In LQG, where do things come from to make the Big Bounce?

[MTd2]

Is it from a Black Hole? Is it from a Cyclic Universe?

 

[marcus]

Is it from a Black Hole? Is it from a Cyclic Universe?

Many cases have been studied, both by computer (numerically) and by solvable equations (analytically). Some cases studied are not even isotropic---there is a lot of variety. Sometimes additional inflation is included, besides what naturally occurs in the bounce.

Some cases studied are cyclic, going through repeated cycles of collapse bounce expansion collapse...

But not all cases look like that. Sometimes there is a single bounce----an infinitely long contraction+bounce+infinitely long expansion.

Black hole bounce has also been studied in a few papers---results have not been as consistent (as those of cosmological LQG bounce.)

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One does not try to answer "where does it come from?" at this stage of the game. One sets up a LQG model like our universe and runs time backwards to see if the singularity is avoided and typically you go back in time past where the singularity used to happen, and find a space similar to ours in crude outlines. As you go back in time this space expands. So the model finds that there was a contracting classical space broadly similar to ours. Nobody tries to say "where it came from." It is just an extension back in time of our own spacetime region, which we normally take for granted.
Because there fails to be a boundary (no singularity) so it has to continue extending on back. That is simply what spacetime does, if there is no singularity in the way to stop it.

 

[tom.stoer]

The basic result is that there is a kind of mirror-symmetry between t and -t of the wave function near the big bounce.

The dynamics for late t (or early -t) is not so clear to me, especially as there is a rather generic indication for slow-roll inflation (which I have never seen in the contraction phase).

 

[physics_931]

The basic result is that there is a kind of mirror-symmetry between t and -t of the wave function near the big bounce.

The dynamics for late t (or early -t) is not so clear to me, especially as there is a rather generic indication for slow-roll inflation (which I have never seen in the contraction phase).

I heard a talk recently by Param Singh where he showed that bounce occurs even in presence of inflation. I don't remember the details though. From the figure he showed, I don't there is a mirror symmetry.

 

[MTd2]

Another question. Is there a big bounce in case of a big rip?

 

[physics_931]

Yes. But then it is not a bounce. It will be a recollapse. Again referring to the same talk of Param Singh, I think he said that he has shown that all such singularities are removed in loop cosmology. Looks like a very solid result if it is correct.

 

[tom.stoer]

yes, ther results are rather robust w.r.t. modifications of the theory, method, approximations and perturbations; it's a general feature of LQC-like models; it's origin is the new quantization method

 

[MTd2]

I never saw a paper or talk about a bounce on the big rip. Do you people have anything about that?

 

[tom.stoer]

I never saw a paper or talk about a bounce on the big rip. Do you people have anything about that?

good point; I am not aware of such a result, either

 

[physics_931]

I never saw a paper or talk about a bounce on the big rip. Do you people have anything about that?

here are the papers by Singh and his collaborators where he showed that big rips and all future singularities are avoided:

Title: Are loop quantum cosmos never singular? arXiv:0901.2750

Title: Avoidance of future singularities in loop quantum cosmology arXiv:0605113

Abstract: We consider the fate of future singularities in the effective dynamics of loop quantum cosmology. Non-perturbative quantum geometric effects which lead to $\rho^2$ modification of the Friedmann equation at high energies result in generic resolution of singularities whenever energy density $\rho$ diverges at future singularities of Friedmann dynamics. Such quantum effects lead to the avoidance of a Big Rip, which is followed by a recollapsing universe stable against perturbations. Resolution of sudden singularity, the case when pressure diverges but energy density approaches a finite value depends on the ratio of the latter to a critical energy density of the order of Planck. If the value of this ratio is greater than unity, the universe escapes the sudden future singularity and becomes oscillatory.

 

[physics_931]

good point; I am not aware of such a result, either

Singh's plenary talk in Loops meeting in Beijing last year had all the results from his works. A Chinese student made a video and showed me. May be we should get that talk online somehow.