# The Big Crunch

## Main Question or Discussion Point

I guess this is a relatively simple question, but depending on the answer can have complex
follow-up questions. So here goes with the first: is there a center of mass to the universe? Or is this not possible?

P.S. Is this site being weird for anyone? It only works on my phone's web browser as of about 3 hours ago...

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mathman
Based on current theory, the answer is probably not. I don't know it applies here, but an analogy often used to derscribe the universe is that it is like the surface of a balloon (in three dimensions, not two). Using the analogy, note that the surface of a balloon has no center.

Okay, now for a follow up. Wouldn't the universe have to have a center of mass to eventually pull everything in the universe back to said center of mass to create the 'Big Crunch' or whatever else it may be called?

ideasrule
Homework Helper
Okay, now for a follow up. Wouldn't the universe have to have a center of mass to eventually pull everything in the universe back to said center of mass to create the 'Big Crunch' or whatever else it may be called?
No. Going back to the balloon analogy, the balloon may deflate and eventually shrink to zero size, but to the people on the surface there wouldn't be any "center" that the balloon is shrinking towards. Every point on the surface would steadily get closer to every other point, and this applies to the entire surface.

Ah, I see! One last one now: What would cause the theoretical 'balloon' to stop expanding and start shrinking?

Cyosis
Homework Helper
Gravity. Just think of it as throwing a rock up on the surface of the earth. Initially it is going up (expanding) but gravity is slowing down its speed until it goes to zero, then reverses its direction and it starts falling (shrinking). Note that it looks like there is another 'force' at work which actually accelerates the expansion so gravity may never be strong enough to actually reverse the expansion.

ideasrule
Homework Helper
Gravity. Just think of it as throwing a rock up on the surface of the earth. Initially it is going up (expanding) but gravity is slowing down its speed until it goes to zero, then reverses its direction and it starts falling (shrinking). Note that it looks like there is another 'force' at work which actually accelerates the expansion so gravity may never be strong enough to actually reverse the expansion.
Yes, and this gravity mostly comes from dark matter, which comprises 90% of the matter in the universe. Some of it also comes from the energy in radiation.

George Jones
Staff Emeritus