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Void "repels" milky way's galaxy group.

ASTRONOMERS HAVE DISCOVERED a giant cosmic void that explains why our Local Group of galaxies is moving through the universe as fast as it is.

The Local Group lies in a filament of a much larger cosmic structure. The galaxy clusters in this cosmic web don't stay still but rather gravitate (literally) toward the largest members. Our Local Group is moving toward what's called the Great Attractor, a dense collection in the vicinity of the Centaurus, Norma, and Hydra clusters about 160 million light-years away. Beyond that and about four times farther away lies another, equally influential attractor called the Shapley Supercluster.

But it turns out that there's another player. Using the Cosmicflows-2 catalog of galaxies, Yehuda Hoffman (Hebrew University, Israel) and colleagues have mapped the motion of more than 8,000 galaxies and confirmed that two titan structures determine how local galaxies flow through the cosmic web: Shapley on one side and a single, as-yet uncharted void in the opposite direction.

Think of the local cosmic structure as a gravitational water park: the twisty slides start high (where the void is) and end up low (where the supercluster is), with the natural motion always being down--that is, with gravity. Galaxies toboggan along the gravitational slides.

How fast the galaxies go depends on how tall the slides are. In the same sense, the fact that there's a big void in one part of the gravitational landscape makes the Local Group flow faster toward the dense concentrations in the other direction than it would otherwise.

This discovery might solve a cosmic conundrum. Astronomers knew that the Local Group moves with respect to the cosmic microwave background (CMB). This motion is called the CMB dipole. But the velocity (630 km/s, or 1.4 million mph) is about double what it should be, if Shapley and the other clusters were solely responsible. The void's effect essentially doubles Shapley's pull, explaining why the Local Group moves as fast as it does. The team thus labels the region "the dipole repeller" in their January 30th Nature Astronomy paper.

* Watch a video explaining the result at

Caption: Local cosmic structure (spans 1.7 billion light-years). The arrow is the cosmic dipole's direction.

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Title Annotation:COSMOLOGY
Author:Carlisle, Camille M.
Publication:Sky & Telescope
Date:May 1, 2017
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