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 Milky Way Galaxy is one of the biggest galaxies in the Local Group, a modest cluster of stellar metropolises. The Local Group, in turn, lies in a filament of the much larger cosmic structure. The galaxy clusters in this cosmic web don’t stay still, but rather slowly gravitate (literally) toward the largest clusters.
Astronomers have known since the 1980s that the 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. They’ve also found another, equally influential attractor called the Shapley Supercluster, a huge structure along roughly the same line of sight but four times farther away.
In 2006, when Dale Kocevski and Harald Ebeling (both then of University of Hawai'i) confirmed Shapley’s influence on the Local Group by mapping out how clusters clump together on the sky, they also saw hints of a void in the opposite direction.
Now, using the Cosmicflows-2 catalog of galaxies, Yehuda Hoffman (Hebrew University, Jerusalem) and colleagues have mapped out the movements of more than 8,000 galaxies and confirmed that, yes, the two titans that determine how local galaxies flow through the cosmic web are Shapley and this single, as-yet unmapped void.
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 cluster is), with the natural motion always being down — that is, with gravity. Galaxies toboggan along the gravitational slides.
But how fast the galaxies go depends on how tall the slides are. In the same way, the fact that there’s a big, "high" void in one part of the gravitational landscape makes the Local Group flow faster toward the dense, “low-lying” regions in the other direction than it would otherwise. The net effect is as though the void is pushing in the same direction as the supercluster is pulling. It may even be that the void, which the team labels “the dipole repeller” in their January 30th Nature Astronomy paper, has more of an effect on the Local Group’s motion than the Shapley region does on its own.
This discovery actually may solve a longstanding cosmic conundrum. Astronomers knew that the Local Group moves with respect to the cosmic microwave background (CMB), the ocean of photons suffusing the universe that is left over from the Big Bang. This motion is called the CMB dipole. But the velocity (630 km/s, or 1.4 million mph) was about double what it should be, if Shapley and the other clusters were responsible. The repeller’s effect essentially doubles Shapley’s pull, explaining why the Local Group moves as fast as it does.
Below, you’ll find a movie explaining the result. Don’t mind the jargon: if it fazes you, the illustrations should carry you through. Credit: Yehuda Hoffman
Yehuda Hoffman et al. “The Dipole Repeller.” Nature Astronomy. January 30, 2017.
Dale D. Kocevski and Harald Ebeling. “On the Origin of the Local Group’s Peculiar Velocity.” Astrophysical Journal. July 10, 2006.
O. Lahav, M. Rowan-Robinson, and D. Lynden-Bell. “The Peculiar Acceleration of the Local Group As Deduced from the Optical and IRAS Flux Dipoles.” Monthly Notices of the Royal Astronomical Society. October 1, 1988.
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