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In reply to the discussion: The universe is much bigger than big [View all]cthulu2016
(10,960 posts)Back when we thought the size of the universe was strictly limited by the speed of light it was a straightforward physics question whether everything flying apart would run out of steam, slow down to a stop and then start collapsing back on itself.
If there was enough mass in the universe then we would eventually collapse back. That is called a closed universe. (And some even figured this collapse to a single point would start another explosion, and that the universe was thus cyclical.)
But with less total mass everything is flying apart fast enough to reach a kind of escape velocity from itself. That's a open universe. The expansion would continue forever, but with less and less energy. Literally colder, as you say.
(Either way, there's no life at the end of the story.)
We thought there wasn't enough mass in stars to "close" the universe, but we are recognizing how much matter there is outside of stars. Lots and lots of dust. Since that stuff doesn't emit light we can't see it, and it is called dark matter.
One way we know there is a lot of it is that the night sky isn't solid white. In between each two stars we see there are a gazillion other stars, some close, some far, in that part of the sky. So why isn't the sky 100% stars? We used to think it was because the universe was small... only so many stars in it. Now we think it is because there is a lot of dust that blocks light. Space is not as transparent as we assumed.