Cosmological "Dark Flow"

Kashlinsky et al, 2008a,
WMAP, GSFC, NASA

Larger map.

A stream of hundreds of
distant galaxy clusters
is moving towards the
edge of the visible
universe towards a
region between
constellations Centaurus
and Vela (more).

Cosmological Stream of Galaxy Clusters

Using data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP), a team of scientists (including: Alexander Kashlinsky, Fernando Atrio-Barandela, Dale Kocevski, and Harald Ebeling) has detected hundreds of distant galaxy clusters moving towards the edge of the visible universe. These clusters show a small velocity that appears to be independent of the universe's expansion, and this velocity does not change as distances to the clusters increase (exhibilting constant motion for at least a billion light-years), according to lead researcher Alexander Kashlinsky. This cosmolgically large stream of matter is assumed to be gravitationally attracted by even larger masses that are apparently located beyond the horizon of the observable universe, as the distribution of matter in the visible universe cannot account for this motion. Kashlinsky refers to this collective motion as a "dark flow," in reference to other mysteries of cosmological significance now called dark energy and dark matter.

Clowe et al, 2006,
STScI, Magellan,
University of Arizona,
NASA

Larger image.

Located some 3.8
billion light-years
away, the Bullet
Cluster (1E 0657-556)
is one of those
galaxy clusters
being carried
along within the
"Dark Flow" (more).

This dark flow was detected using the largest all-sky X-ray cluster catalog assembled to date and three years of WMAP data on the cosmic microwave background (CMB), as "a strong and coherent bulk flow" of nearly 2 million miles (3.2 million kilometers) per hour to the limit of the catalog. The clusters were found to be heading toward a 20-degree wide patch of sky between the constellations of Centaurus and Vela. The study involved a sample of some 700 X-ray clusters that included objects up to 6 billion light-years away — close to half of the observable universe.

Velocities of galaxy clusters can be measured by analyzing fluctuations in the CMB, which are generated by the scattering of the microwave photons by hot X-ray emitting gas within such clusters. As these clusters do not precisely follow the expansion of space, the wavelengths of scattered photons from the CMB — a flash of light emitted 380,000 years after the Big Bang — detected on Earth are altered in a way that reflects each cluster's individual motion as the photons travelled through the "atmosphere" of an intervening galaxy cluster. This results in a minute shift of the CMB's temperature in a cluster's direction, which astronomers call the kinematic Sunyaev-Zel'dovich (SZ) effect. (More discussion is available from: a NASA GSFC press release; Anil Ananthaswamy, New Scientist, September 25, 2008; Kashlinsky et al, 2008a; and Kashlinsky et al, 2008b).

Other Information

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