Is the universe still expanding? If so, how much can it expand and will it eventually implode?
No, no, no. Before the dark energy was discovered there were three possibilities: that it would implode, that it would continue to expand or that it would expand at an ever decreasing rate. Once the dark energy was discovered and discovered to be positive it was shown that it will continue to expand.
The equations as we know them now do not seem to permit the possibility that it will implode. As it expands the dark energy won’t dilute. It’s vacuum energy. In every bit of volume there is the same amount, but the electrons, protons or neutrons will spread out, so what we can look forward to is a universe that is completely empty.
If the universe is expanding there must be something into which it’s expanding?
Ah-ha! No. You are a victim of your own neural architecture which doesn’t permit you to imagine anything outside of three dimensions. Even two dimensions. People know they can’t visualise four or five dimensions, but they think they can close their eyes and see two dimensions. But they can’t. When you close your eyes and try to see two dimensions you’ll always see a surface embedded in three dimensions.
Is there something special about three dimensions? No. There is something special about your neural architecture. You evolved in a world where everything inside your brain is hooked up and geared to be able to see three dimensions and nothing else.
So the universe isn’t something that can be imagined by a human brain?
That’s right. That’s why we’re stuck using abstract mathematics for the simple reason that our visualisation abilities evolved in a certain environment that just wasn’t appropriate for understanding quantum mechanics and general relativity, so we have to get our intuitions from abstract mathematics.
So the universe doesn’t have an outside?
It doesn’t have an outside or an inside. It just has the rubber surface. You have to learn to think of the surface of the balloon as being all there is. It’s all there is.
from The Economist