present, but according to our measurement is not the only – or even dominant – component of the velocity field. As for structures outside of this horizon, the space-time on sufficiently large scales had to be inhomogeneous prior to inflation of the Universe and that overall structure should have been preserved on sufficiently large scales."

It is thought that there should be no preferred direction of large-scale motion relative to the cosmic microwave background (CMB) – "left-over" energy emitted 380,000 years after the Universe formed in the big bang, but Kashlinsky comments that the dark flow may probe the primeval structure of space-time on scales well beyond the present-day horizon. "The standard cosmological model accounts very convincingly for how the present observed properties of our Universe came about as a results of inflationary expansion of our bubble; the dark flow probes physics prior to inflation. The two models are complementary to each other."

The coloured dots are clusters within one of four distance ranges, with redder colours indicating greater distance. Coloured ellipses show the direction of bulk motion for the clusters of the corresponding colour. Image: NASA/Goddard/A. Kashlinsky, et al.

Hot X-ray emitting gas in a galaxy cluster scatters photons from the CMB. Clusters do not precisely follow the expansion of space, so the wavelengths of the scattered photons change to reflect their individual motion, which results in a minute shift of CMB temperature in the cluster's direction. The change (called the kinematic Sunyaev-Zel'dovic effect, or KSZ) is so small it has never been observed in a single cluster. But Kashlinsky and his team found that by studying large groups of galaxy clusters they could tease this signature out.

They studied 700 galaxy clusters using the Wilkinson Microwave Anisotropy Probe (WMAP) three year data set released two years ago, which first brought the mystery motion to light. Now the team has doubled the number of clusters to work with thanks to WMAP's five year data set. The updated results provide further strong evidence that the dark flow is real, with the brightest clusters at X-ray wavelengths holding the greatest amount of hot gas to distort the CMB photons. After data processing, these same clusters also display the strongest KSZ signature, which is unlikely if the dark flow were merely a statistical fluke.

In addition, the team sorted the cluster catalogue into four 'slices' to represent different distance ranges, and examined the preferred flow direction for the clusters in each slice. While the size and exact position of this direction display some variation, the overall trends among the slices exhibit remarkable agreement.

"In order to probe the flow to various distances we bin clusters by their redshift (z) range, so we have 4 z-bins. What is important is that with this catalogue we could bin clusters by the X-ray luminosity at each z-bin and still have enough clusters to beat down the noise," explains Kashlinsky, who says that this process achieves two objectives: "First, we can see more luminous clusters to larger z and hence, by using them, we can probe the flow to larger distance, and secondly, more luminous clusters have more gas and, if all clusters are part of the same flow, they should have a larger KSZ component and a larger CMB signal. We find a strong correlation between the CMB signal we measure and cluster luminosity range which would be expected if all the clusters participate in the same flow."

The team continue to expand their data set to refine their measurements, and it is expected that new data from ESA's Planck mission, which is mapping the CMB in greater detail than ever, will eventually be used to test their models.

Kashlinsky's study appears in the 20 March issue of The Astrophysical Journal Letters.

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