Nominations for 3rd quarter MIP

[marcus]

In another couple of weeks we should have a Most Important Paper poll for 3rd quarter (July-September) of 2008. As of now I have these four in mind, one of which was spotted by Francesca (thanks!)

http://arxiv.org/abs/0809.3190
Evaluation of new spin foam vertex amplitudes
Igor Khavkine
19 pages, 4 figures

"A new numerical evaluation algorithm is proposed for the new spin foam vertex amplitudes proposed by Engle, Pereira & Rovelli and Freidel & Krasnov. The algorithm is applied to compute the high spin behavior of the new vertex amplitudes. Their asymptotics exhibit non-oscillatory, power-law decay, similar to that of the Barrett-Crane model, but with different exponents."

Obviously the ability to calculate the quantum amplitudes of spacetimes (spacetimes are paths from one spatial geometry to another) is essential. The Loll Utrecht group does this for the Triangulations path integral. Christensen's West Ontario (UWO) group with its Beowulf Cluster is one of several outfits with the capability to do this for spinfoam path integrals. Igor Khavkine is with this group. I list this first among the 3rd quarter 2008 papers because of the importance of fast computer algorithms for numerical quantum gravity.

About the next paper, it's clear that Triangulations QG is one of the strongest things going, and their getting a quantum version of de Sitter space to emerge (out of nothing but the chaos of microscopic degrees of freedom, no macro geometry put in by hand) is a major result.

http://arxiv.org/abs/0807.4481
The Nonperturbative Quantum de Sitter Universe
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
37 pages, many figures
(Submitted on 28 Jul 2008)

"The dynamical generation of a four-dimensional classical universe from nothing but fundamental quantum excitations at the Planck scale is a long-standing challenge to theoretical physicists. A candidate theory of quantum gravity which achieves this goal without invoking exotic ingredients or excessive fine-tuning is based on the nonperturbative and background-independent technique of Causal Dynamical Triangulations. We demonstrate in detail how in this approach a macroscopic de Sitter universe, accompanied by small quantum fluctuations, emerges from the full gravitational path integral, and how the effective action determining its dynamics can be reconstructed uniquely from Monte Carlo data. We also provide evidence that it may be possible to penetrate to the sub-Planckian regime, where the Planck length is large compared to the lattice spacing of the underlying regularization of geometry."

The correctness of spinfoam in the semiclassical limit is a key issue. The following paper is an obvious nominee. It uses a powerful technique developed by the authors for translating a spinfoam into a conventional (Feynman-style) action-based path integral. This opens up other technical possibiliites besides solution of the semiclassical limit problem which they are working on here.

http://arxiv.org/abs/0809.2280
On the semiclassical limit of 4d spin foam models
Florian Conrady, Laurent Freidel (Perimeter Inst. Theor. Phys.)
32 pages, 5 figures
(Submitted on 15 Sep 2008)

"We study the semiclassical properties of the Riemannian spin foam models with Immirzi parameter that are constructed via coherent states. We show that in the semiclassical limit the quantum spin foam amplitudes of an arbitrary triangulation are exponentially suppressed, if the face spins do not correspond to a discrete geometry. When they do arise from a geometry, the amplitudes reduce to the exponential of i times the Regge action. Remarkably, the dependence on the Immirzi parameter disappears in this limit."

There is currently an influx of researchers into Loop Cosmology. It has become an attractive field of research, partly because of the strength of observational cosmology and possibilities of empirical verification, partly because of interest in non-singular (prebang) cosmology and removing black hole singularities. This paper takes care of some conceptual matters in LQC and so may turn out to be useful (and cited) in the rising tide of research in this area.

http://arxiv.org/abs/0809.2590
Physical time and other conceptual issues of QG in the example of LQC
Wojciech Kaminski, Jerzy Lewandowski, Tomasz Pawlowski
18 pages, 1 figure
(Submitted on 15 Sep 2008)

"Several conceptual aspects of quantum gravity are studied on the example of the homogeneous isotropic LQC model. In particular: (i) The relativistic time of the co-moving observers is showed to be a quantum operator and a quantum spacetime metric tensor operator is derived. (ii) Solutions of the quantum scalar constraint for two different choices of the lapse function are compared and contrasted. (iii) The mechanism of the singularity avoidance is analyzed via detailed studies of an energy density operator. (iv) The relation between the kinematical and the physical quantum geometry is discussed on the level of relation between observables."

 

[marcus]

to those four, let's add a fifth nominee.
This paper could turn out to be really important:
http://arxiv.org/abs/0809.4464
Effective Theory of Braid Excitations of Quantum Geometry in terms of Feynman Diagrams
Yidun Wan
24 pages, 7 figures
(Submitted on 25 Sep 2008)

"We study interactions amongst topologically conserved excitations of quantum theories of gravity, in particular the braid excitations of four-valent spin networks. These have been shown previously to propagate and interact under evolution rules of spin foam models. We show that the dynamics of these braid excitations can be described by an effective theory based on Feynman diagrams. In this language, braids which are actively interacting are analogous to bosons, in that the topological conservation laws permit them to be singly created and destroyed. Exchanges of these excitations give rise to interactions between braids which are charged under the topological conservation rules."

As a convenience for anyone trying to size up the candidates' potential importance, I will list the other four in brief, without the abstracts and comment contained in the preceding post:

http://arxiv.org/abs/0809.2280
On the semiclassical limit of 4d spin foam models
Florian Conrady, Laurent Freidel (Perimeter Inst. Theor. Phys.)
32 pages, 5 figures
(Submitted on 15 Sep 2008)

http://arxiv.org/abs/0807.4481
The Nonperturbative Quantum de Sitter Universe
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
37 pages, many figures
(Submitted on 28 Jul 2008)

http://arxiv.org/abs/0809.3190
Evaluation of new spin foam vertex amplitudes
Igor Khavkine
19 pages, 4 figures

http://arxiv.org/abs/0809.2590
Physical time and other conceptual issues of QG in the example of LQC
Wojciech Kaminski, Jerzy Lewandowski, Tomasz Pawlowski
18 pages, 1 figure
(Submitted on 15 Sep 2008)

==========
EDIT to reply to next post by MTd2:
I agree that the Braid paper is very interesting. I was planning to post the poll next week: the "MIP poll" for what people think will be the most important or influential paper of this quarter (July thru September). Unless some new paper comes out in the next week, the poll will probably just have these 5 choices. Maybe both you and I will vote for Yidun Wan's paper (I can't say yet, I also like the Renate Loll and the Laurent Freidel papers---actually they all seem pretty good to me and likely to be valuable for future research.)

 

[MTd2]

"Effective Theory of Braid Excitations of Quantum Geometry in terms of Feynman Diagrams" This one! PLEASE!

 

[marcus]

Wan's paper will definitely will be on the poll MTd2. Will it be satisfactory if it is at the top of the poll list?
The Loll paper was just published in Physical Review D. It is likely to be influential too.

 

[marcus]

I will add one by Bojowald which addresses the problem of testing bounce cosmology models (LQC) by observations of structure formation in the early universe. Here's the list of papers nominated for this quarter.

http://arxiv.org/abs/0809.4464
Effective Theory of Braid Excitations of Quantum Geometry in terms of Feynman Diagrams
Yidun Wan
24 pages, 7 figures
(Submitted on 25 Sep 2008)

http://arxiv.org/abs/0809.2280
On the semiclassical limit of 4d spin foam models
Florian Conrady, Laurent Freidel
32 pages, 5 figures
(Submitted on 15 Sep 2008)

http://arxiv.org/abs/0807.4481
The Nonperturbative Quantum de Sitter Universe
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
37 pages, many figures, Physical Review D 78 063544 (2008)
(Submitted on 28 Jul 2008)

http://arxiv.org/abs/0809.3190
Evaluation of new spin foam vertex amplitudes
Igor Khavkine
19 pages, 4 figures
(Submitted on 18 Sep 2008)

http://arxiv.org/abs/0809.2590
Physical time and other conceptual issues of QG in the example of LQC
Wojciech Kaminski, Jerzy Lewandowski, Tomasz Pawlowski
18 pages, 1 figure
(Submitted on 15 Sep 2008)

http://arxiv.org/abs/0808.0701
Effective theory for the cosmological generation of structure
Martin Bojowald, Aureliano Skirzewski
8 pages, 1 figure, Advanced Science Letters 1 (2008) 92-98
(Submitted on 5 Aug 2008)

"The current understanding of structure formation in the early universe is mainly built on a magnification of quantum fluctuations in an initial vacuum state during an early phase of accelerated universe expansion. One usually describes this process by solving equations for a quantum state of matter on a given expanding background space-time, followed by decoherence arguments for the emergence of classical inhomogeneities from the quantum fluctuations. Here, we formulate the coupling of quantum matter fields to a dynamical gravitational background in an effective framework which allows the inclusion of back-reaction effects. It is shown how quantum fluctuations couple to classical inhomogeneities and can thus manage to generate cosmic structure in an evolving background. Several specific effects follow from a qualitative analysis of the back-reaction, including a likely reduction of the overall amplitude of power in the cosmic microwave background, the occurrence of small non-Gaussianities, and a possible suppression of power for odd modes on large scales without parity violation."

 

[francesca]

...one of which was spotted by Francesca (thanks!)
http://arxiv.org/abs/0809.3190 Evaluation of new spin foam vertex amplitudes Igor Khavkine

oh, I've seen now the threat... so I would like to say that if you enlighten this paper, you should also pick up these ones!
1. http://arxiv.org/abs/0809.3718"
Asymptotics of LQG fusion coefficients
Emanuele Alesci, Eugenio Bianchi, Elena Magliaro, Claudio Perini
Comments: 14 pages
2. http://arxiv.org/abs/0808.1971"
Intertwiner dynamics in the flipped vertex
Emanuele Alesci, Eugenio Bianchi, Elena Magliaro, Claudio Perini
Comments: 12 pages, 7 figures
(well, I'm coloured )
Cheers, F

 

[marcus]

oh, I've seen now the threat... so I would like to say that if you enlighten this paper, you should also pick up these ones!
1. http://arxiv.org/abs/0809.3718"
Asymptotics of LQG fusion coefficients
Emanuele Alesci, Eugenio Bianchi, Elena Magliaro, Claudio Perini
Comments: 14 pages
2. http://arxiv.org/abs/0808.1971"
Intertwiner dynamics in the flipped vertex
Emanuele Alesci, Eugenio Bianchi, Elena Magliaro, Claudio Perini
Comments: 12 pages, 7 figures
(well, I'm coloured )
Cheers, F

F, I see that here are several related papers about calculating the new spinfoam vertex. I do not agree that you are biased (that your vision is coloured). I think you are objectively correct to point out what is going on. These are young researchers, many of whom have been in the Marseille group, and they are working out the consequences of the new vertex formulas.

I don't clearly understand what are fusion coefficients. If you want to, explain in a few words.

I was glad you pointed out the paper of Igor Khavkine, which belongs also to this topic---calculating with the new spinfoam vertices. It was you who enlightened me (by highlighting that paper.)

I see that three of the nominated papers have already been cited. The ones by Khavkine, Bojowald, and Freidel.