Outlined in light blue, giant collections of galaxies can be divided up into superclusters. But our supercluster, along with many nearby ones, might still reside in an even larger cosmic void.
If you went to give our cosmic address, you might tell someone that we lived on planet Earth, orbiting our Sun, on the outskirts of a spur of the Milky Way’s spiral arms, in the second largest galaxy in our local group, about 50 million light years from the Virgo Cluster, embedded within the Laniakea supercluster.
Well, you might have to add another line to that address, as Laniakia, along with dozens of other nearby giant clusters, is all embedded within a great cosmic void stretching a billion light years from end-to-end. This below-average region of space is consistent with everything we observe, supported by new observations presented at this week’s American Astronomical Society meeting, and just might provide the solution to one of the Universe’s greatest discrepancies.
The simulated large-scale structure of the Universe shows intricate patterns of clustering that never repeat. But from our perspective, we can only see a finite volume of the Universe, which appears uniform on the largest scales.
On the largest scales, the Universe is uniform, with equal amounts of matter and energy everywhere. If you drew an imaginary sphere a few billion light years wide around any point and measured the total amount of mass inside, you’d get the same number everywhere, to about 99.99% accuracy. But if your sphere were smaller, you’d see you’d start to get different numbers in different locations. Gravitation pulls matter into filaments, groups and clusters of galaxies, and steals matter away from less dense regions, creating great cosmic voids.
Today, matter in the Universe is distributed like a combination of a spider web and swiss cheese. The “holes” in the Universe are stupendous, with some stretching tens of millions of light years across before you run into any galaxies at all. On the other hand, there are places where filaments intersect — a great nexus in the cosmic web — that correspond to the locations and existences of ultra-large galaxy clusters, some of which contain many thousands of times the mass of our galaxy.