It sounds like the premise of a bad sci-fi movie: There's some mysterious entity, beyond the boundaries 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 it's been there. But we do know its name: the dipole repeller.
The name may be a bit dorky, but it's a very real thing. It's also nothing to worry about — just a normal consequence of the usual process of structure formation that's been happening in the universe for [checks watch] 13.8 billion years.
How to build a supercluster
To set the stage for the dipole repeller, we need to go big. And not your usual astronomy-big, with galaxy-scale events and energies. No, we have to go really big.
Beyond the Milky Way sit a few other galaxies. There's Andromeda, 2.5 million light-years away, which everyone knows and loves. There's also Triangulum, which nobody really cares about. Our three galaxies and a few dozen dwarf galaxies combine to form the Local Group, which is a very unassuming name for a structure a few million light-years across.
The nearest big deal to our Local Group is the Virgo Cluster, a massive ball of over a thousand galaxies sitting 60 million light-years away. Our Local Group and other groups in this patch of space aren't part of the Virgo Cluster itself; rather, they belong to a greater structure known as the Virgo Supercluster.
Here's where things get a little tricky. Groups and clusters have decent, understandable definitions: They are gravitationally bound. Superclusters aren't; they're just 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 get a range of segmentations.
It's like a population census trying to define a metro area: Sure, there are the city limits, but what about all the people living near a major city and working in it? Where, exactly, does it stop?
A story of superclusters and voids
Despite these varied definitions, we can draw some general outlines. The Virgo Supercluster appears to be just one branch of an even larger supercluster called Laniakea. Other superclusters surround and connect with Laniakea, like the Shapley Supercluster, the Hercules Supercluster and the Pavo-Indus Supercluster. Each of these massive structures is hundreds of millions of light-years long.
The superclusters are like the foam you see when you add too much soap to your bath. We're just giving different parts of that foam network cool names. But between all those bits of foam are vast, empty regions. In your bath, those empty pockets are the soap bubbles themselves. In cosmology, they're the great cosmic voids.
Every supercluster defines the edge of a corresponding cosmic void. There's the Sculptor Void, the Canis Major Void, the Boötes Void and more. Each of these voids is a vast expanse of not much at all — empty cosmological wastelands containing nothing but a few straggling galaxies, like oasis towns in a desert. The largest of these voids, like Boötes, are over 300 million light-years in diameter.
That's a whole lot of nothing.
The dipole repeller
It's actually kind of hard to map our local vicinity of the universe (and by "local," I mean everything within about a billion light-years). That's because all the dust in the Milky Way obscures our view, and we have to resort to fancy astronomical tricks, like sensitive infrared and radio surveys, to get a sense of what's going on.
It's through these tricks that cosmologists were able to identify the Shapley Supercluster, Laniakea's nearest neighbor. The mass of the Shapley Supercluster is so impressive that it exerts a gravitational pull on this entire region of space. Every galaxy, including the Milky Way, is moving in that direction.
But the estimated mass of the Shapley Supercluster may not be quite enough to account for our velocity. In addition to the pull of the Shapley, there has to be something else, a push, coming from the opposite direction.
This is the dipole repeller, a hypothetical void (and possible supervoid) that sits on the opposite side of the Milky Way as the Shapley Supercluster. As the Shapley pulls us with its massive gravity, the dipole repeller pushes us with its massive … nothingness.
How does that work?
Think about it this way. Let's say you carve out a hole in something — a block of wood, a chunk of cheese or the large-scale structure of the universe. If you place something near that hole, it will feel a gravitational tug in every direction except the hole. So it will tend to move away from the hole, because that hole can't contribute its own gravitational influence.
It will appear as if the hole — or void — is repelling the object, even though it's only sitting there, literally doing nothing.
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