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Dark flow

Dark flow

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Dark flow is an astrophysical
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...

 term describing a peculiar velocity
Peculiar velocity
Peculiar motion or peculiar velocity refers to the true velocity of an object, relative to a rest frame.-Galactic astronomy:In galactic astronomy, the term peculiar motion refers to the motion of an object through space.Local objects are usually related in terms of proper motion and radial...

 of galaxy cluster
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...

s. The actual measured velocity is the sum of the velocity predicted by Hubble's Law
Hubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...

 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
Wilkinson Microwave Anisotropy Probe
The Wilkinson Microwave Anisotropy Probe — also known as the Microwave Anisotropy Probe , and Explorer 80 — is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat — the Cosmic Microwave Background Radiation — across the full sky. Headed by Professor...

 data using the kinematic Sunyaev-Zel'dovich effect
Sunyaev-Zel'dovich effect
The Sunyaev–Zel'dovich effect is the result of high energy electrons distorting the cosmic microwave background radiation through inverse Compton scattering, in which the low energy CMB photons receive energy boost during collision with the high energy cluster electrons...

, 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
Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations.-Stars:...

 and Vela
Vela (constellation)
Vela is a constellation in the southern sky. Its name is Latin for the sails of a ship, and it was originally part of a larger constellation, the ship Argo Navis, which was later divided into three parts, the others being Carina and Puppis.-Stars:...


The authors, Alexander Kashlinsky
Alexander Kashlinsky
Alexander Kashlinsky is an astronomer and cosmologist, better known for discovery of a phenomenon called dark flow.-Biography:*1976-1979: Tel Aviv University, Israel, Departament of Physics & Astronomy...

, 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
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

, 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
Light cone
A light cone is the path that a flash of light, emanating from a single event and traveling in all directions, would take through spacetime...


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.


The dark flow was determined to be flowing in the direction of the Centaurus
Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations.-Stars:...

 and Hydra
Hydra (constellation)
Hydra is the largest of the 88 modern constellations, measuring 1303 square degrees. It has a long history, having been included among the 48 constellations listed by the 2nd century astronomer Ptolemy. It is commonly represented as a water snake...

 constellations. This corresponds with the direction of the Great Attractor
Great Attractor
The Great Attractor is a gravity anomaly in intergalactic space within the range of the Centaurus Supercluster that reveals the existence of a localised concentration of mass equivalent to tens of thousands of Milky Ways, observable by its effect on the motion of galaxies and their associated...

, which was a previous 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
Norma cluster
The Norma Cluster is a rich cluster of galaxies located near the center of the Great Attractor. Although it is both nearby and bright, it is difficult to observe because it is located in the Zone of Avoidance, a region near the plane of the Milky Way. Consequently, the cluster is severely...

, situated merely between 150-250 million light-years away. This may reveal that the source of that attraction might lie even further away, and which the Great Attractor itself is heading towards.

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.


Astrophysicist Ned Wright
Edward L. Wright
Edward L. Wright is an American astrophysicist and cosmologist, well known for his achievements in the COBE project and as a strong Big Bang proponent in web tutorials on cosmology and theory of relativity....

 posted an online response to the study arguing that its methods are flawed. The authors of the "dark flow" study released a statement in return, refuting three of Wright's five arguments and identifying the remaining two as a typo and a technicality that do not affect the measurements and their interpretation.

A more recent statistical work done by Ryan Keisler claims to rule out the possibility that the dark flow is a physical phenomenon because Kashlinsky et al. do not consider primary CMB anisotropies as important as they are.

NASA's Goddard Space Center considered that this could be the effect of a sibling universe or a region of space-time fundamentally different from the observable universe. Data on more than 1,000 galaxy clusters have been measured, including some as distant as 3 billion light-years. Alexander Kashlinsky claims these measurements show the universe's steady flow is clearly not a statistical fluke. Kashlinsky said: "At this point we don't have enough information to see what it is, or to constrain it. We can only say with certainty that somewhere very far away the world is very different than what we see locally. Whether it's 'another universe' or a different fabric of space-time we don't know."

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