A New Vacuum for LQG (Dittrich Geiller)

[marcus]

IMHO as amateur QG watcher, this paper has considerable potential importance. Really impressed. Could be a game-changer:

http://arxiv.org/abs/1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller
(Submitted on 24 Jan 2014)
We construct a new vacuum for loop quantum gravity, which is dual to the Ashtekar-Lewandowski vacuum. Because it is based on BF theory, this new vacuum is physical for (2+1)-dimensional gravity, and much closer to the spirit of spin foam quantization in general. To construct this new vacuum and the associated representation of quantum observables, we introduce a modified holonomy-flux algebra which is cylindrically consistent with respect to the notion of refinement by time evolution suggested in [1]. This supports the proposal for a construction of a physical vacuum made in [1,2], also for (3+1)-dimensional gravity. We expect that the vacuum introduced here will facilitate the extraction of large scale physics and cosmological predictions from loop quantum gravity.
10 pages, 5 figures

So far they've only worked this out for 2+1 dimensional gravity. Their reference [18 ] is to followup paper expected to do it for 3+1.
"[18] B. Dittrich and M. Geiller, to appear."

 

[marcus]

It turns out Bianca Dittrich will be discussing her "new LQG vacuum" in an online seminar a little over a week from now. Tuesday 4 Feb at the ILQGS. Check this link for availability of the audio and the slides PDF files: http://relativity.phys.lsu.edu/ilqgs/

When posted the slides PDF link will probably be
http://relativity.phys.lsu.edu/ilqgs/dittrich020414.pdf
and the audio link will probably be
http://relativity.phys.lsu.edu/ilqgs/dittrich020414.wav

For those interested in the other online seminar talks:
ILQGS SEMINAR SCHEDULE for SPRING 2014

1/21  [B]Matter matters in asymptotically safe quantum gravity[/B]  P. Dona (Sissa)
2/4   [B]A new vacuum for loop quantum gravity[/B]      B. Dittrich (Perimeter)
2/18 [B]Quantum group spin nets:refinement & relation to spin foam[/B] S. Steinhaus (Perimeter)
3/4  [B]Simplicity constraints in spin foam models[/B]  M. Geiller, S. Speziale (PSU,Marseille)
3/18 [B]S-matrices for General Boundary QFT on Anti de Sitter spacetime[/B]  M. Dohse (Morelia)
4/1  [B]Continuum limit and renormalization[/B]  	 L. Freidel (Perimeter)
4/15 [B] Linking canonical and covariant LQG[/B]  	 A. Zipfel (Erlangen)
4/29  [B]LQC and the very early universe[/B] I.Agullo, A.Barrau, G.Mena (LSU, Grenoble, Madrid)
5/13  [B]Hawking radiation in LQG[/B]	 	 	 J. Pullin (LSU)

 

[marcus]

I see that this "new LQG vacuum" paper is BASED on a 4th quarter 2013 paper by Dittrich and Steinhaus which develops an interesting connection between time-evolution of triangulations and their refinement.
http://arxiv.org/abs/1311.7565
Time evolution as refining, coarse graining and entangling
Bianca Dittrich, Sebastian Steinhaus
(Submitted on 29 Nov 2013)
We argue that refining, coarse graining and entangling operators can be obtained from time evolution operators. This applies in particular to geometric theories, such as spin foams. We point out that this provides a construction principle for the physical vacuum in quantum gravity theories and more generally allows to construct a (cylindrically) consistent continuum limit of the theory.
33 pages, 9 figures

It is reference [1] of the "new vacuum" presentation. So this Dittrich Steinhaus paper seems likely to be important. BTW it was on the 4th quarter MIP poll and embarrassingly only got, so far, one vote! Here's the poll, you can see how the papers stack up so far:
https://www.physicsforums.com/showthread.php?t=730750

Regretably, I didn't vote for it To date, the only vote was that of Nonlinearity, who seems (also from his other choices) to have significant insight into current QG research. Hat tip to Nonlinearity

Dittrich's "new vacuum" initiative seems to be moving away from using a bare-bones GRAPH (a spin network) to represent states of spatial geometry and towards using a richer structure, a triangulation.

I see that Sebastian Steinhaus is giving the next ILQGS seminar talk (on 18 February) after the talk by Bianca Dittrich that is coming up in a week or so. It might be relevant.

 

[marcus]

It's interesting that Dittrich and two of her recent co-authors, Marc Geiller and Sebastian Steinhaus, are scheduled to give 3 out of a total of 8 talks planned for the ILQGS this semester.http://relativity.phys.lsu.edu/ilqgs/
Geiller is giving the third talk in collaboration with Simone Speziale.
Three out of eight is a big chunck of the seminar schedule to devote largely to one group. I suspect that the three talks are related and note that Jorge Pullin, who runs the seminar, has scheduled them consecutively.
ILQGS SEMINAR SCHEDULE for SPRING 2014

...
2/4   [B]A new vacuum for loop quantum gravity[/B]      B. Dittrich (Perimeter)
2/18 [B]Quantum group spin nets:refinement & relation to spin foam[/B] S. Steinhaus (Perimeter)
3/4  [B]Simplicity constraints in spin foam models[/B]  M. Geiller, S. Speziale (PSU,Marseille)

Some of us may want to take a look ahead of time at the papers on which the talks are likely to be based, making the talks easier to learn from when they are given. So I'll list some links.

One is relevant to Dittrich's 4 February talk:
http://arxiv.org/abs/1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller

Two relate to the Steinhaus talk:
http://arxiv.org/abs/1311.7565
Time evolution as refining, coarse graining and entangling
Bianca Dittrich, Sebastian Steinhaus

http://arxiv.org/abs/arXiv:1312.0905
Quantum group spin nets: refinement limit and relation to spin foams
Bianca Dittrich, Mercedes Martin-Benito, Sebastian Steinhaus

A Geiler Speziale paper their talk on simplicity constraints could be based on has not yet appeared, but could over the course of the next month. In the meanwhile here are links to work on simplicity constraints they've done separately:
http://arxiv.org/abs/1112.1965 [pdf, other]
Testing the imposition of the Spin Foam Simplicity Constraints
Marc Geiller, Karim Noui

http://arxiv.org/abs/1107.5274
Holomorphic Lorentzian Simplicity Constraints
Maité Dupuis, Laurent Freidel, Etera R. Livine, Simone Speziale

When Dittrich's talk is posted the slides PDF link will probably be
http://relativity.phys.lsu.edu/ilqgs/dittrich020414.pdf
and the audio link will probably be
http://relativity.phys.lsu.edu/ilqgs/dittrich020414.wav
==========
ADDITIONAL note that just came up. Geiller gave a talk at Perimeter on LQG with the original self-dual (Ashtekar) variables. The video was just posted at PIRSA:
http://pirsa.org/displayFlash.php?id=14010098

 

[marcus]

Recently, on 30 January, Dittrich has given a Perimeter colloquium talk to a large audience
http://pirsa.org/14010112/
about her (and her co-workers') new way of constructing the QG vacuum.

The first 10 minutes or so is general QG overview. The focused discussion starts at around minute 14. A colloquium talk is aimed at a wider audience, not just for specialists the way a seminar talk would be. So it is supposed to be more accessible.
She draws a contrast to the standard Ashtekar-Lewandowski construction, and she makes comparisons with condensed matter. The talk is fairly short, under 40 minutes. The PI director Neil Turok asked a series of questions at the end.

There will probably be some overlap with the online ILQGS talk Dittrich will give on Tuesday 4 Feb, a few days from now. Probably some of the same slides, but likely also more technical explanation and more questions.

 

[marcus]

Dittrich's 4 Feb ILQGS slides are now posted at http://relativity.phys.lsu.edu/ilqgs/
the link is http://relativity.phys.lsu.edu/ilqgs/dittrich020414.pdf
When the audio is posted (later today or tomorrow?) the link will presumably be:
http://relativity.phys.lsu.edu/ilqgs/dittrich020414.wav

To get a quick sense of the scope:

==Slide 39 "Conclusions and Outlook" says==
We have an understanding of how to construct the physical vacuum as a continuum object, starting from spin foam amplitudes.

The construction of the BF vacuum/representation is a nice exercise towards this end:
Realization of a condensate state.

Very near to spin foam dynamics.

Might facilitate extraction of low energy physics, cosmology etc.

Many generalizations possible. One is:
Does it allow SL(2,C) Hilbert space, supporting self dual variables?
==endquote==

In a sense the talk is in two parts, with the second half being given online at ILQGS two weeks later, by Dittrich-co-author Sebastian Steinhaus.

==Slide 38 refers to the 18 Feb talk==
…
Continuum limit, i.e. with construction of physical vacuum.
We therefore need coarse graining and refining ...
... coming in the next ILQGS talk.
==endquote==

Relevant papers mentioned earlier:

A new vacuum for Loop Quantum Gravity http://arxiv.org/abs/1401.6441
Time evolution as refining, coarse graining and entangling http://arxiv.org/abs/1311.7565

Relevant Perimeter videos:

Geiller on LQG with the original self-dual (Ashtekar) variables.
http://pirsa.org/displayFlash.php?id=14010098
Dittrich on the new LQG vacuum
http://pirsa.org/14010112/

==Slide 16 describes the SET-UP for the new vacuum representation==
•manifold with auxiliary metric
•set of embedded triangulations
•embedded vertices: carry coordinate labels
•edges: geodesics with respect to auxiliary metric (replaces piecewise linear) •triangles, tetrahedra: given by minimal surfaces
•dual complex (for instance barycentric, however details do not matter) with a root node (fixing a reference frame)
•refining operations given by refining Alexander moves (alternative: set of refining Pachner moves)
•equips the set of triangulations with a partial (directed)* order
==endquote==



 

[marcus]

The audio of Dittrich's talk has been posted. The telephone connection from Perimeter to LSU was not especially good so the sound is a bit blurred. I found I could understand IF I followed with the slides closely. As long as I was looking at the slide that she was discussing, I could understand the spoken commentary, and it was quite helpful!
Here's the audio:
http://relativity.phys.lsu.edu/ilqgs/dittrich020414.wav
Bianca had a lot to cover, and used almost the whole allowance of time. So there was time for only a few questions (all by Carlo Rovelli) at the end.

I'm inclined to think BD and co-workers are on a good track which will lead to important results, for the classical or continuum limit among other things.

The new formulation of QG is called the BF version of Loop gravity, as distinct from the until-now usual AL version (Ashtekar-Lewandowski).

To put it briefly the BF version is obtain by DUALIZING everything in the AL version. For more detail, see the slides for the 4 February talk.
http://relativity.phys.lsu.edu/ilqgs/dittrich020414.pdf

Dittrich's co-author Sebastian Steinhaus will give the ILQGS talk in two weeks, Tuesday 18 Feb.

 

[marcus]

Here's a reminder that Dittrich's co-author Sebastian Steinhaus will give an online ILQGS seminar talk in about 3 days from now, Tuesday 18 February.

The topic is practically the same as the title of the recent Dittrich-Martin-Steinhaus paper, that came out in December of last year.

http://arxiv.org/abs/1312.0905
Quantum group spin nets: refinement limit and relation to spin foams
Bianca Dittrich, Mercedes Martin-Benito, Sebastian Steinhaus

So one could have a look at that as a preparation for listening to Tuesday's talk.

Another important paper basic to this effort to construct an alternative formulation of LQG is:

http://arxiv.org/abs/1311.7565
Time evolution as refining, coarse graining and entangling
Bianca Dittrich, Sebastian Steinhaus

 

[marcus]

The slides PDF for tomorrow's talk (18Feb) by Dittrich's co-author Sebastian Steinhaus has been posted:
http://relativity.phys.lsu.edu/ilqgs/steinhaus021814.pdf

The link to the audio, when it is posted, will presumably be:
http://relativity.phys.lsu.edu/ilqgs/steinhaus021814.pdf

But already the slides are pretty interesting and comparatively easy to understand. I will try to copy the outline here, or at least the list of half-dozen main topics.

As anticipated, the talk will be based on these two papers
http://arxiv.org/abs/1312.0905
Quantum group spin nets: refinement limit and relation to spin foams
Bianca Dittrich, Mercedes Martin-Benito, Sebastian Steinhaus

http://arxiv.org/abs/1311.7565
Time evolution as refining, coarse graining and entangling
Bianca Dittrich, Sebastian Steinhaus
======================
Here is the list of CONCLUSIONS given on slide #37:

• Construction principle for the physical vacuum: dynamical cylindrical consistency
• Definition of quantum group spin net models and the symmetry preserving coarse graining algorithm.
• Conjecture: Intertwiner d.o.f. are relevant (also for spin foams).
• Rich fixed point structure with extended phases and phase transitions.
• No fine tuning needed to flow to interesting fixed points.
• Although starting from a fixed lattice: Fixed points (under coarse-graining) describe fully triangulation invariant models
• Interpretation of spin nets as ‘melonic’ spin foams.
• Glueing (BF) vs. Decoupling (simplicity constraints) → ‘mixed’ fixed point
• 2nd order phase transition?

 

[marcus]

I think it will be a talk worth listening too. Download the slides PDF first, so when you start the audio you will be ready to scroll thru the slides along with the speaker. Here is Sebastian's outline, from slide #4:

Quantum group spin nets: refinement limit and relation to spin foams

1 Constructing the physical vacuum
2 From spin foams to spin nets
3 Spin nets as ‘melonic’ spin foams
4 Quantum group basics
5 Coarse-Graining via Tensor Network Renormalization
6 Results: Fixed points and phase diagrams
7 Conclusion / Outlook

For more information about the ILQGS seminar series, go here:
http://relativity.phys.lsu.edu/ilqgs/

 

[marcus]

I mentioned that the slides PDF for Tuesday's (18Feb) talk by Dittrich's co-author Sebastian Steinhaus had been posted:
http://relativity.phys.lsu.edu/ilqgs/steinhaus021814.pdf

The link to the audio is now online as well:
http://relativity.phys.lsu.edu/ilqgs/steinhaus021814.wav

The audio quality is crisp and clean, a big improvement over the ILQGS talk two weeks ago.
The goal of Sebastian's research (with Dittrich and Martin-Benito) is, as he puts it, to discover the refinement limit of Spinfoam QG.
They have a definite technique of refinement and they really seem to be making progress towards this goal!
He also says Spinfoam can be seen as a generalization of lattice gauge theory, which puts the program in a broader context giving guidance and perspective.
The title of Steinhaus' talk is:
Quantum group spin nets: refinement limit and relation to spin foams

It is based on these two papers:
http://arxiv.org/abs/1312.0905 (with the same title as the talk) and
http://arxiv.org/abs/1311.7565
Time evolution as refining, coarse graining and entangling
=====================

Another thing happening just now.
Bianca Dittrich is teaching a fast introduction to (LQG and Spinfoam) Quantum Gravity. Call it an
Introductory Review===an accelerated self-contained introduction for people who already have some exposure to the subject. The first two lectures just happened and are online video:
Lecture 1 http://pirsa.org/displayFlash.php?id=14020061 (given 18 Feb)
Lecture 2 http://pirsa.org/displayFlash.php?id=14020062 (given 19 Feb)

Here is the link for the whole series, which is just now getting started:
http://pirsa.org/C14009

LQG/Spinfoam has become probably the most active line of research in quantum gravity currently showing the most focused progress, so this introductory startup course by Dittrich could be useful to some people.

Also just today a colloquium talk by Laurent Freidel (another prominent QG researcher at Perimeter) went online:
http://pirsa.org/14020144/
Non equilibrium thermodynamic of gravitational screens
Laurent Freidel
In this talk I will review the evidence for a mysterious and deep relationship between gravitational dynamics and thermodynamics. I will show how we can extend this connection to non equilibrium thermodynamics. Using the fact that the gravitational equations are fundamentally holographic, we express them in a way that shows a deep connection between gravity and the dynamics of viscous bubbles. We will explore some aspects of this surprising correspondence.