Artistic representation of the superclusters of the universe. (Image credit: Mark Garlick / Science Photo Library)
Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, presenter of “Ask a Spaceman” and “Space Radio” and author of “How to die in space. “
It sounds like the premise of a bad sci-fi movie: there is a mysterious entity, beyond the confines of our galaxy, that is pushing against us with incredible force. We don’t know exactly what it is, and we don’t know how long ago it was there. But we do know its name: dipole repellent.
The name may sound silly, but it’s very real. Nor is there anything to worry about, just a normal consequence of the usual process of structure formation that has taken place in the universe per [checks watch] 13.8 billion years.
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How to build a supercluster
To prepare the stage for the dipole repellent, we need to make grains. And not your usual astronomy, with galaxy-scale events and energies. No, we have to go big.
Beyond the Milky Way sit a few more galaxies. There is Andromeda, 2.5 million light-years away, which everyone knows and loves. There’s also Triangulum, which no one really cares about. Our three galaxies and a few dozen dwarf galaxies combine to form the Local Group, which is a very modest name for a structure a few million light-years in diameter.
The closest thing to our local group is the Cluster of the Virgin, a massive ball of more than a thousand galaxies located 60 million light-years away. Our local group and other groups in this space are not part of the Virgo Cluster itself; rather, they belong to a larger structure known as the Virgin Supercluster.
This is where things get a little complicated. Groups i clusters they have decent and understandable definitions: they are gravitationally bound. Superclusters are not; they are only collections of galaxies that are larger than clusters but smaller than, say, the entire universe. Different cosmologists can apply various definitions of the word “supercluster” and obtain a series of segmentations.
It’s like a census of people trying to define a metropolitan area: of course, there are city limits, but what about all the people who live near a big city and work there? Where exactly does it stop?
A story of superclusters and voids
Despite these varied definitions, we can draw some general lines. The Virgin Supercluster appears to be just a branch of an even larger supercluster called Laniakea. Other superclusters surround and connect with Laniakea, such as the Shapley Supercluster, the Hercules Supercluster, and the Turkey-Indus Supercluster. Each of these massive structures is hundreds of millions of light-years long.
Superclusters are like the foam you see when you add too much soap to your bathroom. We just give great names to different parts of this foam net. But among all these pieces of foam are vast, empty regions. In your bathroom, these empty pockets are the same soap bubbles. In cosmologythey are the great cosmic voids.
Each supercluster defines the edge of a corresponding cosmic void. There is the Void Sculptor, the Canis Major Void, the Boötes Void and more. Each of these voids is a vast expanse of not much: empty cosmological wastelands that contain only a few scattered galaxies, like oasis cities in a desert. The largest of these gaps, such as Boötes, are more than 300 million light-years in diameter.
This is very much nothing.
Related: What is the Largest in the Universe?
Dipole repellent
In fact, it’s a little difficult map our local proximity to the universe (And by “local,” I mean everything in about a billion light-years). This is because all the dust in the Milky Way obscures our vision, and we have to resort to fantastic astronomical tricks, such as sensitive radio and infrared surveys, to get an idea of what’s going on.
It was through these tricks that cosmologists were able to identify Shapley’s Supercluster, Laniakea’s closest neighbor. The mass of the Shapley Supercluster is so impressive that it exerts a gravitational pull on this entire region of space. All galaxies, including the Milky Way, move in this direction.
But the estimated mass of the Shapley supercluster may not be enough to explain our speed. In addition to the Shapley pull, there must be something else, a push, coming from the opposite direction.
This is the dipole repellent, a hypothetical hollow (and possibly superbuid) located on the opposite side of the Milky Way like the Shapley Supercluster. As Shapley throws us with its massive gravity, the dipole repellent pushes us with its massive … nothing.
How does this work?
Think of it this way. Suppose you make a hole in something: a block of wood, a piece of cheese, or the big ladder. structure of the universe. If you place anything near this hole, it will feel a gravitational pull in all directions except the hole. So it will tend to move away from the hole, because that hole cannot contribute with its own gravitational influence.
It will look as if the hole, or the void, is repelling the object, even if it is just sitting there, literally doing nothing.
Learn more by listening to the “Ask a Spaceman” podcast available on iTunes (opens in a new tab) and askaspaceman.com. Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter. Follow us on Twitter @Spacedotcom yen Facebook.