Before the big bang

  • Thread starter PhilKravitz
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PhilKravitz

Main Question or Discussion Point

Is there any possibility of coming up with physical measurements that will ever tell us any thing about the universe before the big bang? I can not think of any.
 

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  • #2
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a) We don't know if there was even a Universe before the big bang.

b) Even if there was, I very much doubt any information would "survive" the implosion of all matter, followed by the big bang. And even if it did, inflation would attenuate the "signal" to such an extent it would be undetectable.
 
  • #3
marcus
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There are models which go back before bang, and there are ways of testing at least some, by checking what they predict against finer detail in the ancient light (CMB) as it is observed by improved instruments.

Look up recent papers by Aurelien Barrau, and his coauthors, and also a recent paper by Wen Zhao and his coauthors. They say what to look for in order to disprove, or to confirm, bounce cosmologies. One proposed NASA mission with sufficient capability (out of several) is called "B-pol". As far as I know it has not been funded, and may not be for some time because of budget constraints.

If you need help finding the relevant papers, please ask. This has been discussed in some other threads here at PF. Searching with terms like "Barrau" or "B-pol" might find them.
 
  • #4
PhilKravitz
Marcus thanks for the reference. Here is one of the papers by Barrau

Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters.

Julien Grain, A. Barrau, T. Cailleteau, J. Mielczarek.

Nov 2010
12 pp.

Phys.Rev. D82 (2010) 123520
e-Print: arXiv:1011.1811 [astro-ph.CO]
 
  • #5
marcus
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Excellent find!
In case anyone is interested in taking a look, the abstract is
http://arxiv.org/abs/1011.1811
and has a link to the PDF which is free online.
 
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MArcus, do you know if the inflation generated by LQG bounce is consitent with the eternal inflation?
 
  • #7
marcus
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MArcus, do you know if the inflation generated by LQG bounce is consistent with the eternal inflation?
I don't know the answer to that, Phil. Only a handful of papers have been written so far that study how LQG bounce relates to inflation. I will list some recent ones---because even if you just look at the summary at the beginning, and the conclusions at the end (skipping the technical detail) you can get some idea of where things stand and what they are saying.

It is a research area that has not been developed much so far. AFAICS nobody is claiming, at this point, that the LQG model by itself (with no additional "inflaton" or scalar field) provides the amount of inflation considered adequate to explain what we see.

You've heard that common estimate that "60 e-folds" would be required. Expansion by a factor of e60. The LQG bounce contributes some, but does not by itself provide the full amount.

What is more interesting, I think, is a subtle connection brought out in a few recent papers which argue that the bounce starts things off in a way which makes a satisfactory 60 e-fold inflation more likely without fine tuning.

This is argued in the context of the most simple inflation mechanisms. I'll get a couple of references in case you or anyone is interested in checking out what the researchers are saying.
============================

To give a direct answer about possible (in)consistency, I haven't seen any research where they connect LQG with eternal inflation, but notice that LQG cosmology always involves a bounce----so there is a prior contraction phase.

Maybe I should ask you? In the eternal inflation picture does one expect regions that expand and then collapse?

Because, correctly or not, the LQG model indicates we have a contracting phase in our past history. When you run the model back in time, it reaches a maximum density (where quantum gravity is repellent) and then re-expands as you go back further in time. So the model tells you there was this collapsing phase before current expansion started.

Does this fit in with the eternal inflation picture at all? This is an area of ignorance on my part.
 
  • #8
marcus
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Some recent papers, in case anyone wants to check them out:
http://arxiv.org/abs/0912.4093
Loop quantum cosmology and slow roll inflation
Abhay Ashtekar, David Sloan
(Submitted on 21 Dec 2009, last revised 2 Oct 2010)
"In loop quantum cosmology (LQC) the big bang is replaced by a quantum bounce which is followed by a robust phase of super-inflation. Rather than growing unboundedly in the past, the Hubble parameter vanishes at the bounce and attains a finite universal maximum at the end of super-inflation. These novel features lead to an unforeseen implication: in presence of suitable potentials all LQC dynamical trajectories are funneled to conditions which virtually guarantee slow roll inflation with more than 68 e-foldings, without any input from the pre-big bang regime. This is in striking contrast to certain results in general relativity, where it is argued that the a priori probability of obtaining a slow roll with 68 or more e-foldings is suppressed by a factor e-204."

http://arxiv.org/abs/1011.5516
Inflation and Loop Quantum Cosmology
Aurelien Barrau
5 pages, Proceedings of the 35th International Conference on High Energy Physics, Paris, 2010 (ICHEP 2010)
(Submitted on 24 Nov 2010)
"On the one hand, inflation is an extremely convincing scenario: it solves most cosmological paradoxes and generates fluctuations that became the seeds for the growth of structures. It, however, suffers from a 'naturalness' problem: generating initial conditions for inflation is far from easy. On the other hand, loop quantum cosmology is very successful: it solves the Big Bang singularity through a non-perturbative and background-independent quantization of general relativity. It, however, suffers from a key drawback: it is extremely difficult to test. Recent results can let us hope that inflation and LQC could mutually cure those pathologies: LQC seems to naturally generate inflation and inflation could allow us to test LQC."
 
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  • #9
bapowell
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Marcus, with regards to eternal inflation possibly involving a bounce, see:

http://arxiv.org/abs/gr-qc/9312022
http://arxiv.org/abs/gr-qc/0110012

These works illustrate that while inflation can be eternal to the future, it can't be past eternal and an initial singularity can't be avoided. I suppose putting a bounce in place of the initial singularity would be kosher, but I don't know if it would disrupt any of the assumptions about stress energy the authors are making.
 
  • #10
marcus
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Thanks! that does not completely fill in the gap for me, but it helps.
 
  • #11
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there's a review article on eternal inflation here:
http://arxiv.org/abs/0712.0571
But it was written in 2007 I think it was too soon after the Bounce proposal (2006?) for that to be discussed. Id love it if Agguire or Guth or someone would do a paper discussing this.
As I understand it, if GBV are right then there's a singularity at the begining of eternal inflation. But eternal inflation invovles many pocket universes budding off and so out big bang is not unique. Maybe then the bounce happened at the begining of eternal inflation. I met Steven Feeney at the bar at UCl of this paper fame:
http://arxiv.org/abs/1012.3667
But he said he wasnt knowledgable on LGQ.
Aguire has disputed the GBV theorom though.
One interesting point is that Bojowald in his book "Once Before Time" discusses the GBV theorm and says its been used to argue for a begining of time, but it needs an expanding universe, if LQG is right then the universe is not always exapnding but contracted in the past. Hes clear the bounce scenario not only doesnt contradict inflation but actually makes it more likely. What it means for eternal inflation to me wasnt clear. I think this may be importnat if Feeney finding's are confirmed by Planck.
 
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