From New Scientist:
FOR most of us the universe is unimaginably vast. But not for cosmologists. They feel decidedly hemmed in. No matter how big they build their telescopes, they can only see so far before hitting a wall. Approximately 45 billion light years away lies the cosmic horizon, the ultimate barrier because light beyond it has not had time to reach us.
So here we are, stuck inside our patch of universe, wondering what lies beyond and resigned to that fact we may never know. The best we can hope for, through some combination of luck and vigilance, is to spot a crack in the structure of things, a possible window to that hidden place beyond the edge of the universe. Now Sasha Kashlinsky believes he has stumbled upon such a window.
Kashlinsky, a senior staff scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has been studying how rebellious clusters of galaxies move against the backdrop of expanding space. He and colleagues have clocked galaxy clusters racing at up to 1000 kilometres per second – far faster than our best understanding of cosmology allows. Stranger still, every cluster seems to be rushing toward a small patch of sky between the constellations of Centaurus and Vela.
Kashlinsky and his team claim that their observation represents the first clues to what lies beyond the cosmic horizon. Finding out could tell us how the universe looked immediately after the big bang or if our universe is one of many. Others aren’t so sure. One rival interpretation is that it is nothing to do with alien universes but the result of a flaw in one of the cornerstones of cosmology, the idea that the universe should look the same in all directions. That is, if the observations withstand close scrutiny.
All the same colleagues are sitting up and taking notice. “This discovery adds to our pile of puzzles about cosmology,” says Laura Mersini-Houghton of the University of North Carolina, Chapel Hill. Heaped in that pile is 95 per cent of the universe’s contents, including the invisible dark matter that appears to hold the galaxies together, and the mysterious dark energy that is accelerating the universe’s expansion. Accordingly, Kashlinsky named this new puzzle the “dark flow”.
Kashlinsky measures how fast galaxy clusters up to 5 billion light years away are travelling by looking for signs of their motion in the cosmic microwave background, the heat left over from the big bang. Photons in the CMB generally stream uninterrupted through billions of light years of interstellar space, but when they pass through a galaxy cluster they encounter hot ionised gas in the spaces between the galaxies. Photons scattered by this gas show up as a tiny distortion in the temperature of the CMB, and if the cluster happens to be moving, the distortion will also register a Doppler shift.
In any individual cluster, this shift is far too small to detect, which is why no one had ever bothered looking for it. However, Kashlinsky realised if he combined measurements from a large enough number of clusters, the signal would be amplified to a measurable level.
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