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Asymptotic silence and LQC

  1. Jan 30, 2014 #1
    Ive been reading about something called asymptotic silence
    If Ive understood it correctly its state where the 4 dimensions we see today 3 space and 1 time, transform such as there are 4 space dimension and no time dimension. Bee has a post baout this on her blog:
    but the original paper she mentions is from 2012.
    On her blog she says that this realizes the no boundary proposal from HAwking and HArtle.
    However its derived in loop quantum cosmology which predicts a bounce. What I dont understand is that if we lose the time dimension how do we get a prior contracting universe that then bounces? Is the asymptotic silence state only at the moment of transition or...?
  2. jcsd
  3. Jan 30, 2014 #2
    Not sure, I guess its a momentary vector direction zero judging from the opening page lol.

    I'll have to study the paper itself in the link provided to say further, provided I can understand it well enough to come to a conclusion lmao. I'll look at it as I have time.
  4. Jan 30, 2014 #3


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    Good guess! Aurelien Barrau (and co-authors) say this has to be studied more. There might under some conditions be a brief signature change of the effective 4D metric right around and during the bounce.
    A brief change from Lorentzian to Euclidian metric signature i.e. from -+++ to ++++, might or might not happen.
    I trust Barrau more than anyone else currently to put new LQC developments in perspective and understand the big picture.
    Here is an invited 75 page review of the whole field which includes a brief reference to this sig-change issue at bottom of page 23 and top page 24.
    Observational issues in loop quantum cosmology
    A. Barrau, T. Cailleteau, J. Grain, J. Mielczarek
    (last revised 8 Jan 2014)
    ...The early universe is an invaluable laboratory to probe "Planck scale physics". Focusing on Loop Quantum Gravity as one of the best candidate for a non-perturbative and background-independant quantization of gravity, we detail some expected features.
    75 pages, invited topical review for Classical and Quantum Gravity

    The references to sig-change around top of page 24 are to [30] and [31]:
    [30] T. Cailleteau, J. Mielczarek, A. Barrau and J. Grain, Class. Quant. Grav. 29 (2012) 095010.
    [31] T. Cailleteau, A. Barrau, J. Grain and F. Vidotto, Phys. Rev. D 86 (2012) 087301.

    [30], the main reference to this particular topic in the Barrau et al review article, is to the article that Bee cited in her blog. Not the 4 page conference report by Mielczarek, but the longer source article Bee calls "technical" that the report was based on.
    Anomaly-free scalar perturbations with holonomy corrections in loop quantum cosmology
    Thomas Cailleteau, Jakub Mielczarek, Aurelien Barrau, Julien Grain
    19 pages
    This was published in 2012 in CQG and already has 46 citations http://inspirehep.net/record/945926?ln=en
    This [30] is what she refers to when she says "The full length paper is here. It’s very technical, but the main conclu…"

    Here is reference [31] which the review article cited along with [30].
    Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
    Thomas Cailleteau, Aurelien Barrau, Julien Grain, Francesca Vidotto
    (Submitted on 28 Jun 2012)
    Loop Quantum Cosmology yields two kinds of quantum corrections to the effective equations of motion for cosmological perturbations. Here we focus on the holonomy kind and we study the problem of the closure of the resulting algebra of constraints. Up to now, tensor, vector and scalar perturbations were studied independently, leading to different algebras of constraints. The structures of the related algebras were imposed by the requirement of anomaly freedom. In this article we show that the algebra can be modified by a very simple quantum correction, holding for all types of perturbations. This demonstrates the consistency of the theory and shows that lessons from the study of scalar perturbations should be taken into account when studying tensor modes. The Mukhanov-Sasaki equations of motion are similarly modified by a simple term.
    5 pages
    It was published in physical Review D in June 2012 and has 17 citations

    I certainly can not say confidently what is happening with this. Bee's blog is suggestive, but I would not follow all the leads (old classical ideas that resonate). I think in this case if you want to know what is happening at the heart of this, you have to ask Barrau. He is a brilliant young guy who likes motorcycles and lives in Southern France (Grenoble). He has always focused on the OBSERVATIONAL side as well as the theoretical. He has mentored some of the others even though he is not much older than they. Linda Linsefors is a new member of the group, she impresses me as strong also. Others in Barrau's group you see as co-authors in this listing:
    Last edited: Jan 30, 2014
  5. Jan 31, 2014 #4
    Hi thanks for that MArcus, when you say the metric changes form -+++ to ++++, is that referring to replacing 3dimensions of space and 1 dimension of time with 4 dimension of space and no dimensions of time. If so how do you project the universe before the bounce if there is no time?, does time re enter the metric somehow? if so how? Wouldnt everything be static and unchanging if there is no time dimension? It looks like the Hawking Hartle model to me, where am I going wrong?
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