Which Non-String Quantum Gravity Paper from Q2 2007 Will Shape Future Research?

[marcus]

What's your pick for the most important paper from among second quarter 2007 non-string quantum gravity research? This short list was narrowed down from a larger number of paper which were flagged by several PF people this time, including Francesca, Jal, and myself. Papers that appear in latter half June can be considered with the third quarter (July-August-September) batch.
Here's the list

http://arxiv.org/abs/0705.2388
The loop-quantum-gravity vertex-amplitude
Jonathan Engle, Roberto Pereira, Carlo Rovelli
6 pages

"Spinfoam theories are hoped to provide the dynamics of non-perturbative loop quantum gravity. But a number of their features remain elusive. The best studied one -the euclidean Barrett-Crane model- does not have the boundary state space needed for this, and there are recent indications that, consequently, it may fail to yield the correct low-energy n-point functions. These difficulties can be traced to the SO(4) -> SU(2) gauge fixing and the way certain second class constraints are imposed, arguably incorrectly, strongly. We present an alternative model, that can be derived as a bona fide quantization of a Regge discretization of euclidean general relativity, and where the constraints are imposed weakly. Its state space is a natural subspace of the SO(4) spin-network space and matches the SO(3) hamiltonian spin network space. The model provides a long sought SO(4)-covariant vertex amplitude for loop quantum gravity."

http://arxiv.org/abs/0705.2222
Loop Quantum Gravity: Four Recent Advances and a Dozen Frequently Asked Questions
Abhay Ashtekar
21 pages, to appear in the Proceedings of the 11th Marcel Grossmann Conference

"As per organizers' request, my talk at the 11th Marcel Grossmann Conference consisted of two parts. In the first, I illustrated recent advances in loop quantum gravity through examples. In the second, I presented an overall assessment of the status of the program by addressing some frequently asked questions. This account is addressed primarily to researchers outside the loop quantum gravity community."

http://arxiv.org/abs/gr-qc/0703144
Dynamical coherent states and physical solutions of quantum cosmological bounces
Martin Bojowald
30 pages, 3 figures

"A new model is studied which describes the quantum behavior of transitions through an isotropic quantum cosmological bounce in loop quantum cosmology sourced by a free and massless scalar field. ... an exactly solvable model ... basis for a systematic perturbation theory of loop quantum gravity... remarkably different from what is known for harmonic oscillator coherent states. ... the quantum state ... evolve deterministically through the bounce."

http://arxiv.org/abs/0705.4398
The Dark Side of a Patchwork Universe
Martin Bojowald
24 pages, 2 figures, Contribution to the special issue on Dark Energy by General Relativity and Gravitation

"While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe."

 

[marcus]

I guess we need a fifth option. If anyone cares to they can WRITE IN. I can't go back and edit the poll to include this in the usual way.

Garrett Lisi has put a major paper out on the WEB, in a media format that is alternative to the arxiv-preprint format.

http://deferentialgeometry.org/
Garrett Lisi: Deferential Geometry

"The message from gravity is clear: our universe is geometric. But gravity does not exist alone -- it interacts with the whole zoo of standard model fields. A unification of these fields requires the gravitational connection to be absorbed as part of a larger connection, along with the intimately intertwined Higgs field and frame of GR. By extending this connection further, one generation of fermions may be derived as BRST ghosts, and the dynamics described by a modified BF theory. This description meshes well with recent approaches quantizing gravity perturbatively around a topological theory. But where could such a big, messy connection come from? As it turns out, precisely this connection matches the geometry of the largest exceptional Lie group -- giving exactly three complete generations of fermions, linked by triality! In this fully unified picture, gravity and all fields of the standard model are described by the pure geometry of a group manifold, with no strings attached."

for brief comment by John Baez and Alejandro Rivero, see
https://www.physicsforums.com/showthread.php?p=1359790#post1359790
posts #25 and 26
...

There is a big jump between Garrett's last arxiv posting on this topic which was November 2005
http://arxiv.org/abs/gr-qc/0511120
and what he has just now web-published multimedia-style from his own computer
some of the discoveries he describes were made just in the past month

 

[chrisina]

The dark size of a Patchwork Universe definately gets my vote...
I am still wondering about a most simple cubic lattice ( N(a) =4/3pi (a/lp)^3) ...

 

[marcus]

Chrisina, thank you for offering your view. BTW I think of this kind of poll as a game of forecasting. One can see who of us has a useful mental model of the history of science by our ability to make predictions.

In this case it is predictions about what paper will turn out to be most valuable or influential to future research. As a rough measure, one could simply count the citations after a year or so. But judging the importance is still a little more subjective than that. We will see.

Actually the choice is very difficult this time, I think.

Three people have ventured to make predictions, so far:
Chrisina---whose grandmother was the secretary of Louis de Broglie in the 1930s, someone with a vivid knowledge of the history of science

Birulami---who introduces himself as a software engineer in Düsseldorf

DaveB---who is an applied physicist, born in 1962, who lives in Ohio.

https://www.physicsforums.com/poll.php?do=showresults&pollid=1138

And I, a retired mathematician, am having a terrible time restraining myself from voting. But I want to give others a chance.
Right now it looks like the favorite is Bojowald's second paper where he removes the need for a cosmological constant, or a "dark energy" field, by showing an inherent quantum correction of the kinetic term of the QC hamiltonian. This is the "Dark Side of a Patchwork Universe" paper, which Chrisina and DaveB choose.

But just in terms of citations, Birulami could be right on the money. He says the Ashtekar paper, which was an invited talk at a major international conference---the Marcel Grossmann in Berlin, of summer 2006. It will surely prove a highly visible and strategically important paper because it confronts the issue of how the emerging field of Quantum Gravity is seen by people in more traditional specialties of particle physics and classical relativity.

 

[jal]

Hi marcus!
I'm going to take a GIANT STEP.
I can see into the future.
I can see all the small steps that everyone is taking.
Everyone is going in the same direction …. A structure for spacetime/quark sea .

--------------
My qualifications:
I've read ftp://ftp.alainconnes.org/bookjune4.pdf[/URL]
(Alain cones is a mathematician trying to explain to other mathematicians the Standard Model.)
Of course, I missed most of what he had to say.
I've read [url]http://arxiv.org/abs/0706.2702[/url] papers by Eyo Eyo Ita III
and missed all his details.
I've read http://www.phys.psu.edu/~cteq/handbook/v1.1/handbook.pdf
Handbook of perturbative QCD
and found out that I know very little but got a good idea of what the experimenteurs are doing.
-------------
I would say that everyone is trying to understand QCD and the "quark sea".
[b]It would be enlightening if someone could demonstrate that the ultraviolet and an infrared cutoff could be as a result of the minimum length and the resulting structure. [/b]
-------------
My vote:
Bojowald's second paper, "Dark Side of a Patchwork Universe". A model which can be explored.
jal

 

[marcus]

-------------
My vote:
Bojowald's second paper, "Dark Side of a Patchwork Universe". A model which can be explored.
jal

Jal, please don't forget to actually register your prediction by voting in the POLL
otherwise I'm afraid it won't be counted.

 

[jal]

Oppps!
I would like to mention
http://arxiv.org/PS_cache/arxiv/pdf/...706.1534v1.pdf
Coupling gauge theory to spinfoam 3d quantum gravity
Simone Speziale
June 11, 2007
as a runner up.
jal

 

[marcus]

Thanks,
and as a runner up I certainly agree with your suggestion of Simone Speziale's paper. He's super: I have a very high estimation of him. Here is the abstract link in case anyone wants to check out the one-paragraph summary
http://arxiv.org/abs/0706.1534
Coupling gauge theory to spinfoam 3d quantum gravity

 

[jal]

Marcus?
Iwas looking over some of the other papers that you have been supplying for our attention.
http://arxiv.org/abs/0706.3688
Why the Standard Model
Authors: Ali H. Chamseddine, Alain Connes
(Submitted on 25 Jun 2007)
-------------
http://arxiv.org/abs/0706.3690
A Dress for SM the Beggar
Authors: Ali H. Chamseddine, Alain Connes
(Submitted on 25 Jun 2007 (v1), last revised 26 Jun 2007
-------
The question that comes to my mind...
Would not the most influential paper be the one that can supply the most usefull math approach?
Since your a retired mathematician, I assume you would be able to evaluate the importance of those 2 papers better than most.
----------
My understanding is that the above two papers are a condensed version of ftp://ftp.alainconnes.org/bookjune4.pdf[/URL]
-----------
jal

 

[marcus]

Let's use the Connes and Chamseddine papers to start the third quarter list.
Summer is usually slack months, so it will be good to have some strong candidates,

we can just arbitrarily say that the second quarter ended on 22 June, or the solstice or something
and everything after that is third quarter.

Yeah, the Connes et al work is outstanding and very important, as you obviously realize! the main papers came out Fall 2006 and I haven't checked in detail but i think that these papers are giving more the survey of what's been accomplished rather than new results. If I missed something maybe someone will point it out. I would nevertheless expect them to be highly cited and influential.

It is a mystery to me how people are going to put Connes schemata together with a fundamental quantum dynamics of spacetime and matter.
AFAICS Connes schematizes all the particles and forces including gravity, and can even predict certain masses, so that his model is testable.

But even if it is tested and survives testing and gains credibility it is still not entirely satisfactory because it does not AFAICS have a basic picture of space time and matter describing their fundamental degrees of freedom and how it all interacts at a microscopic level----why matter curves spacetime etc.
It doesn't tell me what was happening right before the big bang or why the expansion of the universe seems to be accelerating.

Loop Quantum Cosmology offers me answers to those questions.

So the problem for me is, suppose Connes schematic picture of particles and forces is tested and prevails. How are they going to put it together with some more basic dynamic model?

BTW doesn't it look as if Garrett Lisi has done something very similar to Connes? or started to do that? Garrett's schematic seems more intuitively geometrical to me. If it works out, it might be easier to assimilate into a quantum dynamics of geometry and matter.
It works with manifolds, Lie groups, and the cousins of differential calculus.

I didn't get Lisi's paper on the poll, but did want to suggest it as a write in. If nobody picks up on it, maybe we can include it on the third quarter poll along with Connes.

 

[marcus]

It's time for nominations for the third quarter MIP (most important paper) forecasts.

My inclination is to limit it to a very few DARK HORSE LONGSHOTS

Because this quarter there were a large number of potentially important papers including several by highly prominent people like Carlo Rovelli and Alain Connes which would either be obvious picks or would be hard to choose between.

So what I'm inclined to do is narrow down to a few papers that are perhaps not so well known but each of which in some way represents a long-odds gamble. What I'm thinking is just these four

AASTRUP GRIMSTRUP (missed them when they came out, but better late than never)
They have a way to put the Connes Standard Particle Physics, not on a simple 4D spacetime manifold but instead on the LQG configuration space of all geometries of a given spacetime continuum. So they put Connes NCG together with LQG.

BOJOWALD
His *Nature Physics* article says yes in his model there was a bounce and there are certain features of the Before quantum state which cannot be determined from knowledge of the After quantum state. He has identified an area of fundamental uncertainty about what happened before the big bang. Is he right?

BONANNO REUTER
They get the desired amount of inflation without exotic matter (without invoking a negative pressure scalar field myth), plus their inflation makes a graceful exit without a contrived "slow roll" potential, and reheating is automatic. IOW they get a completely natural inflation without adding an "inflaTON" or anything else to their model. All their model does is renormalize Einstein's gravity in the simplest imaginable way. That done, the rest comes free. They also get the correct entropy of the Microwave Background. Are they right?

FREIDEL KRASNOV
They have a dynamics for the LQG spinfoam formulation. If it is the right one, this culminates a search that has lasted some 10 years. There are rival spinfoam candidates.

Here are the links
Aastrup and Grimstrup
Intersecting Connes Noncommutative Geometry with Quantum Gravity
http://arxiv.org/abs/hep-th/0601127

Bojowald
What happened before the big bang?
http://npg.nature.com/nphys/journal/v3/n8/full/nphys654.html
(abstract page with citation count: http://adsabs.harvard.edu/abs/2007NatPh...3..523B )

Bonanno and Reuter
Entropy signature of the running cosmological constant
http://arxiv.org/abs/0706.0174

Freidel and Krasnov
A New Spin Foam Model for 4d Gravity
http://arxiv.org/abs/0708.1595================
some past M.I.P. polls
https://www.physicsforums.com/showthread.php?t=116791
https://www.physicsforums.com/showthread.php?t=124951
https://www.physicsforums.com/showthread.php?t=134513
https://www.physicsforums.com/showthread.php?t=149466
https://www.physicsforums.com/showthread.php?t=162163
https://www.physicsforums.com/showthread.php?t=174427

 

[marcus]

Six predictions have been registered in the poll so far.

William Donnely and I think Rovelli et al (their new spinfoam vertex) will prove the most influential.

Birulami favors Ashtekar's survey of current LQG progress.

Three others, Chrisina, DaveB, and Jal, think it will be Bojowald's Dark Energy paper that has the most impact on future research. (I shouldn't call the paper that since Bojowald finds a cumulative quantum correction in LQG that could explain accelerated expansion without resorting to an exotic "dark energy" field.)

It will take a year or so for the dust to settle, so that we can see which papers actually are the most influential, as show for instance by their citations. So anyone who wishes is still welcome to register their guess as to the outcome.