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On the infinity of the Universe 
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#19
Mar2112, 03:32 PM

P: 93

Erm... not really. What quantum gravitational effects would do anything there? I'd thought quantum gravity was still in the limbo of unfinished theories...



#20
Mar2112, 04:10 PM

P: 526

Here's a video of Alan Guth explaining that a "bubble" universe arising from inflation can still appears to be infinite: http://www.youtube.com/watch?v=rfeJhzPq3jQ 


#21
Mar2112, 04:58 PM

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P: 1,676




#22
Mar2112, 05:09 PM

P: 93

Yes, I did know both that singularities are a breakdown in the description of a particular physical event, and that a final theory should explain that sort of thing, but I'd never really connected the two pieces of knowledge together. That is... great. Thank you :) 


#23
Mar2112, 05:21 PM

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P: 1,676

Cool. In fact, while admittedly not complete, loop quantum cosmology (based on  you guessed it  loop quantum gravity) predicts a bounce at the Planck scale. I'm not an expert (I'm hoping Marcus drops by at some point...he can shed much more light on this), but I think the bounce solution is quite generic in lqc.



#24
Mar2112, 05:37 PM

P: 526




#25
Mar2112, 07:44 PM

P: 93




#26
Mar2112, 08:14 PM

P: 526

You are making a small mistake in confusing a dimension being "small" and a dimension being curled up. In string theory (or more precisely, now MTheory) the other 7 spatial dimensions are curled up into tiny CalabiYau manifolds. When you talk about the length of a dimension, you are talking about how much distance must be covered to traverse the entire dimension. Saying that a dimension is curled up is essentially saying you can return to your starting position after traversing that dimension. So you are correct that when the 3 large dimensions shrink down to a planck length, the other dimensions become noticeable. It is not known if our dimensions are curled up or not, but if the universe if unbounded it would suggest that they are curled up. But if a dimension is curled up, it is curled up  period. Even if it changes size, it still retains it's shape. (Though the string theorist Brian Greene showed in the 90's that CalabiYau shapes can tear and recombine, changing dimensional arrangement. But that's getting off topic.) 


#27
Mar2112, 08:24 PM

P: 93

I'm sorry, I have a... strong enough grasp of physics up until basic Quantum Mechanics, but once we get to the point of discussing MTheory or other potential TOEs, I have no idea what I'm talking about. 


#28
Mar2112, 08:48 PM

P: 526




#29
Mar2112, 09:20 PM

P: 93




#30
Mar2212, 01:38 AM

P: 570

I think that perhaps you are assuming the singularity was finite. It could have been infinite. If the Universe is infinite now then I would think the singularity was infinite as well. But now I too am guilty of guessing. 


#31
Mar2212, 05:23 AM

P: 93

2. Also yes, as I said in my last post, I realised I was committing the Mind Projection Fallacy with singularities, assuming they were a property of the world instead of a property of the theory. My mindset is adjusted now, so that problem was also dissolved. 3. And yeah, when I said that I meant 'The Universe or only the Observable Universe', that is, I was asking whether it was the origin of our whole bubble or exclusively the part we can see of it. 


#32
Mar2212, 09:09 AM

P: 451

A few things I thinks its worth mentioning.
1) Eternal inflation and chaotic inflation are not the same. read here: http://arxiv.org/pdf/grqc/0409055.pdf 2) Inflation has good evidence for it but its not a done deal yet, we still need to see the B mode polarisation for it to passs its final hurdle. Well nothing is ever final, but this is a key test, see here: http://www.nature.com/news/2009/0904...l/458820a.html 3)Borde , Guth and Vilenkin have argued that eternal inflaiton has an initial singualrity, but that has been disputed by Aguirre and Gratton. see here: http://arxiv.org/abs/0712.0571 If anyone knows of any way to observationally resolve this dispute, I'd love to hear it but I suspsect there is not. 4) Guth claims most inflationary models are eternal , but I note he doesnt say all.So if ifnaltion is shown beyond reaosnable doubt, there is stil a chance its not eternal. see here http://arxiv.org/abs/hepth/0702178: 5 LQC resolves singualrities inlcuding the one proposed at the beginning of eternal inflation by Borde Guth and Vilkenkin , see here: http://arxiv.org/abs/0812.4703 


#33
Mar2212, 09:24 AM

P: 93

2) I didn't think it was a done deal, but I kinda like the idea. Plus it kind of appeals to my physical sense. 3 & 5) I'm pretty sure bapowell addressed this with a pretty good point: singularities state the breakdown of a theory, not the breakdown of physics. If a theoretical model possesses a singularity, that's evidence for its incompleteness, so saying that 'Eternal Inflation has an initial singularity' is pretty much the same as saying 'Eternal Inflation is still not a good enough theory.' So even if LQC doesn't turn out to be true, the final theory should have as a property the ability to dissolve singularities. 4) Yes, I understand that, too, although I really like the idea of an Eternal Inflation. Of course, me liking it and it being true are not correspondent, and I haven't studied science for long enough to know whether my scientific hunches are anywhere on spot. I'm just going to follow them where science doesn't know what it's doing. 


#34
Mar2212, 09:27 AM

Sci Advisor
P: 1,676

And James, for a good technical introduction to string theory try Zwiebach. I've not read it myself but it seems highly praised by many in the field. More advanced standards are the texts by Polchinski and Green, Schwarz, and Witten. The latter is a now outdated by has its unique strengths. 


#35
Mar2212, 09:35 AM

P: 93

And to be clearer, I'd thought the Chaotic Model was the only Eternal Inflation Model, I didn't know there were other Eternal Inflation theories. But I didn't really do enough research on the field as of yet so I'm just going with the flow for now until I obtain more knowledge on that and can actually participate more actively in such discussions.



#36
Mar2212, 10:31 AM

P: 451

Thats fascinating, i didnt realise that matter bounce in Horava gravity prodcued B mode polarisation. To be fair thats a pretty new paper. I do note that the author concluded we can still assess the different models via their tilt, so I think its still true that if we can produce the spectrum from the B mode it could be the decider on inflation, but I guess not in the same way that the Nature article implied. As far as disputing the initial singualrity in eternal inflaiton, this is a an exmaple within the eternal inflation community: http://arxiv.org/abs/0712.0571 Of course the loop guys wouold dispute from a very different angle ie a bouncing comsology 


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