Is Asymptotic Silence in Loop Quantum Cosmology the End of Time?

[skydivephil]

Ive been reading about something called asymptotic silence
If I've 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:
http://backreaction.blogspot.co.uk/2014/01/a-moment-of-silence-replaces-big-bang.html
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 don't 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...?

 

[Mordred]

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.

 

[marcus]

... What I don't 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...?

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.
http://arxiv.org/abs/1309.6896
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.
http://arxiv.org/abs/1111.3535
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].
http://arxiv.org/abs/1206.6736
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
http://inspirehep.net/record/1120269?ln=en

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:
http://arxiv.org/find/grp_physics/1/au:+barrau_a/0/1/0/all/0/1

 

[skydivephil]

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?

 

[sardar]

I can provide some insights on the concept of asymptotic silence and its connection to loop quantum cosmology (LQC). Asymptotic silence refers to a state in which there are no time dimensions and the four spatial dimensions become equivalent. This idea was first proposed by Carlo Rovelli and Francesca Vidotto in 2012 in their paper "Asymptotic silence in loop quantum cosmology".

In this state, the universe is described by a quantum state that is independent of time, which means that there is no notion of time passing. This is in contrast to the traditional understanding of the universe, where time is considered to be a fundamental aspect of reality. In LQC, this state is achieved through a quantum bounce, where the universe contracts to a minimum size and then expands again. This bounce replaces the singularity of the Big Bang in traditional cosmological models.

The connection between asymptotic silence and the no-boundary proposal of Stephen Hawking and James Hartle is that both propose a universe without a beginning or end, and in both cases, time is not a fundamental aspect of reality. However, the mechanism for achieving this state is different. In the no-boundary proposal, time is eliminated through a mathematical trick, whereas in LQC, it is a result of the quantum dynamics of the universe.

To address the question of how a prior contracting universe can bounce without a time dimension, it is important to understand that in LQC, time is not completely eliminated, but it loses its fundamental status. The universe is still described by a quantum state that evolves over a "pseudo-time" parameter, which is not the same as the time we experience in our everyday lives. This pseudo-time parameter is responsible for the bounce, as it determines the dynamics of the universe in the asymptotic silence state.

In conclusion, asymptotic silence in LQC is a fascinating concept that challenges our traditional understanding of time and the beginning of the universe. It is a promising avenue for further research and could potentially provide new insights into the fundamental nature of our universe.