It is known that our galaxy is being pulled through space, but cosmologists suspected it was being pushed as well – and new research might confirm it

The Milky Way is being “pushed” through space by a cosmic dead zone that lurks half a billion light years from Earth, researchers claim.

Located on the far side of the constellation of Lacerta, the Lizard, the vast patch of nothingness appears to have a striking dearth of galaxies compared to the rest of its cosmic neighbourhood.



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The paucity of stars, planets and other matter in the region may explain as much as half of the force that propels our home galaxy through the heavens at a speed of two million kilometres per hour.



Scientists have long known that the Milky Way and its neighbouring galaxy, Andromeda, are being pulled across the heavens by the gravitational attraction of the most massive structure in the observable universe. The Shapley attractor, as it is known, is a dense “supercluster” of galaxies some 750m light years from the Milky Way.



But despite the enormous draw of the Shapley attractor, cosmologists knew that it was not the whole story when it came to the Milky Way’s motion. They suspected that the galaxy was not merely being pulled, but was being pushed along as well.



In the latest search for an answer, Yehuda Hoffman, a cosmologist at Hebrew University in Jerusalem, worked with scientists in France and Hawaii to build a 3D map of the nearest galaxies. They drew on measurements from an array of observatories, including the Hubble space telescope, to work out how more than 8,000 galaxies were moving in the ever-expanding universe.



The map revealed a steady flow of galaxies towards the Shapley attractor and away from another region of space almost directly behind the Milky Way on the same axis. The scientists called the newly-identified region of space the “dipole repeller.”



The movement of the Milky Way is dominated by the gravitational attraction of the galaxies around it. If galaxies were scattered randomly through space, the pull would be the same in every direction. But galaxies are not evenly spread out in the universe. As a result, patches of space that are dense in galaxies draw others towards them, while regions that are emptier than normal literally fail to pull their weight: they effectively push objects away from them.



“We show that the Shapley attractor is really pulling, but then almost 180 degrees in the other direction is a region devoid of galaxies, and this region is repelling us,” said Hoffman. “So now we have a pull from one side and a push from the other. It’s a story of love and hate, attraction and repulsion,” he said. Details of the work are published in Nature Astronomy.



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In the coming years, Hoffman said he hoped scientists would use more sensitive instruments to examine the dipole repeller to check whether it is home to as few galaxies as expected.



Michael Rowan-Robinson, an astronomer at Imperial College London, said that while the scientists were right to stress how voids can apparently repel galaxies as much as clusters of galaxies attract, they may have over-emphasised the importance of the Shapley attractor and the so-called dipole repeller. In 2000, his team used galaxy surveys from the Infrared Astronomical Satellite to show that there are scores of superclusters and voids of similar size affecting the flow of galaxies through the cosmos.