A Huge Cosmic Void Is Repelling Our Galaxy
Today, in the journal Nature Astronomy, a team of researchers led by Hebrew University cosmologist Yehuda Hoffman has reported that they've found... nothing. To be clear, they've found something, but that something is essentially nothing.
Specifically, Hoffman and his colleagues have spotted an immense, naked region of space which they predict to be largely devoid of galaxies. Since this region is relatively empty of matter, it is exerting a repelling effect on the Local Group of galaxies, which includes Andromeda, the Triangulum Galaxy, and our own Milky Way.
The researchers made the discovery by visualizing the velocities of various galaxies in an area of space extending 1.7 billion light years across. They found that the flow of galaxies was largely directed towards the Shapley Supercluster 650 million light years away. The supercluster is populated by a large number of galaxies, making it gravitationally dense, and thus attractive. Moreover, the researchers found that the flow of galaxies emanated from a distinct region of space, which they dubbed the "Dipole Repeller." They believe this region to be a cosmic void of sorts. Together, it seems that the Shapley Supercluster pulls our Local Group of 54 galaxies while the Dipole Repeller pushes. The result is that our Local Group moves with a velocity of roughly 631 kilometers per second relative to the Cosmic Microwave Background, the leftover radiation from the Big Bang, toward the Shapley Supercluster.
The cosmic void now called the Dipole Repeller was previously mentioned offhandedly more than a decade ago, when Dale D. Kocevski and Harald Ebeling at the University of Hawaii noted a significant "underdensity" of galaxy clusters in an area of space in the northern hemisphere 490 million light years away.
Source: Hoffman, Y., Pomarède, D., Tully, R. B & Courtois, H. M. The dipole repeller. Nat. Astron. 1, 0036 (2017).