Dipole Repeller: A Gigantic Extragalactic Void That’s Pushing Us Through Space

We, and the universe around it, are in a state of constant motion.

Unless you are Sherlock Holmes, you’d probably know that the planet you live is orbiting the sun. What is less well-known is that the sun — and our entire solar system, for that matter — is also orbiting the center of the Milky Way, which is itself hurtling through space at a breakneck speed of roughly 1.2 million miles per hour.

Astronomers had long assumed that our galaxy’s race through space was due to the gravitational pull exerted by two dense regions of the universe — the Great Attractor, which is a region of a half dozen clusters of galaxies about 150 million light-years from the Milky Way, and the Shapley Concentration, an area of over two dozen clusters, located 600 million light-years beyond the Great Attractor.

However, according to a new study published in the latest edition of the journal Nature Astronomy, this is not the complete picture. The Milky Way and the Local Group of galaxies, it turns out, are not only being pulled by the dense regions in their extragalactic neighborhood, they are also being pushed by a previously undetected “void” that is largely devoid of galaxies.

“By 3-d mapping the flow of galaxies through space, we found that our Milky Way galaxy is speeding away from a large, previously unidentified region of low density. Because it repels rather than attracts, we call this region the Dipole Repeller,” study lead author Yehuda Hoffman from the Hebrew University in Jerusalem said in a statement. “In addition to being pulled towards the known Shapley Concentration, we are also being pushed away from the newly discovered Dipole Repeller. Thus it has become apparent that push and pull are of comparable importance at our location.”

In order to create a 3D map of the so-called galaxy flow field — which shows the movement of matter away from regions that are relatively empty and toward regions of mass concentration — the researchers used powerful telescopes like Hubble. Specifically, they focused on galaxies with “peculiar” velocities, which were moving faster than the universe was expanding, and were thus able to infer the underlying mass distribution — including overdense regions that attract this flow, and the underdense regions that repel it.

This, in turn, allowed them to gather the first real evidence of the existence of the Dipole Repeller, which sits in the same axis as the Shapley Concentration.

“There was a hint of the void from studies of the distribution of rich clusters of galaxies that emit X-rays, discussed in articles over a decade ago by Dale Kocevski, Harald Ebeling and myself at the University of Hawaiʻi, but the statistics were not sufficient to be convincing,” study co-author Brent Tully from the University of Hawaii said in a statement.

Scientists hope that now that the Dipole Repeller has been identified, they will be able to glean both the direction and the speed of Milky Way’s motion. The researchers expect that ultrasensitive surveys at optical, near-infrared and radio wavelengths will, in the future, directly detect the sparse concentration of galaxies theorized to exist in this void.