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).

Breaking News

On October 26, 2009, astronomers revealed evidence in a pre-print that their previous finding of a "dark flow" of matter towardsthe edge of the universe is supported by analysis of even more galaxies and of those further away than their earlier study, using five years of observational data from the Wilkinson Microwave Anisotropy Probe (WMAP) and a "significantly expanded" X-ray cluster catalogue. Their latest analysis found that 1,400 galaxy clusters are part of the flow that continues to some 2.6 billion light-years from Sol, which is twice as far as observed during the previous study. According to the astronomers, the flow appears to have been generated shortly after the Big Bang that is no longer detectable in the observable universe (Kashlinsky et al, 2009; and Marcus Chown, New Scientist, November 16, 2009).

In late 2008, some astronomers (including the discovery team) proposed that the massive flow of galactic clusters towards a small region in constellations Vela and Centaurus could be evidence for the presence of another universe which inflated outside our own universe. On the other hand, some astronomers have proposed that the standard cosmological model is wrong, and that a different model might explain the motion of galaxy clusters found. While two other teams of astronomers have confirmed the presence of so-called "dark flow" of galaxy clusters up to 200 million light years, the discovery team has been working with additional astronomers to verify that the flow continues through to the horizon of the visible universe (Amanda Gefter, New Scientist, January 22, 2009; Mersini-Houghton and Holman, 2008; and Kashlinsky et al, 2008a and 2008b).

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

(Not yet available.)