Dark flow is a theoretical non-random component of the peculiar velocity of galaxy clusters. The actual measured velocity is the sum of the velocity predicted by Hubble’s Law plus a possible 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 Wilkinson Microwave Anisotropy Probe (WMAP) data using the kinematic Sunyaev–Zeldovich effect, astronomers Alexander Kashlinsky, Fernando Atrio-Barandela, D. Kocevski and H. Ebeling 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 researchers had suggested that the motion may be a remnant of the influence of no-longer-visible regions of the universe prior to inflation.
The dark flow was determined to be flowing in the direction of the Centaurus and Hydra constellations. This corresponds with the direction of the Great Attractor, which is a gravitational mystery originally discovered in 1973.
However, the source of the Great Attractor’s attraction was thought to originate from a massive cluster of galaxies called the Norma Cluster, located about 250 million light-years away from Earth.
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.
In a study from March 2010, Kashlinsky extended his work from 2008, by using the 5-year WMAP results rather than the 3-year results, and doubling the number of galaxy clusters observed from 700.
The team also sorted the cluster catalog into four “slices” representing different distance ranges. They then examined the preferred flow direction for the clusters within each slice.
While the size and exact position of this direction display some variation, the overall trends among the slices exhibit remarkable agreement. “We detect motion along this axis, but right now our data cannot state as strongly as we’d like whether the clusters are coming or going,” Kashlinsky said.
The team has so far catalogued the effect as far out as 2.5 billion light-years, and hopes to expand its catalog out further still to twice the current distance.
In 2013, data from the Planck space telescope showed no evidence of “dark flow” on that sort of scale, discounting the claims of evidence for either gravitational effects reaching beyond the visible universe or existence of a multiverse. However, in 2015 Kashlinsky et al claim to have found support for its existence using both Planck and WMAP data.