In cosmic terms, the universe is the same everywhere. It is, in scientific terms, uniform. But zooming in, we can see that galaxies are organized in strands and nodes, a structure we call the cosmic web. The web is surrounded by regions of lower density known as cosmic voids.
One such void surrounds the local group of galaxies to which the Milky Way, Andromeda, and their satellites belong. The Local Void, as the region is known, was discovered three decades ago but both its shape and size remained a mystery. Well, not anymore.
Researchers at the University of Hawaii have used the motion of galaxies and their distances to work out the properties of the Local Void. As reported in The Astrophysical Journal, the Local Void is a blobby irregular structure that at its widest point is 225 million light-years across. To give a sense of scale, Andromeda is roughly 2.54 million light-years away.
The volume of the void is a staggering 7 trillion trillion cubic light-years, yet it is only one-seventh of the Virgo Supercluster, the larger family of galaxies to which the local group belongs. And the Virgo Supercluster is just one part of the Laniakea Supercluster. R. Brent Tully, lead author of the study, has previously used the same approach to describe the size and shape of Laniakea.
The presence of such a large void nearby has an impact on the local group of galaxies. Even if galaxies are very far apart, the influence of gravity still reaches across these cosmic distances. A dense region will pull celestial objects in. An emptier region will push objects away over time. And this motion has an impact.
Astronomers measured the motion of Andromeda and the Milky Way against the Cosmic Microwave Background, the light echo of the Big Bang. This gives us an idea of how much they deviate in their motion in comparison to the expansion of the universe at large scales. This deviation is roughly 600 kilometers per second (1.3 million miles per hour).
Thanks to this research, scientists can provide a better explanation for this deviation; 50 percent of it is due to the combined pull and push actions of the Local Void and the Virgo Supercluster.