There's a Huge Void at the Edge of the Milky Way and It's Getting Bigger and Emptier

An international team of astronomers has cast new light on a huge, empty void that encircles our galaxy, the Milky Way.

Scientists first described this vast structure—known as the Local Void (LV)—back in 1987, however, we know little about it, in part, because it is mostly obscured from the view of the Earth by our galaxy's nucleus.

But now, a team led by Brent Tully from the University of Hawaii has mapped the Local Void, highlighting its boundaries where collections of matter—such as galaxies—begin to appear, according to a study published in The Astrophysical Journal.

If you looked at an image of the entire universe, you would notice that it mostly consisted of colossal collections of galaxies—huddled together in what are known as clusters and superclusters—interspersed with vast empty spaces, known as voids.

"The galaxies in the universe are found in a network of clusters, filaments, and voids—'the cosmic web,'" Tully told Newsweek. "Our Milky Way Galaxy lies in a filament but it is immediately bounded by a large empty region that I identified in 1987 and called the 'Local Void.'"

We know that the universe is expanding and that in general, the distance between any two given points within it will increase over time. While this is true on a macro scale, on smaller scales the influence of other forces also come into play.

Galaxies tend to be moving toward the densest areas of the universe—those with the most mass—and away from the voids, meaning that the LV will gradually become bigger and emptier over time.

"The dominant pattern of motion in the universe is the cosmic expansion," Tully said. "In places, the mass concentration is high enough to over-ride the expansion locally. The consequences are bound clusters and groups of galaxies—and on smaller scale we have the solar system and the Earth."

"So on small scales gravity can win, while on very large scales, cosmic expansion wins," he said. "The interest in our study is in the intermediate regimes where cosmic expansion is winning but gravity—the lumpiness in the mass distribution—is causing perturbations."

For their study, Tully and his colleagues examined the motions of 18,000 galaxies, which they used to create a three-dimensional picture of the region of the universe surrounding the Milky Way.

"In the past, we studied the structure of the universe by simply using observed galaxies as tracers," Tully said. "The new technique focuses on the motions of galaxies. Gravity attracts. Galaxies are moving toward where there is a lot of mass and away from empty areas. Just by looking at the patterns of galaxy motions we can build a model of concentrations of matter and of the voids."

This map has helped to shed light on a mystery which scientists have been trying to unravel for three decades.

Astronomers have long been puzzled by the fact that the motions of the Milky Way, Andromeda (the nearest large galaxy to our own) and smaller galaxies in the surrounding neighborhood deviate from the overall expansion speed of the universe by 1.3 million miles per hour.

However, the latest study shows that much of this motion originates from the gravitational influence of the Virgo Cluster—in which the Milky Way is located—and the expanding Local Void.

"Our study shows that about half of this motion is caused by the combination of two local influences—attraction toward the Virgo Cluster, the largest nearby concentration of mass, and 'repulsion' by the absence of matter in the Local Void," Tully said. "At any rate, the Local Void is very large, about 200 million light years across."

Tully and his team have also used the same mapping technique previously to reveal the true scale of the supercluster that we are part of, known as Laniakea.

Superclusters are large groups of smaller galaxy clusters which are among the largest known structures in the universe. The Virgo Cluster is part of the Laniakea Supercluster which is thought to contain around 100,000 galaxies.

This article was updated to include comments from Brent Tully.

Local Void map
A smoothed rendition of the structure surrounding the Local Void. Our Milky Way galaxy lies at the origin of the red-green-blue orientation arrows (each 200 million light years in length). We are at a boundary between a large, low-density void, and the high-density Virgo cluster. Brent Tully