What would happen if the universe stopped expanding?
Hi sozme! In the case of a hyperbolic of flat universe, the universe never stops expanding in finite cosmic time. In the spherical case, the expansion does stop in finite cosmic time and results in the "big crunch".
Seems like a fine question...
The old idea was that space did not expand, then the idea was that it did. There was the idea that the expansion had a beginning, and also the idea that the expansion was cyclic and alternated through epochs of expansion and collapse, and another idea that the expansion was continuous and never even had a beginning, and another idea that maybe in the early beginning that some parts of the universe expanded earlier than others, and then there was an idea that early in the beginning there was a short but huge expansion before the slower later and present expansion. Lately, the present expansion that was thought to be certainly not accelerating is now thought to be accelerating.
So the dynamics of space expansion has really been all over the map.
I think what phinds and dedishrfu are suggesting, rightly, is that there are some theoretical problems with posing this question.
For example, if you are imagining that the expansion stopped suddenly, everywhere at once, then maybe a problem arises about relativity of simultaneity... and probably extends to scenarios where the stopping of the expansion happens gradually by slowing down "everywhere" at the same rate.
On the other hand, if the expansion of the universe is presently uniform everywhere, and apparently accelerating uniformly everywhere, I'm not sure how relativity of simultaneity handles that either. Maybe phinds and dedishrfu could touch on that.
Now if you imagine that the expansion changes at different rates in different parts of the universe, I suspect that would have some effects on the geometry of space-time, maybe some peculiar ones.
Maybe someone could verify, but I believe current thinking is that co-movement of mass along with and at rest with respect to the local expansion of space is not subject to inertial effects of that movement.
Very good replies. It's still over my head, but I asked it just because I wanted to understand a little bit more about the Big Crunch theory - and wondered if expansion had anything to do with it.
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