Dark flow is an astrophysical term describing a peculiar velocity of galaxy clusters. The actual measured velocity is the sum of the velocity predicted by Hubble’s Law plus a small and unexplained (or dark) velocity flowing in a common direction.
According to standard cosmological models, the motion of galaxy clusters with respect to the cosmic microwave background should be randomly distributed in all directions. However, analyzing the three-year WMAP data using the kinematic Sunyaev-Zel’dovich effect, the authors of the study found evidence of a “surprisingly coherent” 600–1000 km/s flow of clusters toward a 20-degree patch of sky between the constellations of Centaurus and Vela.
The authors, Alexander Kashlinsky, F. Atrio-Barandela, D. Kocevski and H. Ebeling, suggest that the motion may be a remnant of the influence of no-longer-visible regions of the universe prior to inflation. Telescopes cannot see events earlier than about 380,000 years after the Big Bang, when the universe became transparent (the Cosmic Microwave Background); this corresponds to the particle horizon at a distance of about 46 billion (4.6×1010) light years. Since the matter causing the net motion in this proposal is outside this range, it would in a certain sense be outside our visible universe; however, it would still be in our past light cone.
The results appeared in the October 20, 2008, issue of Astrophysical Journal Letters. Since then, the authors have extended their analysis to additional clusters and the recently released WMAP five-year data.
Video showing direction of travel of galaxy clusters at four distances from Earth. The colored dots are clusters within one of four distance ranges, with redder colors indicating greater distance. Colored ellipses show the axis of bulk motion for clusters of the corresponding color. Images of representative galaxy clusters in each distance slice are also shown. Credit: NASA/GSFC/A. Kashlinsky et al.
Distant galaxy clusters mysteriously stream at a million miles per hour along a path roughly centered on the southern constellations Centaurus and Hydra. A new study led by Alexander Kashlinsky at NASA’s Goddard Space Flight Center in Greenbelt, Md., tracks this collective motion — dubbed the “dark flow” — to twice the distance originally reported, out to more than 2.5 billion light-years.
The study used a new technique to determine the motion of X-ray-emitting galaxy clusters. The clusters appear to be moving along a line extending from our solar system toward Centaurus/Hydra, but the direction of this motion is less certain. Evidence indicates that the clusters are headed outward along this path, away from Earth, but the team cannot yet rule out the opposite flow.
The video shows the team’s catalog of galaxy clusters separated into four “slices” representing different distance ranges. A colored ellipse shows the flow axis for the clusters within each slice. While the size and exact position of the ellipses vary, the overall trends show remarkable agreement. The video includes images of representative clusters in each distance slice.
The dark flow is controversial because the distribution of matter in the observed universe cannot account for it. Its existence suggests that some structure beyond the visible universe — outside our “horizon” — is pulling on matter in our vicinity. See: Dark Flow