Undergrad Would complexity re-emerge if cosmic expansion reversed?

[Swamp Thing]

Imagine that for some reason the current slow expansion of the universe is going to reverse, and the universe is going to collapse back to the pre-inflation state. [Let's make this an assumption without worrying about the mechnism]

So we have a very dilute and cold distribution of energy, since everything has been processed by black holes in the crunch-and-evaporate process. As this universe begins concentrate down again, would we see particles, stars and galaxies forming, and life evolving, and so on? Or would it remain featureless until maybe a "deflation" and a singularity develops?

 

[phinds]

Imagine that for some reason the current slow expansion of the universe ...

I have a hard time getting past that sentence. If you consider nothing more than our Observable Universe, the recession rate of the most distant objects is about three times the speed of light. I REALLY can't accept that that is "slow". Also, the recession is accelerating, which also makes it hard to think of it as slow.

[Let's make this an assumption without worrying about the mechnism

I don't think you CAN ignore the mechanism since even assuming your assumption is possible, the answer to your question might well depend on the mechanism.

 

[Swamp Thing]

My bad.

Would it be okay to say, "assuming that the current acceleration is going to reverse at some stage" ?

Edit: I'm no expert, but I gather that this has to do with the curvature of our spacetime. If the curvature could change, presumably the acceleration would reverse ... ??

 

[phinds]

My bad.

Would it be okay to say, "assuming that the current acceleration is going to reverse at some stage" ?

Sure. There's no reason to believe that it IS, but you can posit that if you like. You STILL need a mechanism in order for your question to be meaningful. Right now, your question amounts to asking "if the laws of physics are not at all as we understand them to be then what do those laws say about <insert ANYTHING>". This is not a meaningful question really since it contains its own self destruction.

 

[Swamp Thing]

That makes sense. I think you're saying indirectly that no known physics can reverse the overall curvature of any universe.

Thanks.

 

[PeterDonis]

Imagine that for some reason the current slow expansion of the universe is going to reverse, and the universe is going to collapse back to the pre-inflation state. [Let's make this an assumption without worrying about the mechnism]

Sorry, you can't just make assumptions and not worry about the mechanism, because if we don't have a theory that allows the scenario you assume as a solution, which I don't think we do (see below), we have no way of saying what will happen in your scenario.

we have a very dilute and cold distribution of energy, since everything has been processed by black holes in the crunch-and-evaporate process.

Since this certainly isn't the case now, I assume you're talking about some time very, very, very far in the future (something like $10^70$ years or more). How would the expansion of the universe reverse at that point?

Would it be okay to say, "assuming that the current acceleration is going to reverse at some stage" ?

No. Reversing the expansion is not the same as reversing the acceleration. The latter is a much less stringent condition than the former.

I'm no expert, but I gather that this has to do with the curvature of our spacetime.

No, it has to do with what kinds of stress-energy are present. Currently the universe is dominated by dark energy, and we expect that to remain the case forever into the future, in which case the expansion will continue to accelerate forever.

For your scenario to work, you would have to find a way to (a) get rid of the dark energy so the expansion can decelerate, and (b) somehow increase the overall density of stress-energy in the universe enough to be above the critical density, so the universe will stop expanding and start recollapsing. I don't know of any way to do either of those things within our current understanding of physics. If you can find a reference that seems to suggest a way, post it. But in the absence of one I don't think we have a well-defined model to discuss.