You Live Inside a Cosmic Hole... And there's a problem

[00:00:03]There is a giant hole in the universe.

[00:00:07]Imagine a big empty ball that measures 2 billion light-years across, meaning it would take half the age of planet Earth for light to travel from one side to the other.

[00:00:19]And right here, [music] in the very center of this cosmic void, is you and me and the entire Milky Way galaxy.

[00:00:28]Now, the idea that we live in the middle of a big hole in space might sound weird, even kind of terrifying, but there's actually a good chance that this hole is our key to finally understanding the mysterious nature of our own infinite [music] universe.

[00:00:45]Because when we look up at the night sky, even from the darkest place in the world or through the lens of the most powerful telescope in space, we never see the full picture.

[00:00:55]>> [music] >> The universe is not an endless blanket of stars. The real universe looks like this, a three-dimensional structure of voids and clusters that stretches out like an infinite web. It's actually not so different from the way that neurons move through your brain, which is interesting and probably not a coincidence. But going back to the universe, there are two opposing forces that are working against each other to create this web. Those are gravity and dark energy.

[00:01:23]Most of us are pretty familiar with gravity. It's the reason why things fall down when you drop them, and gravity is a product of mass. The Earth is very massive, so therefore it creates the pull of gravity that you are experiencing right now. Even astronauts in space are still caught up in Earth's gravity, and so is the moon. That's what keeps it close to us. Gravity can stretch out through space over long distances, and the more massive the object creating the pull, the further it can reach. That's how the sun's gravity is able to hold on to planets as far away as Neptune. And the most massive object in our galaxy is a black hole located at the very center.

[00:02:03]>> [music] >> It's called Sagittarius A star.

[00:02:07]Now, it's not a star like the way that our sun is a star. The star actually refers to this asterisk in the name, and that is just a way to mark this object as an important one. But, saying Sagittarius A asterisk is pretty awkward. So, instead we use star. Now, the gravity of our galaxy, the Milky Way, holds our sun in place, but it's also powerful enough to reach out and pull on other galaxies. This is our closest neighbor galaxy, Andromeda. It's stuck in the hole with us. One thing you might find comforting is that we're not alone in this giant void. The Milky Way is actually part of a cluster of galaxies that we call our local group.

[00:02:46]There's one large spiral called Triangulum that's just a little further away than Andromeda. Then there are a few dozen much smaller dwarf galaxies that cluster around us. The combined gravity of the three large galaxies is actually so strong that all three of us are being pulled together into one epic collision event that will merge the three spirals together into one supergalaxy.

[00:03:09]That's like two or three billion years from now, though. So, don't worry too much about it. But, the inevitability of the merger does give us a problem that we need to solve right now. If galaxies have a tendency to pull themselves together, [music] then how do we end up with a giant void where hardly any galaxies exist and just one small cluster gets stuck in the middle?

[00:03:32]Were we rejected by the other galaxies?

[00:03:34]Are we the misfits of the cosmos?

[00:03:38]Maybe. But, to find a more reasonable answer, we have to look at the big opposing force to gravity, dark energy.

[00:03:45]It's very spooky-sounding stuff, but dark energy is actually just a label that we use to describe the mysterious force that is causing the universe to expand.

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[00:05:17]This is one of the biggest problems with our current understanding of gravity. If mass attracts other mass, even over incredibly long distances, then in theory, all of the mass in the universe should be pulling itself together. But instead, we've seen evidence that the universe actually does the opposite. It expands, [music] and more recently, we've noticed that the speed of that expansion appears to be accelerating, which would mean that dark energy is not only an opposite to gravity, but it's actually a more powerful force that is able to push objects away from each other faster than gravity can pull them together.

[00:05:53]Now, in the same way that gravity is powered by mass, dark energy appears to be powered by emptiness, which is pretty weird. And the smartest people on Earth are still trying to figure out how that works. But, we know that the result looks like this. If two massive objects are close together, like the Milky Way and Andromeda, then gravity will pull them closer. But, if two objects are very far away, like the galaxies on the other side of our big hole, then regardless of their mass, dark energy will push them apart.

[00:06:23]And the crazy thing is that this all started with the Big Bang itself, which seems like an impossible thing to be able to know. But, we can actually show you a picture of the universe from 13.8 billion years ago. That's about 400,000 years after the Big Bang. We call this picture the cosmic microwave background or CMB. It's the oldest light that we're able to detect, so old and faint that [music] it's fallen down out of the visible spectrum of the human eye. It's even below infrared, all the way down to microwaves of light.

[00:06:56]Now, the Big Bang was not yellow and blue. It wasn't an oval, either. Like a map of the world, this is a two-dimensional representation of a 3D image, and the colors were added to show differences in temperature. Red for hot spots, blue for cold, which also correspond to areas of higher density and lower density.

[00:07:15]One of the most interesting parts of the CMB image is down here in the lower right. This is known as the cold spot.

[00:07:22]It doesn't look like much just because of where it lands on the 2D projection, but it is a massive region of space where light from the Big Bang is significantly colder than the average temperature.

[00:07:34]What really makes this spot stand out, though, is the fact that it is surrounded by areas that are hotter than average. So, there's a massive contrast in this one location, like falling off a mountain cliff into a deep valley.

[00:07:48]No one is sure how that happened, but theories run from a supervoid of deep nothingness to a bruise or imprint from the bubble of our universe colliding with a parallel universe in the distant past.

[00:08:00]But, what it does tell us for sure is that from the very moment of creation, there have been powerful cosmic forces at work. [music] And that's how we end up with a universe that is not just one consistent blanket of stars. This web structure is shaped by pockets of dark energy that expand into voids and concentrations of gravity that pull galaxies into long strings.

[00:08:24]>> [music] >> This means that there is a natural opposite to the void. That's what we call an attractor, and this is ultimately where we are headed. So, the good news is that the Milky Way, or the inevitably fused Milky Way Andromeda Triangulum, will not be stranded in the middle of this hole forever.

[00:08:41]We are being pulled. The mass of the galaxies outside the hole is large enough and close enough to be drawing us out with their gravity. But, even those are being drawn towards an even greater force in the universe. In fact, every galaxy in our general area of space, whether they're inside the hole or outside the hole, is being reeled in like a fish on a line towards one singular point in space.

[00:09:06]This is the [music] Great Attractor.

[00:09:09]As with most things in deep space, we're not exactly sure what the source of this attraction [music] actually is, but we can say that there is some kind of gravitational anomaly located right around here on the edge of our giant hole.

[00:09:23]For it to be creating enough gravity to power this flow of galaxies, it would need to have a concentration of mass about 10 quintillion times greater [music] than the Sun. That is 10 million billion Suns.

[00:09:37]The problem with the Great Attractor is that even though it is humongous, we can't see it from our location on Earth, and that's because our own galaxy is in the way. From our point of view, the dense core of the Milky Way covers about 1/5 of the visible universe, making it very difficult to see what's on the other side. We call this the zone of avoidance, and that is where the Great Attractor lies.

[00:10:02]This one probably is a coincidence, but it's still kind of spooky. And it turns out that this contrast between voids and attractors is responsible for creating the shape of our known universe.

[00:10:13]Now, when we pull back and look at the bigger picture, we see that the universe is actually full of holes, kind of like a sponge or Swiss cheese, with the most infamous of these voids being this one right here, the Boötes void, also known as the Great Nothing. [music] And this is where we start to find some perspective on our own place in the cosmos.

[00:10:34]At 400 million light-years across, Boötes is not the biggest hole in the universe, but it is the deepest and darkest.

[00:10:43]Going by the average density, a region of space this size should probably contain about 2,000 galaxies. Instead, the Boötes void is home to just 60.

[00:10:55]That is some empty space. And when we start to compare that against our own local hole, which is named the KBC void, we start to notice something. Our hole isn't anywhere near as dark. So, what's the deal with that? Well, as with most things in science, this is all about relativity. The KBC void is much larger than Boötes, but it's not nearly as empty.

[00:11:18]Our local hole contains the Milky Way and our local group of galaxies, and it also contains most of the Laniakea Supercluster, which is [music] this massive structure made from hundreds of thousands of galaxies. So, our void is actually better described as an under-dense region of space. And depending on where you are inside the void, there is going to be somewhere between 20 and 80% less density than the cosmic average. So, it's not empty, but it's not full either, meaning that we're not as isolated as you might think.

[00:11:48]Instead of being stranded in the middle of a cosmic desert, we're in more of a galactic suburb. And that can really help us to understand our place in the universe. Maybe we're not alone. It's just that all of the aliens are partying at the Great Attractor, and we're stuck out here at the end of a very long line waiting [music] to get in. Or it could be the opposite, and life just tends to thrive in the suburbs. Think about it.

[00:12:13]Cities are fun to be in, but they're also dangerous. People get murdered, they get hit by cars, stuff falls on them. The same goes for these dense galactic clusters. They are going to be loaded with radiation, supernovas, interstellar objects ripping around and smashing into things. It could be that the middle of nowhere is actually the center of life as we know it, which would mean that these aren't voids, they are just islands of civilization. Now, if you want to get really weird, there's no better place to go than the edge of the universe itself. We can begin that journey by clicking the box on screen right now.