The Sciences

Mysterious "Dark Flow" Is Tugging Galaxies Beyond the Universe's Horizon

80beatsBy Eliza StricklandSep 25, 2008 9:40 PM

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In a bizarre finding that has disrupted the current understanding of the universe, astronomers have detected evidence of a massive gravitational force beyond the horizon of the observable universe.

What's being called a dark flow appears to be pulling vast clusters of galaxies toward a 20-degree-wide patch of sky between the constellations of Centaurus and Vela. "It does fly in the face of everything we know," said astronomer Dale Kocevski.... "I'm sure it's going to be controversial" [Discovery News].

When scientists talk about the observable universe, they don't just mean as far out as the eye, or even the most powerful telescope, can see. In fact there's a fundamental limit to how much of the universe we could ever observe, no matter how advanced our visual instruments. The universe is thought to have formed about 13.7 billion years ago. So even if light started travelling toward us immediately after the Big Bang, the farthest it could ever get is 13.7 billion light-years in distance. There may be parts of the universe that are farther away (we can't know how big the whole universe is), but we can't see farther than light could travel over the entire age of the universe [SPACE.com].

In the surprising new study, which will be published in a forthcoming issue of Astrophysical Journal Letters [subscription required], astronomers looked at enormous clusters of galaxies that contain very hot, X-ray emitting gases. After researchers located these clusters, they

looked at the same spots on a map of what's called the cosmic microwave background -- the attenuated glow from the first light that was free to travel through space just 380,000 years after the universe was born. This glow was mapped in detail by NASA's Wilkinson Microwave Anisotropy Probe [Discovery News].

Astronomers believed that the microwaves change temperature when they pass through galactic clusters that are moving relative to the background glow, and they wanted to test that hypothesis. Researchers expected to find some movement, but not much. Instead, the

velocity of these clusters was computed to be around 2 million miles per hour [Ars Technica]

, and the galactic clusters were found to be moving in a coherent direction. Researchers say that the distribution of matter in the observable universe cannot account for this strong pull, meaning that there must be something truly massive over the horizon that's tugging the galaxies in its direction. And what might that something be? Lead researcher Alexander Kashlinsky suggests that there may be regions of the space beyond the observable universe that had a very different experience after the Big Bang. A theory called inflation suggests

that our universe went through a brief period of hyper expansion soon after the Big Bang. It explains how matter managed to spread out so evenly in space, rather than get stuck clumped in just one corner of space, as would happen in a more gradually expanding universe. Inflation moves everything apart faster than gravity could clump it. It could be, then, that there was another, less effective inflation next door to our observable universe and that other blob from the Big Bang remained clumpier. If so it could be out there, loaded with matter, and it is exerting a powerful gravitational pull on every observable thing in our universe [Discovery News].

Go much deeper into this strange cosmic phenomenon with the Bad Astronomer's latest post, "Trans-cosmic flow broadens our horizon." Image: NASA/STScI/Magellan/U.Arizona/D.Clowe et al.

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