Mystery 'dark flow' extends towards edge of universe

Galaxies going places (Image: NASA/M.Markevitch et al/STSCI; Maggellan/U.Arizona/D.Clowe et al)

SOMETHING big is out there beyond the visible edge of our universe. That's the conclusion of the largest analysis to date of over 1000 galaxy clusters streaming in one direction at blistering speeds. Some researchers say this so-called "dark flow" is a sign that other universes nestle next door.

Last year, Sasha Kashlinsky of the Goddard Space Flight Center in Greenbelt, Maryland, and colleagues identified an unusual pattern in the motion of around 800 galaxy clusters. They studied the clusters' motion in the "afterglow" of the big bang, as measured by the Wilkinson Microwave Anisotropy Probe (WMAP). The photons of this afterglow collide with electrons in galaxy clusters as they travel across space to the Earth, and this subtly changes the afterglow's temperature.

The team combined the WMAP data with X-ray observations and found the clusters were streaming at up to 1000 kilometres per second towards one particular part of the cosmos (The Astrophysical Journal Letters, vol 686, p L49).

Many researchers argued the dark flow would not turn up in later observations, but now the team claim to have confirmed its existence. Their latest analysis reveals 1400 clusters are part of the flow, and that it continues to around 3 billion light years from Earth, a sizeable fraction of the distance to the edge of the observable universe (arxiv.org/abs/0910.4958). This is twice as far as seen in the previous study.

The dark flow appears to have been caused shortly after the big bang by something no longer in the observable universe. It has no effect today because reaching across this horizon would involve travelling faster than light.

One explanation for the flow would be the gravity of a huge concentration of matter, but this is very unlikely. Within the standard big bang picture, massive cosmic structures were "seeded" by random quantum fluctuations, so overall, matter should be spread evenly.

There could be an exotic explanation. Laura Mersini-Houghton of the University of North Carolina, Chapel Hill, thinks the flow is a sign of a neighbouring universe. If the tiny patch of vacuum that inflated to become our universe was quantum entangled with other pieces of vacuum - other universes - they could have exerted a force from beyond the present-day visible horizon (see "Nosey neighbours").

Yet despite the new findings, the existence of the dark flow remains disputed. Charles Bennett, principal investigator of WMAP says the cluster analysis is not statistically significant. "There is no evidence for the large-scale dark flow, using all of the best data available."

Nosey neighbours
Was our universe once entangled with a neighbour? The observation of "dark flow" in galaxy clusters was predicted in 2006 by Laura Mersini-Houghton of the University of North Carolina at Chapel Hill and colleagues. She proposes that the effect occurs because our universe was once influenced by neighbouring domains (arxiv.org/abs/0810.5388).

Mersini-Houghton reasoned that if a force exerted by other universes squeezed ours, it could generate a repulsive effect that would impede the shrinkage of matter into clusters but not leave an imprint on smaller scales. "This skews the distribution of lumps so they are not the same in all directions," she says. "There is a preferred direction - the dark flow."

She also predicted in 2006 that there should be two "holes" - regions with fewer galaxies than expected. Sure enough, there does appear to be a hole - the so-called "cold spot" identified by the WMAP probe. The hole is a very large region of space where the afterglow is cooler than average. However, its cause - and even existence - is disputed, and Mersini-Houghton's hypothesis remains controversial.

Source: http://www.newscientist.com/article/mg20427345.000-mystery-dark-flow-extends-towards-edge-of-universe.html?full=true&print=true