The Universe: You Are Here
We all know where the Earth is. And most people know the shape of our Solar System, and maybe our galaxy. But few really understand the shape of the galaxies around us and our entire Universe. Let’s dive in, to our Universe.
Where are we? Our Galaxy
We live on the planet Earth, which rotates around our Sun.
There are other stars near us, which we can see at night. These include the various constellations that people know and love: Orion, Ursaes and Aquarius.
Our Sun along with 200 billion other stars, make up our galaxy the “Milky Way”. Galaxies come in lots of different shapes – the Milky Way is a spinning spiral disk galaxy. Our solar system is not in the center of our galaxy, it is a bit off to the side, just at the base of one of the larger arms. We don’t have any good pictures of it, because we are sitting right in the middle of it, so they are hard to take. But we know what shape the Milky Way is, and we have some good pictures of some other similarly shaped galaxies. The long streak of stars we see at night is one of the arms of the Milky Way.
There are lots of exciting things happening at the center of our galaxy. Most galaxies are a fair bit denser near their center, there are more stars and large amounts of dust swirling around that will eventually form stars and often a few black holes. This is true with the Milky Way, it bulges in the middle a bit. Most of our galaxy is very flat (that’s what happens when you spin something – think pizza dough) but near the middle where there is a lot more mass, a bit of a bump has survived.
How much of our Universe can we see?
The first thing to know about our Universe is that we can’t see all of it. Light takes time to travel from one place to another. For example, when you look up at the Sun, you don’t see the Sun as it is “right now”, instead you see the Sun as it was roughly eight minutes ago. This means if the sun suddenly disappeared, you wouldn’t see it for eight minutes until the final light of the Sun reached you.
Now, this is true with things appearing in the Universe as well. If a massive spaceship appeared right next to the sun right now, you wouldn’t be able to see it for eight minutes, until the light reached us.
This also means that anything that is more than 13.8 billion light-years away from us, we can’t see yet. There is an ‘observable wall’ that we can’t see passed yet in our Universe (see our post here). This occurs because right after the Big Bang, when all matter came into existence, the light started traveling from distant stars towards us. However it hasn’t all gotten to us yet, there are some really far away galaxies that the light is only midway on its journey to us. That means our observable universe is 13.8 billion light-year sphere with us at the center.
It is not easy for telescopes to see things on this 13.8 billion year boundary. For most of the history of astronomy, we have not been able to see them, however recently, with our new telescopes we have been able to. The Hubble Space Telescope, one of our best, regularly observes things near this boundary. This is actually just incredible to think about, that we can look across space and back into time.
Okay – so what does our observable Universe look like?
Firstly, there are an enormous number of galaxies in the Universe – probably trillions. Every time we point a telescope at some sky, we seem to find more. They come in all sorts of weird and wonderful shapes. On a dark night, you can see a few of them, but they are much much further away than stars, so they just look like stars or dimly glowing dust.
These galaxies don’t cluster together in the Universe, in the way that stars do in a galaxy. Instead, the Universe itself has a remarkably even structure – it’s very uniform, very thin, and very empty – basically a galaxy soup. In fact, the Universe has an average density of 3 atoms per centimeter. And when measured on a large scale, this density is almost perfectly uniform (there is less than a 0.001% variation). This sounds weird because stars, planets, and basically, everything we’ve ever seen in our entire lives are much denser than this. But the vast vast majority of the Universe is instead open space. The tiny variations in density that make up our galaxy, our Sun, and our entire world are essentially rounding errors. Again, our Universe is just a thin galaxy soup.
The only little bit of structure our Universe seems to have at a large scale are ‘galactic filaments’. These are long strands of clusters of galaxies that form a large web across the universe. This is the biggest thing we know. We don’t have any good pictures of these, because they are just too big. But we can visualize them by combining lots of smaller pictures together (see on the left). However, they have to be exaggerated a lot to make the picture look good. Really what we can see of our Universe is a sphere of thin galaxy soup.
