We don’t know what the universe is expanding into, so the universe will probably never stop expanding
The universe will only ever stop expanding if there is so much mass and gravity that stops the edges from getting bigger. But this won’t happen, because the universe is expanding quicker than planets are being made. There are 25 billion galaxies the size of the Milky Way (that’s massive) and scientists have shown us that the universe is traveling and expanding so fast, we can’t even see it, its using a different light form we have never sen before. So if we have this many planets already and the universe is already expanding, then it will never stop expanding…
But if it did? No one knows. Hopefully it will just stop and we will all live happily, but something tells me the universe won’t rest happily :S
Maybe it would rip holes through time and space and start expanding through time – this is what space scientists think of all the time 😀
As I understand it, the universe expands because the energy that blew it into existence in the Big Bang is still spreading matter outwards. When the energy dissipates (spreads itself so thin it has no effect on the matter at the edge of the universe) then the expansion will stop.
There are a lot of theories as to what will happen next but I like this one:
When this happens, there’s a theory that the universe will then start contracting and being drawn inwards. This might be because there is an incredibly big black hole at the centre of the universe where the big bang took place. If that’s the case then the universe might start being pulled back to it’s origin. When, after billions of years, the matter in the universe is all pulled back to this impossibly dense and small space, it’ll explode outwards again in another Big Bang. This idea suggests that the universe is on some kind of infinite loop, which I think is kind of awesome!
There are different ideas of what could happen… but what most scientist think is that it will eventually start going backwards and shrink instead of expand… it will then become smaller and smaller and explode again in something like another Big Bang!