Our galaxy is headed towards a massive region of space. Fast.
Our galaxy, and 100,000 of our neighboring galaxies, are hurtling towards a massive point in the universe known as “The Great Attractor”. This region of space is about 220 million light years away and is what scientists call a gravitational anomaly. Just how massive and powerful is this region? Take into consideration that of the 300 billion stars in the Milky Way, most of them are a lot smaller than our sun. The Great Attractor is believed to have the mass of a quadrillion suns.
First, we should set the stage here by acknowledging that the universe is expanding. It has been expanding since the Big Bang and is growing by 2.2 million km per hour. That means that our galaxy and Andromeda, our neighboring galaxy, should also be moving at that speed, right? Not exactly.
In the 1970’s we created detailed maps of the Cosmic Microwave Background (CMB) of the universe and noticed that one side of the Milky Way was warmer than the other. This was a surprising find in an otherwise homogenous universe. It was a less than one one-hundredth of a degree F difference and yet it was enough to let us know that we were moving at 600 km per second right towards the Centaurus constellation. This speed of 600 km/s is what’s known as our peculiar velocity — movement other than that measured by the Hubble flow which accounts for the growing space between galaxies during the natural expansion of the universe.
A few years later we figured out that it wasn’t just us but everything within 100’s of millions of lightyears of us that was also heading in the same direction.
There’s only one thing that could oppose expansion across such vast distances, and that’s gravity. Andromeda, for example, should be moving away from us and yet we’re set to collide with it in 4 billion years. Enough mass can combat expansion.
Scientists first thought this peculiar velocity was due to our place just on the outskirts of the Virgo Supercluster which could be drawing us in. But even though it’s comprised of 1,300 galaxies, it still wasn’t massive enough to be causing the anomalies. While we are headed toward the Virgo Supercluster, it’s due to the same phenomenon as our collision with Andromeda — groups and clusters naturally condensing.
Why is it so hard for us to simply look and see this mysterious Great Attractor?
There’s a portion of universe around us that isn’t visible because our own galaxy blocks our view. The sight of the Milky Way in the night sky is undeniably beautiful but also a hindrance to astronomers attempting to decipher what lay beyond it. This 20% of the universe that’s blocked out by our galaxy just so happens to be exactly where the Great Attractor is, now covered by gasses, dust, and stars. This area is called the zone of avoidance. The only way to bypass it is to use x-rays and infrared light, though these methods don’t give us the clearest pictures.
We found that behind the zone of avoidance there was a supercluster of galaxies — now known as the Norma Cluster — in one area of the Great Attractor 150 million light years out. And behind that, an even more massive supercluster 650 million light years away containing the mass of 10,000 Milky Way galaxies. This Shapely Supercluster is one of the largest things in the observable universe.
A “Supercluster” is not just a collection of galaxies but rather a section of space where all the galaxies in that area are heading towards a common center. By this definition, the Virgo Supercluster isn’t its own object but is instead an arm in a much bigger structure — the Laniakea Supercluster.
The Great Attractor is thought to be the gravitational center of just this supercluster, comprised of our galaxy and 100,000 others. The Great Attractor is not, then, a celestial body in and of itself but instead a location where everything else collects like the bottom of a bowl.
Other theories include the Great Attractor being a confluence of dark energy or an area of over-density with an immense gravitational pull. Some scientists believe this is simply a taste of the universe’s eventual end. The Big Crunch would see a condensing of the universe after a few trillion years when expansion slows down and begins to reverse. This would, after some time, lead to a supermassive black hole that would devour everything, including itself.
So is the Great Attractor a threat to us?
No, not really. Expansion will in this case win against condensing worlds. The peculiar velocity is only at 20% of what it should be to seal our fate with the Great Attractor. Everything that is not small clusters tends to fall apart, even the huge Laniakea which will one day dilute and fail as a supercluster. At the rate that we’re expanding, we will never actually come into contact with the mysterious Great Attractor, though we continue to study it.