Is the Big Collapse a Black Hole?

[haael]

In the "closed" Friedmann model, there is a Big Bang in the past and Big Collapse in the future. Big Bang singularity is something different than a Black Hole singularity.

Now my question: is the Big Collapse singularity mathematically equivalent to a Black Hole (localized) singularity, or is it rather a non-localized singularity like a Big Bang one? Or maybe something else?

My another question: in the "closed" collapsing Friedmann model the Universe becomes smaller at some point. From QM we know that we can not stuff infinite information in finite region. Would it mean that time would need to move, uhm, backwards near the end of the collapsing Universe? The information will need to disappear as different states would need to evolve into the same state, since there's no room to hold their information.

My real question is: is the collapsing Friedmann model symmetric with regards to time reversal? Does the Big Collapse look just like the Big Bang played backwards?

 

[PeterDonis]

is the Big Collapse singularity mathematically equivalent to a Black Hole (localized) singularity, or is it rather a non-localized singularity like a Big Bang one?

It's the time reverse of the Big Bang; it is not a black hole.

in the "closed" collapsing Friedmann model the Universe becomes smaller at some point. From QM we know that we can not stuff infinite information in finite region.

The closed collapsing FRW model is a classical model, not a quantum model. In a classical model you *can* "stuff infinite information in a finite region".

I'm not sure how much work has been done in trying to construct a quantum model of a closed collapsing universe.

is the collapsing Friedmann model symmetric with regards to time reversal? Does the Big Collapse look just like the Big Bang played backwards?

Yes, in the sense given in my first response above.

 

[bapowell]

In what sense do you think black hole singularities are "local"? Like the big bang singularity, the Schwarzshchild black hole singularity is a spacelike surface. The difference between these two is simply that the big bang singularity occurred in the past (past-spacelike) while the black hole singularity occurs to the future of worldlines moving across the event horizon (future-spacelike). In these terms, the big crunch singularity is future-spacelike.

 

[PeterDonis]

the black hole singularity occurs to the future of worldlines moving across the event horizon (future-spacelike). In these terms, the big crunch singularity is future-spacelike.

Yes, but unlike the black hole singularity, the big crunch singularity is in the future of *all* worldlines in the spacetime, not just the ones that pass inside the event horizon. I assumed that that was what the OP meant by the black hole being "local".

 

[bapowell]

OK. My concern is that the OP was considering the black hole singularity to be localized in space.

 

[PAllen]

OK. My concern is that the OP was considering the black hole singularity to be localized in space.

Well, it's worth noting the Kerr singularity is timelike rather than spacelike, thus localized, and can be bypassed. It is, of course, unknown whether a more realistic (not perfectly axisymmetric) collapse in classical GR produces a timelike or spacelike singularity (so far as I know, this is one of the remaining open questions in GR).