Although we can’t feel it, our universe is in constant motion: the earth spins on its axis at about 1,600 kilometers per hour (1,000 mph) and it orbits around the sun at about 100,000 km/h (67,000 mph); the sun orbits our Milky Way galaxy at about 850,000 km/h (530,000 mph); and the Milky Way galaxy and its companion galaxy Andromeda are moving with respect to the expanding universe at roughly 2 million km/h (1.24 million mph), or 630 kilometers (391 miles) per second.
But what is causing the Milky Way’s race through space? This is a question that has intrigued scientists for more than 40 years, since they discovered that our galaxy was on the move.
“The big question was, what causes the motion at that speed and in that direction?” Prof. Yehuda Hoffman of the Hebrew University of Jerusalem said in an interview.
Until now scientists assumed that a dense region of the universe is pulling us toward it, in the same way gravity made Newton’s apple fall to earth.
In a new study in the latest issue of Nature Astronomy, Prof. Hoffman and his team of researchers now say that our galaxy is not only being pulled, but also pushed. “Our motion versus the universe is a tale of both attraction and repulsion,” he said.
In their research the scientists describe a previously unknown, very large region in our extragalactic neighborhood, sparsely populated by galaxies. This void exerts a repelling force on our local group of galaxies.
Scientists at first assumed that a dense region of the universe was pulling us toward it — the main reason being something they described as “the Great Attractor,” a region of a half-dozen rich clusters of galaxies 150 million light years from the Milky Way. Soon after that, attention was drawn to an area of more than two dozen rich clusters, called the Shapley Concentration, which sits 600 million light years beyond the Great Attractor. Both of these attractors were pulling the galaxies toward them.
Now, however, Hoffman’s team says that besides being pulled, our galaxy is also being pushed.
“By 3D-mapping of 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,” said Hoffman. “In addition to being pulled toward 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.”
The presence of such a low density region has been suggested previously, but confirming the scarcity of galaxies by observation has proved challenging. In this new study, Hoffman, at the Hebrew university’s Racah Institute of Physics, working with colleagues in the United States and France, tried a different approach.
Using powerful telescopes, among them the Hubble Space Telescope, they constructed a 3-dimensional map of the galaxy flow field. Flows are direct responses to the distribution of matter, away from regions that are relatively empty and toward regions of mass concentration.
“We looked at the irregularity of velocities of the galaxies — and mapped them. Some galaxies move faster, others slower than expected,” Hoffman said. “From this mapping we inferred what causes the motion: the over-dense regions attract, while the under-dense regions repel. The decisive role of the repulsion came as a surprise.”
The discovery made by the Hebrew University researchers “helps us better understand the cosmic web that surrounds us,” said Prof. Adi Nusser, a cosmologist and professor of physics at the Technion – Israel Institute of Technology who was not involved in the study but knew about it. “It is a major step forward to resolving the puzzle about the origin of the motion of our galaxy and its immediate neighbors. The discovery tells us that we have been looking in the wrong place. The missing piece of the puzzle is in a large region with fewer galaxies than average rather than the impressive-looking chains of clusters of galaxies in the cosmic web.”
By identifying the Dipole Repeller, the researchers were able to reconcile both the direction of the Milky Way’s motion and its magnitude. They expect that future ultra-sensitive surveys at optical, near-infrared and radio wavelengths will directly identify the few galaxies that they believe lie in this void, which would directly confirm the void associated with the Dipole Repeller.
“Since the dawn of civilization people have been preoccupied by questions about where we live and what is happening around us,” Hoffman said. “Now we know with high certainty how and why we are moving. The pull and push is affecting everything in the universe. ”
Hoffman’s collaborators include Daniel Pomarède, Institut de Recherche sur les Lois Fondamentales de l’Univers, CEA, Université Paris-Saclay, Gif-sur-Yvette, France; R. Brent Tully, Institute for Astronomy (IFA), University of Hawaii, USA; and Hélène M. Courtois, IPN Lyon, University of Lyon, France.