Why do stars group into galaxies?

We received this question from Lloyd via the forum, Chris Smith put it to Astrophysicist Matt Bothwell...

马特:这是一个很大的问题:为什么universe look the way it does? It's also a really good question - Why do stars form these structures we call galaxies rather than just being a uniform sea of stars filling the universe? The very very short answer is because gravity made it that way. If we go back to the very very early universe, so before stars and before galaxies formed, the universe actually was very very uniform. But it wasn't completely uniform. There were little differences in the distribution of matter just because of random chance. So, this little bit of the universe over here might have a bit more matter than average and this part of the universe over there might have a bit less matter than average. And the action of gravity kind of magnifies or amplifies those differences. So the bit of the universe that has a bit more matter than average will have a bit of a stronger gravitational pull because of all of the matter and so it will kind of gobble up more matter than the less dense regions. And so there's a bit of a runaway positive feedback thing where the more dense regions grow and grow and grow and all the other regions get emptier and emptier and emptier. So eventually you fast forward and the universe ends up looking quite blobby, with these very very dense regions which eventually turn into galaxies separated by big empty spaces.

Chris: And that's where we are today.

Matt: Exactly yes. That's where we are today. So once stars start forming, these blobby regions eventually turn into galaxies which is why the universe looks the way it does.

Chris:Now that's a pretty basic question so let's ask you something a little bit harder then.
What's the ultimate fate of the universe then, because it is growing all the time isn't it?
It's getting bigger. As far as we know, the older it gets the faster it's growing as well. So what's the ultimate fate? Does it just get bigger and bigger and bigger ad infinitum.

Matt:That's a very good question that I don’t think we totally know the answer to. It all depends on what dark energy is doing and we don’t know very much about dark energy at all. It was only discovered in the late 1990s. We’ve known for about 100 years now that the universe is getting bigger and bigger and bigger - expanding after the Big Bang. But when astronomers in the late 90s went to measure the rate of this expansion, it actually turns out, against all expectations, that the expansion is getting faster and faster and faster - that there’s some mysterious stuff in the universe that seems to be pushing it apart at the seams. We’ve called this ‘Dark Energy’ that’s the name for the effect. We don’t really understand what it is. And so it could be that in the very far distant future, dark energy gets stronger and stronger and stronger and eventually just tears the universe apart at the seams. But like I said we've only known about this effect for about 20 years and I think we have a lot more to learn before we can say any real answer.

Chris: So if the universe is inflating in this way, does this mean then that given enough time, when you get your telescope out, it could just be a very boring thing that you see? Because everything could have got so far away and is moving apart from us so fast, that it's going faster than light can travel to us, so you just see black space?

Matt: That could be true. So there's this concept in cosmology called the cosmic horizon. So, very much like we have a horizon on Earth, it's part of the earth that we can’t see because the surface kind of curves away from us, there are regions of the universe that we completely cannot see because the light hasn't had time to reach us. So any light that needs more than the current age of the universe to reach us hasn’t had much chance to reach us, so we can't see it. And as the universe kind of accelerates and accelerates and dark energy gets stronger and stronger, the cosmic horizon is going to actually get closer and closer to us. More things are going to fall over the cosmic horizon. So as time goes by, we're actually going to be able to see less and less of the universe.