The Dallas Geek's Quantum Universe

The 'dark flow' of wayward galaxy clusters.

Much attention has been recently given to dark flow. We know that the universe is expanding in an ever accelerating rate. We know the expansion is in all directions. However, there exist galaxy super custers which are moving in a direction independent of the surrounding clusters. What force could exist that could attract such large masses as a super cluster?

Studies have been published suggesting that something massive on a large cosmic scale may exist whose gravity is attracting the galaxy super clusters. Some suggest that is gravity is that of an adjacent universe near the edges of our universe. This may be a clue to what exists beyond our light horizon, beyond the edges of our universe.

Super galaxy cluster map.

http://en.wikipedia.org/wiki/Dark_flow

Dark flow is a term from astrophysics 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.

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 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, 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.

The flow of such clusters

The team has so far catalogued the effect as far out as 2.5 billion light-years, and hope to expand their catalog out further still to twice the current distance.

Astrophysicist Ned Wright 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.

In March 2010 it was reported by Discovery News that NASA’s Goddard Space Center confirmed this could be the effects 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. Said Kashlinsky: “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.”

Dark flow mapped with WMAP.

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 clusters over a region hundreds of millions of light years across.

These galaxies are all redshifted, in accordance with the Hubble Flow, indicating that they are receding relative to us and to each other, but the variations in their redshift are sufficient to reveal the existence of the anomaly. The variations in their redshifts are known as peculiar velocities, and cover a range from about +700 km/s to −700 km/s, depending on the angular deviation from the direction to the Great Attractor.

The first indications of a deviation from uniform expansion of the universe were reported in 1973 and again in 1978. The location of the Great Attractor was finally determined in 1986, and is situated at a distance of somewhere between 150 and 250 Mly (million light years) (47–79Mpc) (the latter being the most recent estimate) from the Milky Way, in the direction of the constellations Hydra and Centaurus. While objects in that direction lie in the zone of avoidance (the part of the night sky obscured by the Milky Way galaxy) and are thus difficult to study with visible wavelengths, X-ray observations have revealed that the region of space is dominated by the Norma cluster (ACO 3627),a massive cluster of galaxies, containing a preponderance of large, old galaxies, many of which are colliding with their neighbours, and/or radiating large amounts of radio waves.

Debate over apparent mass

In 1992, much of the apparent signal of the Great Attractor was attributed to the effect of Malmquist Bias. In 2005, astronomers conducting an X-ray survey of part of the sky known as the Clusters in the Zone of Avoidance (CIZA) project reported that the Great Attractor was actually only one tenth the mass that scientists had originally estimated. The survey also confirmed earlier theories that the Milky Way galaxy was in fact being pulled towards a much more massive cluster of galaxies near the Shapley Supercluster which lies beyond the Great Attractor.