It sounds like the premise of a bad sci-fi movie: there is a mysterious entity, beyond the borders of our galaxy, pushing us with incredible force. We don’t know exactly what it is, and we don’t know how long it’s been around. But we know its name: dipole repellent.
The name might be a little corny, but it’s a very real thing. It’s also nothing to worry about – just a normal consequence of the usual structure-forming process that occurs in the universe for [checks watch] 13.8 billion years old.
How to build a supercluster
To set the stage for dipole repellent, we need to think big. And not your usual astronomy, with galaxy-wide events and energies. No, we have to go very far.
Beyond Milky Way sit down a few others galaxies. There is Andromeda, 2.5 million light years away, which everyone knows and loves. There’s also Triangle, which nobody 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 big problem to our local group is the Virgo Cluster, a massive ball of more than a thousand galaxies located 60 million light-years away. Our local group and other groups in this patch of space are not part of the Virgo cluster itself; rather, they belong to a larger structure known as the Virgo Supercluster.
Here’s where things get a little tricky. Groups and groups have decent and understandable definitions: they are gravitationally bound. Superclusters are not; they are just collections of galaxies that are larger than clusters but smaller than, say, the entire universe. Different cosmologists may apply various definitions of the word “supercluster” and get a range of segmentations.
It’s like a population census trying to define a metropolitan area: sure, there are city limits, but what about all the people living near and working in a major city? ? Where, exactly, does it stop?
A story of superclusters and voids
Despite these varied definitions, some broad outlines can be drawn. The Virgo supercluster appears to be just a branch of an even larger supercluster called Laniakea. Other superclusters surround and connect to Laniakea, such as the Shapley Supercluster, the Hercules Supercluster, and the Pavo-Indus Supercluster. Each of these massive structures is hundreds of millions of light-years long.
Superclumps are like the suds you see when you add too much soap to your bath. We just give cool names to different parts of this moss network. But between all these bits of foam are vast empty regions. In your bath, those empty pockets are the soap bubbles themselves. In cosmologythese are the great cosmic voids.
Each 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 nothing at all – empty cosmological wastelands containing only a few scattered galaxies, like oasis cities in a desert. The largest of these voids, like Boötes, is over 300 million light-years in diameter.
It’s a lot of nothing.
The dipole repellent
It’s actually a bit difficult to map our local neighborhood of the universe (and by “local” I mean anything 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 readings, to get a sense of what’s going on.
It was thanks to these tricks that cosmologists were able to identify the Shapley 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 pull of the Shapley, there must be something else, a push, coming from the opposite direction.
This is the repelling dipole, a hypothetical vacuum (and possible supervacuum) that lies on the other side of the Milky Way than the Shapley Supercluster. As the Shapley pulls us with its massive gravity, the dipole repulsor pushes us with its enormous… nothingness.
How it works?
Think of it this way. Let’s say you drill a hole in something – a block of wood, a piece of cheese, or the tall ladder structure of the universe. If you place anything near this hole, it will feel a gravitational pull in all directions except The hole. It will therefore tend to move away from the hole, because this hole cannot bring its own gravitational influence.
It will appear that the hole – or void – is pushing the object away, 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 following Paul @PaulMattSutter and facebook.com/PaulMattSutter. Follow us on twitter @Espacedotcom and on Facebook.
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