From Sky & Telescope: “Cosmic Void “Pushes” Milky Way”

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Sky & Telescope

January 30, 2017
Camille M. Carlisle

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.

Local Group. Andrew Z. Colvin 3 March 2011
Local Group. Andrew Z. Colvin 3 March 2011

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This visualization is a slice of the local cosmic structure, roughly centered on the Local Group. The black arrows show the “flow” matter follows in this gravitational watershed. Analysis of these flow patterns has revealed that there’s probably a large, unseen void (gray-brown at right) that is “pushing” us toward the Shapley Supercluster (green), which is in turn gravitationally pulling us toward it. The yellow arrow is the direction of the so-called cosmic dipole.
Yehuda Hoffman

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


Access mp4 video here .

See the full article here .

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Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

“Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”