The bounce and many universe theories

[wolram]

In some theories the universe ends/starts with a bounce, is this true with the many universe theory, ie all universes bounce at the same time.
this has had me thinking for some time as ,AFAIK there is nothing to say that alternate universes start and end at the same time.
In other words some universes could bounce before others.

 

[mathman]

Brian Greene's latest book "The Latest Reality" discusses this subject thoroughly.

 

[skydivephil]

I think you need to calrify which many universe theory you are referring too. Eternal inflation is the probably the most relevant version of the mutliverse here. Brian Greene has a very good chapter on eternal inflation in the above mentioned book.
A quick summary is that a peiod of rapid expansion in the early universe is the leading candidate theory to solve a number of comsological problems , read more here:
http://arxiv.org/abs/hep-th/0702178
Howver inflation has been shown likely to produce not one universe but an infinite number of them as once it starts it never stops.
The big bounce has been proposed primarily from loop qaunutm gravity, this paper shows that loop quantum gravity may make inflation a "sure thing"
http://arxiv.org/abs/0912.4093
However the question is is the big bounce consistent with eternal inflation. I have tried to find some literature on this myself and come up a blanck. Maybe the bounce happens at the beginning of eternal inflaiton where a singualrity is often thought to reside. Having found no literature I contacted some loop researchers who agreed the two theories may well be consitent but Id like to see if anyone knows of any published papers showing this and how they might fit together.

 

[DavidMcC]

Wolram, there is another view of bouncing universes.
IMO, it could be an artifact of the way Ashtekar framed his version of loop quantum gravity. I suspected that he had left out the hyperspace continuum (background), though marcus hotly denied it! Another possibility is that he was not allowing for loops to be unlinked to the those of the pre-crunch space. Either way, the extraordinary rate of expansion during inflation looks more like a phase velocity of the front of equal density in the collaping Bose-Einstein condensate of matter produced by the "big crunch" phase. A phase velocity can be as high as you like, depending on the density profile of the collapsing matter.

 

[wolram]

Thanks for the replies, i have a lot to read now.

 

[marcus]

In other words some universes could bounce before others.

That seems right, Woolram. I don't know of any scientific evidence of there being more than one universe. But if there were several, they certainly wouldn't have to bounce at the same time.

Brian Greene's latest book "The Latest Reality" discusses this subject thoroughly.

Howver inflation has been shown likely to produce not one universe but an infinite number of them as once it starts it never stops.

I don't think this has been shown. Eternal inflation, with repeated episodes, is just one type of speculative scenario. You cite a paper by Guth which is specifically about Eternal type, not about all.

The big bounce has been proposed primarily from loop qaunutm gravity, this paper shows that loop quantum gravity may make inflation a "sure thing"
http://arxiv.org/abs/0912.4093

That is a good paper! But it does not show that LQG makes inflation a sure thing. First it does not deal with eternal inflation at all! It deals with a simple no-frills generic kind of inflation, and it ASSUMES what is needed to produce that. The problem there is that the inflation may STOP TOO SOON unless there is some fine-tuning. What the paper shows is that the LQG bounce tends to automatically adjust the parameters so there will be adequate expansion before the field causing inflation decays and the process stops.

So it does not cause Eternal. We are talking about a one-shot episode. And it does not make that oneshot a "sure thing". You have to assume something. But if you do assume the necessary field and potential, the bounce makes it a "sure thing" that the outcome will be sufficient to produce observed conditions that inflation was invented to explain. It won't peter out.

... the way Ashtekar framed his version of loop quantum gravity. I suspected that he had left out the hyperspace continuum (background), though marcus hotly denied ...

I don't "hotly" anything because I can't fathom what you mean by "leaving out the hyperspace continuum (background)".

As far as I can tell, you seem to have your own private idea of what "hyperspace continuum" means, and you use the term repeatedly without defining it.

Here you seem to equate continuum with "background". But the two words mean altogether different things. Ashtekar and co-workers have typically employed a continuum when they set up the LQG model of the world, but they have carefully avoided using a background, because General Relativity itself, as Einstein developed it, was also background-independent. The GR setup does not require a background.

Ashtekar and his crew basically just imitate 1915 General Relativity on that score.

 

[DavidMcC]

I don't "hotly" anything because I can't fathom what you mean by "leaving out the hyperspace continuum (background)".

As far as I can tell, you seem to have your own private idea of what "hyperspace continuum" means, and you use the term repeatedly without defining it.

Here you seem to equate continuum with "background". But the two words mean altogether different things. Ashtekar and co-workers have typically employed a continuum when they set up the LQG model of the world, but they have carefully avoided using a background, because General Relativity itself, as Einstein developed it, was also background-independent. The GR setup does not require a background.

Ashtekar and his crew basically just imitate 1915 General Relativity on that score.

Thanks, marcus. As for "my version" of the "hyperspace continuum", you could say it corresponds to the paper on which the lines and nodes of LQG are drawn. By leaving that out, I suspect that he also leaves out the possibility of more than one set of those lines and nodes, and therefore the multiverse. It follows from that that he could only predict a "bounce" of one universe, and could not distinguish between the "pre-crunch" universe and the "post-crunch" one, IMO. The Smolin formulation of LQG seemed to be more flexible in that respect, although I am not sure who introduced the crucial concept of LINKED loops, which implies the possibility of unlinked ones (ie different universes) as well.

 

[DavidMcC]

... PS, wolram, are you missing an "o" or an "f"?

 

[skydivephil]

That seems right, Woolram. I don't know of any scientific evidence of there being more than one universe. But if there were several, they certainly wouldn't have to bounce at the same time.





I don't think this has been shown. Eternal inflation, with repeated episodes, is just one type of speculative scenario. You cite a paper by Guth which is specifically about Eternal type, not about all.


That is a good paper! But it does not show that LQG makes inflation a sure thing. First it does not deal with eternal inflation at all! It deals with a simple no-frills generic kind of inflation, and it ASSUMES what is needed to produce that. The problem there is that the inflation may STOP TOO SOON unless there is some fine-tuning. What the paper shows is that the LQG bounce tends to automatically adjust the parameters so there will be adequate expansion before the field causing inflation decays and the process stops.

So it does not cause Eternal. We are talking about a one-shot episode. And it does not make that oneshot a "sure thing". You have to assume something. But if you do assume the necessary field and potential, the bounce makes it a "sure thing" that the outcome will be sufficient to produce observed conditions that inflation was invented to explain. It won't peter out.



I don't "hotly" anything because I can't fathom what you mean by "leaving out the hyperspace continuum (background)".

As far as I can tell, you seem to have your own private idea of what "hyperspace continuum" means, and you use the term repeatedly without defining it.

Here you seem to equate continuum with "background". But the two words mean altogether different things. Ashtekar and co-workers have typically employed a continuum when they set up the LQG model of the world, but they have carefully avoided using a background, because General Relativity itself, as Einstein developed it, was also background-independent. The GR setup does not require a background.

Ashtekar and his crew basically just imitate 1915 General Relativity on that score.

The Guth papers main focus is eternal inflation however there is a very easy to read introduction to standard inflation in the frist few pages before any mention of eternal inflation. So I still think that's a good paper to read. According to Guth the vast majority of inflationary models are eternal and this is what i meant by the phrase :
"Howver inflation has been shown likely to produce not one universe but an infinite number of them as once it starts it never stops."
if it was misleading I apologise but I think the paper makes this clear.

I don't believe I claimed that the Ashketar/Sloan paper mentioned eternal inflation and I specially said I couldn't find any papers on the consistency or inconsistency of eternal inflation and the bouce in LQC which i think is at the heart of the question.
Ill reiterate I can't find any papers on this subject. If you know of any Marcus I would love to read them. I did correspond with Martin Bojowald and he said he thought LQC and eternal inflation were consistent but I guess his opinion is just that.

 

[DavidMcC]

Howver inflation has been shown likely to produce not one universe but an infinite number of them as once it starts it never stops."
if it was misleading I apologise but I think the paper makes this clear.

Actually, that was what I suspected. It's the reason I think the Ashtekar version of LQG will be falsified (IMO) - the entire universe isn't going to bounce, only little bits of it (as BHs), and then we don't see the result with telescopes.