The Milky Way Is Running Away From An Extragalactic Void

A schematic view of what the local cosmic web looks like. Daniel Pomarede/Hoffman et al.

Astronomers have discovered a large void in the universe and it appears that the Milky Way and our neighboring galaxies are running away from it at about 630 kilometers per second (1.5 million miles per hour).

In a paper published in Nature Astronomy, an international group of astronomers has studied the velocities of the galaxies around our own and how they compare to the cosmic microwave background. By combining the observations with rigorous statistical analysis, the researchers have been able to map the gravitational distribution of the (somewhat) local universe.

Astronomers know that what is called the "local group" of galaxies are moving towards a dense region called the Shapley attractor. The team, led by Yehuda Hoffman from the Hebrew University of Jerusalem, realized how the gravitational lines seemed to all point towards the Shapley attractor and away from an unknown region. They suspect this region is a large void we are “escaping”. 

The region has been dubbed the dipole repeller, as it looks a bit like a magnet. Although gravity is exclusively attractive, both the void and the Shapley attractor play a role in how the nearby galaxies move.

"The universe started from an initial state with some density irregularities – some regions were slightly denser and some slightly under-dense," Professor Hoffman told IFLScience. "After subtracting out the mean expansion of the universe, the net gravitational force of the overdense regions is that of an attraction and that of the under-dense regions is that of repulsion."

The discovery of this void actually helps explain the Milky Way's peculiar velocity, and more importantly why it's not going directly toward the Shapley attractor but slightly to the side of it. A 3D map of the volume of space stretching 1.5 billion light-years away from us shows that the flow of local galaxies is instead well aligned with dipole repeller.  

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