Dark flow

In astrophysics, dark flow is a controversial hypothesis to explain certain non-random measurements of peculiar velocity of galaxy clusters.

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 Wilkinson Microwave Anisotropy Probe (WMAP) data using the kinematic Sunyaev–Zeldovich effect, a team of astronomers led by Alexander Kashlinsky found evidence of a "surprisingly coherent" 600–1000 km/s[1][2] flow of clusters toward a 20-degree patch of sky between the constellations of Centaurus and Vela.

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 report concluded that while the size and exact position of this direction display some variation, the overall trends among the slices exhibit remarkable agreement.

[10] A more recent statistical work done by Ryan Keisler[11] claims to rule out the possibility that the dark flow is a physical phenomenon because Kashlinsky et al. did not consider the primary anisotropies of the cosmic microwave background (CMB) to be as important as they are.

[13] In 2013, data from the European Space Agency's Planck satellite was claimed to show no statistically significant evidence of existence of dark flow.

Panoramic view of galaxies beyond the Milky Way, with the Norma cluster and the Great Attractor shown by a long blue arrow at the bottom-right in image near the disk of the Milky Way
The dark flow. The colored dots are clusters within one of four distance ranges, with redder colors indicating greater distance. Colored ellipses show the direction of bulk motion for the clusters of the corresponding color. Images of representative galaxy clusters in each distance slice are also shown