(Intro)

I'm sure you have found yourself looking up into the night sky and wondering just how big the universe is. How could you not? It's just so vast, and we're just so tiny.

It's almost impossible not to ponder just how vast it really is, how far it reaches, and I wish I could tell you exactly how big the universe is, but we don't really know.

I mean not only do we not know, but the most logical guess you might make, based on all the latest highest tech observations that we've made turns out to be not even close.

And the unknowability of the size of the universe comes from the fact that light takes time to get to us. The sun for example is so far away from earth that it takes light about eight minutes to get here. 

So when you look at the sun, which you should never actually do by the way, you're seeing it the way it looked eight minutes ago when the light that is now hitting your eyes left its surface. 

Then there's Proxima Centauri, our nearest neighbor star. Light from there takes 4.2 years to get here which is why we say it is 4.2 light-years away. So we're always seeing it how it looked 4.2 years ago, not what it looks like now. 

So basically the farther away you look into space, the farther back in tome you're seeing, which is really cool when you think about it.

Now the first light in the universe that we'd be able to see started traveling about 13.8 billion years ago, that's around 300,000 years after the Big Bang. Before that the universe was so dense that is was basically opaque. There was light, but it couldn't get very far. 

So we can only see as far as that first visible light started traveling from. This is the edge of what's called the observable universe. But here's the thing: the universe is almost certainly bigger than the observable universe.

Cosmologists think that there's a lot more out there beyond that edge, and if you could somehow see the rest of the universe it would look pretty much the same as the observable one, with the same kinds of stars and planets and galaxies. It's just so far away that it's light hasn't had a chance to reach us yet, even though it's been traveling for billions of years. 

So we know that there is more of the universe, but again nobody knows how much more. To get a little closer to understanding what's all out there let's start we do know: how big is the observable universe at least? 

From everything we were just talking about you would think that the observable universe would be basically a sphere that extends 13.8 billion light-years from earth because the universe is about 13.8 billion years old so that's how long the first light would have taken to reach us.

By this reasoning from end to end the sphere would be twice that size, 27.6 billion light-years wide. But it's not, because light might have left the edge of the observable universe 13.8 billion years ago but by now, the spot where it came from is actually much farther away.

Okay, so how did that happen? It's because of another endearingly fascinating trait of our universe: it's expanding, and fast. Everything in space is flying away from everything else, because space itself is expanding. 

It's kind of like dots on a balloon. The farther away the dots are from each other, the more the space between them expands as the balloon inflates.

So we might be seeing light that's been traveling for 13 billion years but during that time its source has moved much farther away from us. Astronomers can figure out how far because light that's traveling toward us through expanding space will be shifted toward the redder end of the spectrum. 

And that is how we know that even though light left the edge of the observable universe 13.8 billion years ago, that spot is now 46 billion light-years from us, which means that the observable universe is twice as big across, 92 billion light-years wide.

So, all of this has left us able to see only a tiny part of the universe with no real sense of how far it goes, which is why you will hear all kinds of mind-blowing ideas about what the universe might be like; maybe it's infinite, maybe it isn't, maybe there are actually lots of universes and we're just living in one version of the infinite variations of history. 

We simply don't know what's out there, and we probably never will because most of the light out there beyond the edge of observability will never reach us. The universe is just expanding too quickly. Past a certain point, everything is moving away from us faster than the speed of light.

And for us light equals information; without it, we are just left in the dark.

Thank you for watching this episode of SciShow Space and thank you especially to our patrons on Patreon who help make this show possible. If you want to help us keep making episodes like this you can go to patreon.com/scishow to learn more, and don't forget to go to youtube.com/scishowspace and subscribe.