Paul Sutter is an astrophysicist at The Ohio State University and the chief scientist at COSI Science Center. Sutter is also host of the podcastsAsk a Spaceman and RealSpace, and the YouTube series COSI Science Now.
Somewhere, in the deepest reaches of the cosmos, far from the safe confines of our home galaxy, the Milky Way, lies a monster. Slowly, inevitably, it is pulling. Over the course of billions of years, it draws us and everything near us closer to it. The only force that acts over such immense distance scales and through cosmic periods of time is gravity, so whatever it is, it's massive and unrelenting.
We call it the Great Attractor, and until recently, its true nature has been a complete mystery. Note that it's still a mystery, just not a complete one.
The Great Attractor was first discovered in the 1970s when astronomers made detailed maps of the Cosmic Microwave Background (the light left over from the early universe), and noticed that it was slightly (and "slightly" here means less than one one-hundredth of a degree Fahrenheit) warmer on one side of the Milky Way than the other — implying that the galaxy was moving through space at the brisk clip of about 370 miles per second (600 km/s).
Even though astronomers could measure the rapid velocity, they couldn't explain its origin.
The Zone of Avoidance
First, why is there a mystery in the first place? Astronomers are fantastically good at looking at stuff in space — it is, after all, their one job. So you'd think by now someone would've pointed a telescope in the direction of our motion and … well, figured it out. But there's a problem: whatever the Great Attractor is, it lies in the direction of the constellation Centaurus, and the disk of our own Milky Way cuts right through our view that way. Our galaxy is full of junk — stars, gas, dust, more gas — and all that junk blocks the light from the more distant universe.
So we're fantastically good at mapping most of the large-scale structure of the universe, except where we're forced to look through our own galaxy. Ever the dramatic bunch, astronomers have called this region the Zone of Avoidance.
And dang it, the Great Attractor sits right back there, deep in the Zone, difficult to characterize. Thankfully, that's been starting to change, as X-ray and radio astronomers have peered through the murky depths of the Milky Way and begun a hazy, uncertain sketch of that hitherto unknown patch of universe.
Go Big and go home
To understand what's going on with the Great Attractor, we need to look at the big picture. And I mean Big: The biggest picture of all. Beyond our Milky Way galaxy is our nearest decent-size galactic neighbor, the Andromeda Galaxy. A little over 2.5 million light-years away, it's practically down the street at the scales I'm talking about.
The Milky Way, Andromeda, the Triangulum Galaxy, and a few dozen hangers-on form the Local Group, a gravitationally bound clump about 10 million light-years across.
The Next Big Thing down the way is the Virgo Cluster, the Downtown of our local patch of universe: More than 1,300 galaxies packed into a dense clump only 65 million light-years away. The Virgo Cluster is gravitationally bound, too, which means about what you think it would mean: Its member galaxies tend to hang out near each other, tied up by their mutual gravity.
Going bigger than that and it gets a little fuzzy, in terms of defining extra-galactic structures. There are enormous collections of galaxies called "superclusters," and for a long time they were loosely defined as "Eh, it's larger than a cluster, but smaller than a universe." They got sweet names, too, based on what constellation we looked through to map out the structure, or named after old astronomers: Virgo Supercluster, Hydra-Centaurus Supercluster, Shapley Supercluster, etc. That definition worked fine until we needed to start getting serious work done; e.g., figuring out what the heck is going on with the Great Attractor.
Go with the Flow
We live in a hierarchical universe. That is, over the past 13-and-change billion years, matter has been accumulating into small clumps, which merged into bigger clumps, which merged into even bigger clumps. The party came to a stop, however, about 5 billion years ago when dark energy started to dominate … but that's the subject of another article.
Our universe has already formed galaxies, groups and clusters. Our own Local Group is condensing, with the Milky Way and Andromedaheaded for a collision in about 5 billion years. The Local Group itself, along with some other groups and smaller clusters, are cruising along the gravitational highways to the downtown Virgo Cluster, which is at the center of the conveniently-named Virgo Supercluster.
And all the nearby stuff — including the Milky Way, Andromeda, the Virgo Cluster, and environs — are heading toward the Great Attractor. A combination of more sophisticated (read: any) surveys within the Zone of Avoidance, and a more sophisticated (read: any) understanding of what exactly is a "supercluster," have begun to unravel the mystery of the Great Attractor.
Instead of just being a "large blob of galaxies," studies of the velocities of galaxies in our local neighborhood of the universe have led to a better working definition of "supercluster:" a volume of space where all the galaxies in that space are "flowing" to a common center. And this definition has reworked our understanding of the local universe. The Virgo Supercluster isn't an isolated object, but just an arm (to be fair, a tremendously huge arm) of an even larger structure: the Laniakea Supercluster.
The Not-So-Great Attractor
Looking at super-galactic structures through the lens of flows of matter, it's easy to see what's going on with the Great Attractor. We live in a hierarchical universe, with small structures assembling like galactic Lego blocks into larger ones. The Milky Way and Andromeda are headed toward the center of the Local Group as it condenses. All the stuff in the Virgo Supercluster is falling toward its center: the Virgo Cluster.
And all the stuff in the Laniakea Supercluster is falling toward its center, currently occupied by the Norma Cluster, which is the accumulation of all the gas and galaxies that already beat us there.
So the Great Attractor isn't really a thing, but a place: the focal point of our patch of the universe, the end result of a process set in motion more than 13 billion years ago, and the natural result of the flows and buildup of matter in our universe. How did this process begin? Well, that, too, is another article….
And before I go: The Great Attractor won't stay that Great for long. In fact, we'll never reach it. Before we do, dark energy will rip the Norma Cluster away from us. Clusters will stay like they are, but superclusters will never live up to their names. So take comfort in that: we have nothing to fear from the Great Attractor.
Learn more by listening to the episode "What is the Great Attractor?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Jone L. for the question that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.