Our home galaxy is right on the border of a void so vast it's hard to picture. To get a better idea of its shape and size, a team of astronomers has mapped out the edges of this extragalactic emptiness, and calculated just how much gravitational influence it has on the Milky Way.
Just like the Earth orbits the Sun and the Sun orbits the center of the Milky Way, our home galaxy is itself racing through the cosmos at tremendous speeds. In fact, it seems to be moving much faster than it should be, were it only under the influence of the expansion of the universe. The Milky Way, along with nearby neighbors like Andromeda and a host of smaller galaxies, has been clocked at about 2 million km/h (1.3 million mph).
This discrepancy could be explained by the distribution of mass on gigantic scales. Galaxies aren't evenly spread out across the cosmos – they tend to clump together into clusters, connected by thin strands of material like a spider web. That leaves large sections of space relatively empty – and as astronomers discovered in 1987, the Milky Way is right on the fringe of one of them.
This Local Void, as it's known, has been estimated to be between 146 and almost a billion light-years wide, which is unfathomably empty. And it seems to be getting bigger. A few years ago astronomers found that a particularly low-density area dubbed the Dipole Repeller is pushing away the Milky Way and other galaxies in the Local Sheet – an almost flat cluster of galaxies that makes up one wall of the void.
But the Local Void is hard to study, mostly because it's a region of nothing hiding behind the huge concentration of stars and matter at the center of the Milky Way galaxy. For the new study, the team measured the movements of 18,000 galaxies detailed in a dataset called Cosmicflows-3. Using that, the researchers constructed a 3D cosmographic map and found that the walls of the Local Void came into sharp focus.
The team also used this information to calculate how much influence the Local Void is exerting on our galaxy. After accounting for the speeds expected as the universe expands, the researchers found that about half of the motion of our galactic cluster is created "locally," as the massive Virgo cluster pulls us towards it and the Local Void pushes us away.
It's also been argued that empty regions like the Local Void and the Dipole Repeller aren't actively pushing matter away. Instead, areas of high density naturally attract more matter towards them, making the voids larger and therefore give the appearance that they're repelling matter.
The new research was published in the Astrophysical Journal. The team details the shape and structure of the Local Void with 3D models in the video below.
Source: University of Hawaii
