If LQG now satisfactory, how to add matter?

[flatcp]

I hope that in this discussion thread we can get back to the main topic---the ways currently being considered to include matter."

You opened the thread with this description "If LQG now satisfactory, how to add matter?" . Seems reasonable for people who are definitely smarter then me (and from what I have read on this forum, you as well) to focus on the qualifier prior to considering the smuggled in concept.

If the moon is made of cheese, what kind?

 

[marcus]

...

If the moon is made of cheese, what kind?

Heh heh, great comment.

Actually the theory is essentially ready to test (with very simple matter) and ready to add matter. "If" is just an attention-getter. I have to go. Back later today.

 

[marcus]

I could have titled the thread Since matterless LQG satisfactory, how to add matter?

The theory has reached the point where it is reasonably coherent (LQG cosmology is being done with spinfoam) and makes a robust prediction of cosmo bounce---something that can be tested.

A concise simple discussion of this begins here:
http://www.math.columbia.edu/~woit/wordpress/?p=3262&cpage=1#comment-67952
Bee Hossenfelder, a reputable QG phenomenologist, entered the discussion here:
http://www.math.columbia.edu/~woit/wordpress/?p=3262&cpage=1#comment-67988

A more credible objective expert on QG phenomenology can't be found. Bee has organized two conferences on "Experimental Search for Quantum Gravity"---the world's first. One at Perimeter Institute, when she was there. The second one where she is working now, at NORDITA in Stockholm. She invited QG and string alike, across the board. She is a phenomenologist---that means develops and evaluates TESTS of theories---not playing favorites.

It's clear. You can falsify LQG if the CMB shows no evidence of cosmic bounce. The theory has to face the music of the ancient light---the CMB music. Bee is not the kind that takes prisoners or pulls punches.

It's not like some of Smolin's gambits, where people like Rovelli and Ashtekar didn't see the point and declined to sign on. There was never a proof that LQG implies energydependent speed of light, even when some people tried hard to derive one. But the bounce is robust. Ashtekar's people get it every time they solve the equations or run a computer simulations of the early universe. Time doesn't stop, in LQG, as you go back. A top density is reached and contracting distances re-expand.

I think it may be personally difficult for people like Rovelli and Thiemann to sign on to the bounce as an implication of LQG (because it puts the theory at risk of falsification) but I don't see any way they can avoid doing that. Rovelli already hinted, or mentioned that in his October paper 1010.1939.

Anyway, reluctantly or not, matterless LQG is going to be tested---actually most of the early universe models have some kind of simplified matter, like a scalar field. What I mean by "matterless" LQG is the theory with only this radically simplified form of matter.

And it may survive. That's why I say the next question to ask is how to add matter to the picture.

 

[MTd2]

Why aren't the tetrahedron when connection to other tetrahedron free to permute connecting vertices, change the chirality of connecting edges amd orientation of connecting faces?

 

[atyy]

It's clear. You can falsify LQG if the CMB shows no evidence of cosmic bounce. The theory has to face the music of the ancient light---the CMB music. Bee is not the kind that takes prisoners or pulls punches.

It's not like some of Smolin's gambits, where people like Rovelli and Ashtekar didn't see the point and declined to sign on. There was never a proof that LQG implies energydependent speed of light, even when some people tried hard to derive one. But the bounce is robust. Ashtekar's people get it every time they solve the equations or run a computer simulations of the early universe. Time doesn't stop, in LQG, as you go back. A top density is reached and contracting distances re-expand.

I think it may be personally difficult for people like Rovelli and Thiemann to sign on to the bounce as an implication of LQG (because it puts the theory at risk of falsification) but I don't see any way they can avoid doing that. Rovelli already hinted, or mentioned that in his October paper 1010.1939.

So if Rovelli has not yet signed on, how do we know this is not another "prediction" that will falsify LQG?

 

[marcus]

So if Rovelli has not yet signed on, how do we know ...?

We don't know for sure. He's careful and will not base anything on guesswork. He won't say something is a prediction until there is a watertight case, all spelled out. But it looks unavoidable to me.

Maybe I should be more cautious!

What must be shown is that a bounce occurs in the full spinfoam theory.

Let's glance at two October 2010 papers to gauge how far we are from that:

http://arxiv.org/abs/1010.1258
Big Bounce in Dipole Cosmology
Marco Valerio Battisti, Antonino Marciano
(Submitted on 6 Oct 2010)
"We derive the cosmological Big Bounce scenario from the dipole approximation of Loop Quantum Gravity. ... This model thus enhances the relation between Loop Quantum Cosmology and the full theory."

The dipole cosmology is simplified spin foam. It is not the full theory. The initial and final states are restricted. OK so the bounce has been derived only in a TOY spinfoam model, so far.

Then also, the authors of the next paper have found something wrong with the way time is handled in LQC. This also applies to the Battisti Marciano paper although it is not usual LQC---they treated time the same way.

http://arxiv.org/abs/1010.0502
Local spinfoam expansion in loop quantum cosmology
Adam Henderson, Carlo Rovelli, Francesca Vidotto, Edward Wilson-Ewing
(Submitted on 4 Oct 2010)
"...In this paper we consider a vacuum Bianchi I universe and show that by choosing an appropriate regulator a spinfoam expansion can be obtained without selecting a clock variable and that the resulting spinfoam amplitude is local."

I think this paper points out a technical matter that needs fixing. By my reckoning it doesn't invalidate the general impression that the bounce is a robust characteristic of Loop early universe. The Penn State work under
Ashtekar's direction has repeatedly confirmed this. I have a hard time imagining that it will not finally be confirmed.
I'll have to look at this again in the morning when I am fresh.

 

[marcus]

Atyy, I "slept on it" and can return a little fresher. One way to address the question is that it is now to a considerable extent out of Rovelli's hands and over in the court of the phenomenologists.

On the theorists' part there is an ongoing effort to link the full theory with LQC. Rovelli's group will continue doing that---there are already two years of papers. Thiemann has written on that too and he has a group at Erlangen. You already see Erlangen and Marseille people collaborating on completing the job. I think it is a done deal. The full theory (spinfoam) will be applied to cosmology.

For that matter, you see Penn State people working on the same thing: full theory-->cosmo.
Specifically it is the spinfoam formulation applied to cosmo. The theorists are bound to do that, it is out of anyone person's hands.

We have 10 years of experience teaching us to expect that the full theory applied to cosmo will give a bounce. They've tried all kinds of variations already including non-isotropic and that feature appears robust. As you saw, Battisti Marciano just tried it with spinfoam dynamics (toy version) and got a bounce.

So what happens after that is ultimately up to the phenomenologists.

I think there is a kind of moral wisdom in having a division of labor here. Phenomenologists have a professional interest in seeing if a theory is "ready" and if it smells ready to them they go about seeing how to test it.

The parent of a theory may not even want to see his construct go to the front and take its chances. I don't know what it feels like---it could actually be hard. The way professional specialization works, the parent is relieved of some of the responsibility of deciding. The theory goes up for testing when the phenomenologists decide---or so I think. That is one way it can work anyway.

 

[marcus]

So let's see who some of these phenomenologists are, who have recently weighed in. It makes a big difference what we think of them.
http://arxiv.org/abs/1007.2396
Constraints on standard and non-standard early Universe models from CMB B-mode polarization
Yin-Zhe Ma, Wen Zhao, Michael L. Brown

The paper was recommended by Bee Hossenfelder (NORDITA) whom we know, along with one of her own. Looks like she might think the work is solid, otherwise why recommend it? Who are the authors?

Wen Zhao has 35 papers going back to 2005. A substantial number of them are in observational early-universe cosmology, CMB analysis. So this is "right down his alley".
http://arxiv.org/find/astro-ph/1/au:+Zhao_W/0/1/0/all/0/1
He is at Cardiff U with joint appointment at the Wales Institute of Mathematical and Computational Sciences.

Yin-Zhe Ma and Michael Brown are Cambridge. Both are at the Kavli Institute for Cosmology. YZM has joint appointment at the Inst. of Astronomy. MB belongs to the Cavendish Lab Astrophysics group.
I guess the main institutional handle for both would be KICC (Kavli Inst. Cosm. Cambridge)

Webpage at Cavendish Astrophysics for Michael Brown:
http://www.mrao.cam.ac.uk/people/mbrown.html
(title is Senior Research Associate)

Yin-Zhe Ma papers going back to 2007 when YZM was at Beijing Kavli Inst. for Theoretical Physics (KITP China):
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=a+Ma%2C+Yin-Zhe&FORMAT=WWW&SEQUENCE=
I'll get back to this as time permits.

 

[marcus]

...
http://arxiv.org/abs/1007.2396
Constraints on standard and non-standard early Universe models from CMB B-mode polarization
Yin-Zhe Ma, Wen Zhao, Michael L. Brown

The paper was recommended by Bee Hossenfelder (NORDITA) whom we know, along with one of her own. Looks like she might think the work is solid, otherwise why recommend it? Who are the authors?

Wen Zhao has 35 papers going back to 2005. A substantial number of them are in observational early-universe cosmology, CMB analysis. So this is "right down his alley".
http://arxiv.org/find/astro-ph/1/au:+Zhao_W/0/1/0/all/0/1
He is at Cardiff U with joint appointment at the Wales Institute of Mathematical and Computational Sciences...

This is the real sign that Loop has reached a satisfactory state---phenoms are spontaneously gathering around scrutinizing it. They want to test (whether or not Loop people like the idea, opinions may differ) and think that they can.

I just learned that SHINJI TSUJIKAWA a Tokyo U phenomenologist has a "Loop falsifiable by CMB" paper in preparation. In this case it will be co-authored with a central Loop cosmology figure, Martin Bojowald.

I'll get the tip-off quote. It is reference [51] on page 34 of a Bojowald Calcagni that just appeared

http://arxiv.org/abs/1011.2779
Inflationary observables in loop quantum cosmology
Martin Bojowald, Gianluca Calcagni
40 pages
(Submitted on 11 Nov 2010)
"The full set of cosmological observables coming from linear scalar and tensor perturbations of loop quantum cosmology is computed in the presence of inverse-volume corrections. Background inflationary solutions are found at linear order in the quantum corrections; depending on the values of quantization parameters, they obey an exact or perturbed power-law expansion in conformal time. The comoving curvature perturbation is shown to be conserved at large scales, just as in the classical case. Its associated Mukhanov equation is obtained and solved. Combined with the results for tensor modes, this yields the scalar and tensor indices, their running, and the tensor-to-scalar ratio, which are all first order in the quantum correction. The latter could be sizable in phenomenological scenarios. Contrary to a pure minisuperspace parametrization, the lattice refinement parametrization is in agreement with both anomaly cancellation and our results on background solutions and linear perturbations. The issue of the choice of parametrization is also discussed in relation with a possible superluminal propagation of perturbative modes, and conclusions for quantum spacetime structure are drawn."

==quote==
In this final section we discuss how they can be used to restrict models of loop quantum cosmology, making the framework falsifiable. Details will be provided in a separate publication [51]. For such an endeavor, it is crucial to obtain independent information on the main correction parameter δ_Pl...
==endquote==

[51] is a paper by Bojo Calcagni and Tsujikawa "to appear"

I think I might start compiling an "Honor Role" of phenomenologists who have published papers on this topic (most without collaboration by Loop people) or otherwise got the word out. Outstanding would be Sabine Hossenfelder (NORDITA Stockholm) who has organized two conferences on the experimental search for QG and published a number of papers on QG phenom. She's the one who pointed Wen Zhao out to me. Also outstanding are Aurelien Barrau, and a former PhD student of his, Julien Grain.

You can look up these people's work by name on Arxiv. Something that matters, I think, is that they don't play favorites. They explore testing possibilities of theories with implications for cosmology, any and all alike (including "string-inspired", braneworlds and all that.) Professional attitude .

Here's my provisional Early-Universe QG Phenomenologist Honor Role alphabetized by surname :

Aurelien Barrau
Julien Grain
Sabine Hossenfelder
Shinji Tsujikawa
Wen Zhao

 

[marcus]

To recap, if one just considers Loop QG with limited or no matter, the theory has to significant extent reached a stable configuration where it makes robust predictions that can be tested. The application of the full theory to cosmology is being carried out--one knows generally what the theory's consequences are and what to look for. Phenomenologists have taken over part of the job.

I do not expect the formulation of the theory to change much except as it changes to accommodate more complex realistic matter. From now on, I'm suggesting, what drives the development of the theory will be the need to add matter to the picture.

I'm not talking about "unification". I mean simply putting additional fields into the existing quantum-geometrical framework and having them interact with the geometry. So far Loop cosmology simulations have tended to use massless scalar fields---simple toy matter, not the real stuff---and the same with analytic solvable models.

There are some exceptions and Atyy has pointed out a bunch of them. Feynman diagrams for conventional field theory unearthed in a spinfoam QG context by Freidel, Livine, and others. But still the situation isn't clear enough for me to know, or even guess, what to expect.

I think I will make a tentative bet that the following paper, when it appears, will have some clues. This could be something that MTd2 has hinted at but I wasn't sure at the time if he was talking about this or something else.

This paper is in preparation:

Quantum Twisted Geometries and Coherent States
Laurent Freidel and Simone Speziale

I'll give some background on this. The paper was cited in a January 2010 paper by the same authors called:
Twisted Geometries: A Geometric Parametrization of SU(2) Phase Space.
http://arxiv.org/abs/1001.2748

 

[marcus]

As with any prediction I am foolish enough to make, you are welcome to make fun of me if proven wrong---assuming you remember what I say today and can compare it with the Freidel Speziale when it finally comes out. Just keep an eye open for something called
Quantum Twisted Geometries and Coherent States.

The main topic of that paper will of course not be matter, but I'm betting that it will contain a hint as to how Freidel and Speziale think matter can be brought in.

The January paper http://arxiv.org/abs/1001.2748 has an limitation worth noticing: it seems to be restricted to 4-valent spin networks. Correct me if you know otherwise. I don't see this explicitly stated. "Twisted" could also be called "squished". In the dual to the network, where two tetrahedra butt up against each other, the two triangle faces don't necessarily match. You might have to squish one of them in order to make it like the other.
=====================

One or more people in this thread mentioned Bilson-Thompson and braid matter. I haven't heard much of anything about braid matter for over 2 years and I don't expect the subject to be brought up. Let's put that one on "ignore" until further notice.

Last I heard, Song He (one of those who worked earlier on braid matter) was doing something with covariant Regge at Albert Einstein Institute---Dittrich's group. It actually relates to this Freidel Speziale work. From Song He track record I have a lot of expectations from him---if there were immediate results to be gotten from braid he would be on that but he is doing something else. It doesn't mean that something LIKE braid in some unknown sense couldn't have potential. I have no clue what that could be.

The main thing for now is probably just to decide simply how to put matter into the spin network and spinfoam picture, not any kind of "unification" or recovery of the standard model. Please correct me if you see that I'm wrong about that for some reason.
==============
The Freidel Speziale paper of January 2010 has been cited 16 times.
http://www.slac.stanford.edu/spires/find/hep?c=PHRVA,D82,084040
What did they say they were going to do in the paper that is in preparation? Let's call it something, for short, like QTG+CS (for quantum twisted geometries and coherent states.)

 

[marcus]

More on the adding matter front:

John Barrett posted a November 2010 update to this April 2010 entry in his blog:

http://johnwbarrett.wordpress.com/

Quantum gravity with matter

I gave a short talk at IHES in December (and a rather longer one in Marseille, too) on the topic of modifying quantum gravity models so that they contain realistic matter. A lot of work on quantum gravity is done without any matter fields and one gets the impression that matter fields are an optional extra which just make the system more complicated. The icing on the cake, as Chris Isham used to say about topology.

In my talk I suggested that, on the contrary, quantum gravity models with matter can actually be rather simpler than models without matter. This is because the Einstein action is induced by the matter fields, so removing the requirement to put the Einstein action into the theory from the beginning.

Some slides from my talks at Bayrischzell and Oxford are available. I am writing a short paper expanding this.

Update (Nov ’10) I’ve found a good result about this since those talks, hence the delay (and also I’m trying to finish a different paper first).

​

http://johnwbarrett.wordpress.com/2010/04/22/hello-world/

 

[atyy]

Ha, ha! So much for (Rovellian) LQG!

OK, that's premature, but I'm glad Barrett is going this way too!

http://arxiv.org/abs/1009.4475
"The above discussion suggests that it would be more natural to have some fundamental quantum theory of spin networks or spin foams which knows nothing about the Einstein action, except that it appears in the infrared limit, and is defined instead using some natural symmetry or other principles."

http://arxiv.org/abs/1004.0672
"Now, it was not our intention that this work would or should settle this debate, but we find that this theory is more in line with the arguments of the former way."

http://arxiv.org/abs/0909.1861
"In this essay we have taken a new step: geometry is nothing but the collective organization of emergent matter. This leads to a new way to view the Einstein equations: there is no surprise that T and R are inter-related, they are different facets of the same thing. In quantum graphity, matter becomes both geometry and matter."

http://arxiv.org/abs/0906.1313
"If gravity is induced [9], which means that Newton’s constant is zero at tree level and arises as a one loop correction, then the entanglement entropy is responsible for all of the entropy, and reproduces the area law with the correct coefficient [7,10]."

 

[marcus]

OK, that's premature,...

Indeed.

And Roche, Livine, Markopoulou, and Strominger don't make a very coherent ensemble.
You might find that current (Rovelli) LQG was more compatible with each one separately than the four are amongst themselves.

Your post just now appeared to be in response to this one of mine, and yet does not really connect to it:

More on the adding matter front:

John Barrett posted a November 2010 update to this April 2010 entry in his blog:

http://johnwbarrett.wordpress.com/

Quantum gravity with matter

I gave a short talk at IHES in December (and a rather longer one in Marseille, too) on the topic of modifying quantum gravity models so that they contain realistic matter. A lot of work on quantum gravity is done without any matter fields and one gets the impression that matter fields are an optional extra which just make the system more complicated. The icing on the cake, as Chris Isham used to say about topology.

In my talk I suggested that, on the contrary, quantum gravity models with matter can actually be rather simpler than models without matter. This is because the Einstein action is induced by the matter fields, so removing the requirement to put the Einstein action into the theory from the beginning.

Some slides from my talks at Bayrischzell and Oxford are available. I am writing a short paper expanding this.

Update (Nov ’10) I’ve found a good result about this since those talks, hence the delay (and also I’m trying to finish a different paper first).

​

http://johnwbarrett.wordpress.com/2010/04/22/hello-world/

Barrett works closely with Rovelli, whose PhDs may postdoc either at Nottingham or at Perimeter. Barrett just announced the setting up of a QG Masters degree program at Nottingham (see his blog, I gave the link above.)
And he says that he gave a long talk at Marseille in December 2009 laying out his ideas, as they were then, about how to include matter. Now, November 2010, he says he has found a result, material for a follow-up paper. It might be an agreeable surprise, I hope so.

What I was thinking was you might have some clue as to what direction Barrett is going, what his ideas on putting matter into the QG picture might be. If so please spell it out a bit for me. Paraphrase in your own words. So we have more than isolated short quotes out of context.

 

[atyy]

What I was thinking was you might have some clue as to what direction Barrett is going, what his ideas on putting matter into the QG picture might be. If so please spell it out a bit for me. Paraphrase in your own words. So we have more than isolated short quotes out of context.

I couldn't understand a word of his talk, once the category theory started! It seems to have something to do with http://arxiv.org/abs/hep-th/0608221. But all I got was that it has something to do with Sakharov's induced gravity, the exact same reference as Strominger's.

 

[marcus]

I couldn't understand a word of his talk, once the category theory started!...

You didn't indicate what talk. Maybe you could tell me the title. There are several of Barrett's talks online but the only Barrett video I can get my computer to play is the 2009 Planck Scale conference one.

Maybe if you say the title I can get the PDF slides and glean some idea what you are talking about.

 

[marcus]

It seems to have something to do with http://arxiv.org/abs/hep-th/0608221...

The coin just dropped. You have access to those online video lectures that I can't play. I'm making a wild guess that Barrett is trying to include matter in LQG using the paper you just mentioned:

http://arxiv.org/abs/hep-th/0608221
A Lorentzian version of the non-commutative geometry of the standard model of particle physics
John W. Barrett
14 pages
(Submitted on 31 Aug 2006)
"A formulation of the non-commutative geometry for the standard model of particle physics with a Lorentzian signature metric is presented. The elimination of the fermion doubling in the Lorentzian case is achieved by a modification of Connes' internal space geometry so that it has signature 6 (mod 8) rather than 0. The fermionic part of the Connes-Chamseddine spectral action can be formulated, and it is shown that it allows an extension with right-handed neutrinos and the correct mass terms for the see-saw mechanism of neutrino mass generation."

This is a result that Barrett and Alain Connes got at right about the same time, and their two papers reporting the result came out within a week of each other.

From my perspective it seems way too good to be true---well ahead of my expectations---that Barrett would at this point be trying to load the Standard Particle Model into a LQG spinfoam model of geometry using the NCG form of the SM.
Matilde Marcolli gave a paper at Oberwolfach about that, this year.

I'd be very surprised if that is what you were talking about---or what you caught a video of Barrett talking about---but it seems to follow from part of your message.

If anyone else is reading this thread and is interested, here is the list of video and PDF slides for Barrett talks that I think Atyy was watching one of. I don't know which one. Not all would be relevant to this topic.
http://johnwbarrett.wordpress.com/talks/

 

[atyy]

The slides for the talk are at http://hep.itp.tuwien.ac.at/~miw/bzell2010/Barrett-2010.pdf

In induced gravity, the action has only matter classically, but gravity is induced by quantum corrections (slide 5).

So which matter action should we take? Presumably Barrett is suggesting an action similar to the one he proposed in http://arxiv.org/abs/hep-th/0608221 (also slides 3 and 6).

And the quantization would be a spin foam based on that action. So no EH action, as Alexandrov and Roche would like. (All just my guesses.)

 

[marcus]

The slides for the talk are at http://hep.itp.tuwien.ac.at/~miw/bzell2010/Barrett-2010.pdf

In induced gravity, the action has only matter classically, but gravity is induced by quantum corrections (slide 5).

So which matter action should we take? Presumably Barrett is suggesting an action similar to the one he proposed in http://arxiv.org/abs/hep-th/0608221 (also slides 3 and 6).

And the quantization would be a spin foam based on that action. So no EH action, as Alexandrov and Roche would like. (All just my guesses.)

Smart guesses! I feel a bit like Dr. Watson confronted by one of Holmes' reasoned conjectures. Thanks!

============================

So Barrett's funding agency for QGQG (quantum geometry quantum gravity) a branch of ESF (euro sci. foundation) supported this QGQG workshop in May 2010 at a picturesque spot south of Munich, called BayrischZell--call the workshop Bzell 2010:
http://hep.itp.tuwien.ac.at/~miw/bzell2010/
And Barrett gave a talk, and some other people we know of gave talks:
http://hep.itp.tuwien.ac.at/~miw/bzell2010/program2010.html
And what you posted was the PDF for the slides of that May 2010 Bzell talk...

And in his home website he puts that Bzell talk in context:
http://johnwbarrett.wordpress.com/talks/
and says that although there is no video for the Bzell, there is an Oxford version of the same talk also given in May 2010 and there is a video for that...

have to go, back later.

 

[marcus]

To repeat the point I am making in this thread, some kind of LQG has reached the stage where it is ready to be tested using astronomical data.
Right or wrong, it has gotten to where you can see it as ready, mature, complete enough to test.

I've called this version "matterless" LQG but that is not quite right--it is common to include some simple toy matter like a single scalar field. It is this "toy matter" LQG is now fairly mature---makes definite testable predictions---and is beginning to be taken over by phenomenologists as a subject of interest to them. To recap, here is an earlier post in this thread:

...
It seems fairly obvious that matterless (or simple scalar matter) LQG has matured to the point of being testable with the next generation of CMB spacecraft . The proposed NASA B-Pol mission for example--how soon such steps are taken depends mainly economic and political conditions, there are no technical barriers.
http://www.b-pol.org/index.php

Since there are evidently differing opinions regarding the theory's maturity, I'll copy two recent abstracts bearing on that:

http://arxiv.org/abs/1011.1811
Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters
Julien Grain, A. Barrau, T. Cailleteau, J. Mielczarek
12 pages, 5 figures
(Submitted on 8 Nov 2010)
"Cosmological models where the standard Big Bang is replaced by a bounce have been studied for decades. The situation has however dramatically changed in the last years for two reasons. First, because new ways to probe the early Universe have emerged, in particular thanks to the Cosmic Microwave Background (CMB). Second, because some well grounded theories -- especially Loop Quantum Cosmology -- unambiguously predict a bounce, at least for homogeneous models. In this article, we investigate into the details the phenomenological parameters that could be constrained or measured by next-generation B-mode CMB experiments. We point out that an important observational window could be opened. We then show that those constraints can be converted into very meaningful limits on the fundamental Loop Quantum Cosmology (LQC) parameters. This establishes the early universe as an invaluable quantum gravity laboratory."
...

As a followup, here is a recent paper by Barrau. It is a write-up of the talk he delivered at the ICHEP in Paris (International Conference on High Energy Physics):

http://arxiv.org/abs/1011.5516
Inflation and Loop Quantum Cosmology
Aurelien Barrau
5 pages, Proceedings of the 35th International Conference on High Energy Physics, Paris, 2010 (ICHEP 2010)
(Submitted on 24 Nov 2010)
"On the one hand, inflation is an extremely convincing scenario: it solves most cosmological paradoxes and generates fluctuations that became the seeds for the growth of structures. It, however, suffers from a 'naturalness' problem: generating initial conditions for inflation is far from easy. On the other hand, loop quantum cosmology is very successful: it solves the Big Bang singularity through a non-perturbative and background-independent quantization of general relativity. It, however, suffers from a key drawback: it is extremely difficult to test. Recent results can let us hope that inflation and LQC could mutually cure those pathologies: LQC seems to naturally generate inflation and inflation could allow us to test LQC."

 

[marcus]

Here is the concluding paragraph of Barrau's new paper

==quote page 5 http://arxiv.org/abs/1011.5516 ==

My view is that the LQC-inflation paradigm is becoming "convincing". LQC (probably) generates inflation and inflation (possibly) allows us to test LQC. This is a tantalizing picture. Some important points nevertheless need to be investigated. First, scalar modes (and the resulting temperature power spectrum of the CMB) must be studied into the details. This is on the way ([14]) but computations are far from trivial as it is not straightforward to obtain an anomaly-free algebra in this case. Then, Inverse-Volume (IV) corrections should be included. All what has been said before is related to holonomy corrections only. This should not be very difficult and dramatic new effects are not expected as most of the observable features are associated with the bounce itself (which will basically remain the same with IR corrections) and not with subtle loopy corrections to the propagation of physical modes. Finally, and most importantly, inhomogeneities have to be taken into account as they are known to grow very fast during the contraction phase. This point, of course, questions the reliability of the picture.
==endquote==

 

[ensabah6]

what about NCG+LQG?

http://arxiv.org/abs/1012.0713
Quantum Gravity coupled to Matter via Noncommutative Geometry
Johannes Aastrup, Jesper M. Grimstrup, Mario Paschke
15 pages, 1 figure
(Submitted on 3 Dec 2010)
"We show that the principal part of the Dirac Hamiltonian in 3+1 dimensions emerges in a semi-classical approximation from a construction which encodes the kinematics of quantum gravity. The construction is a spectral triple over a configuration space of connections. It involves an algebra of holonomy loops represented as bounded operators on a separable Hilbert space and a Dirac type operator. Semi-classical states, which involve an averaging over points at which the product between loops is defined, are constructed and it is shown that the Dirac Hamiltonian emerges as the expectation value of the Dirac type operator on these states in a semi-classical approximation."

 

[marcus]

Ensabah, thanks for giving links to the LQG+NCG work of the two Danes = Aastrup and Grimstrup.

I don't know how matter will fit into the picture---I'm watching for the next papers from people like Tom Krajewski and J.W. Barrett. and the people they have co-authored with. Their papers might give some clue. (Partly I am helped by Atyy's intuition in this.)

In the meantime for the past 2 years LQG-lite (the version with at-best-rudimentary matter) has been undergoing a rapid "tying up of loose ends". It is looking more like a "wrap"----like it's wrapped up, ready to ship, or in the case of a physical theory, ready to test.

Krajewski just became a permanent member of the Marseille qg team. Not a postdoc, mind you. There are several PhD students and postdocs at Marseille that are already publishing valuable papers. But this is like a junior faculty appointment. So now there are four permanents (Perez, Krajewski, Speziale, Rovelli).

You have seen the recent work of Bianchi and Smerlak---one is is just a postdoc and the other still a PhD student. To me they already look like junior faculty grade.

Krajewski's appointment has to have something to do with how the inclusion of matter is likely to happen. He has co-authored with Rivasseau among others. You might want to check out the general research topics he's been into.

In line with this observed process of tying up loose ends, this paper was just posted today:http://arxiv.org/abs/1012.1739
Lorentz covariance of loop quantum gravity
Carlo Rovelli, Simone Speziale
6 pages, 1 figure
(Submitted on 8 Dec 2010)
"The kinematics of loop gravity can be given a manifestly Lorentz-covariant formulation: the conventional SU(2)-spin-network Hilbert space can be mapped to a space K of SL(2,C) functions, where Lorentz covariance is manifest. K can be described in terms of a certain subset of the 'projected' spin networks studied by Livine, Alexandrov and Dupuis. It is formed by SL(2,C) functions completely determined by their restriction on SU(2). These are square-integrable in the SU(2) scalar product, but not in the SL(2,C) one. Thus, SU(2)-spin-network states can be represented by Lorentz-covariant SL(2,C) functions, as two-component photons can be described in the Lorentz-covariant Gupta-Bleuler formalism. As shown by Wolfgang Wieland in a related paper, this manifestly Lorentz-covariant formulation can also be directly obtained from canonical quantization. We show that the spinfoam dynamics of loop quantum gravity is locally SL(2,C)-invariant in the bulk, and yields states that are preciseley in K on the boundary. This clarifies how the SL(2,C) spinfoam formalism yields an SU(2) theory on the boundary. These structures define a tidy Lorentz-covariant formalism for loop gravity."

This paper refers to 1010.1939 the October "A Simple Model..." paper as giving the current definitive version of the theory.
What they are doing is showing its mathematical equivalence to whatever alternative formulations may need to be constructed in order to prove the desired results, like in this case Lorentz covariance.

Interestingly, today's paper draws on the work of Sergey Alexandrov, who has been a constructive critic of LQG. Several papers by Alexandrov are cited.

 

[marcus]

Back in post #44, mid November, I was beginning to keep track of some of the Early Universe Phenomenologists working on testing LQG and related.

This is the real sign that Loop has reached a satisfactory state---phenoms are spontaneously gathering around scrutinizing it. They want to test (whether or not Loop people like the idea, opinions may differ) and think that they can.

I just learned that SHINJI TSUJIKAWA a Tokyo U phenomenologist has a "Loop falsifiable by CMB" paper in preparation. ...
...
...
Here's my provisional Early-Universe QG Phenomenologist Honor Roll alphabetized by surname :

Aurelien Barrau
Julien Grain
Sabine Hossenfelder
Shinji Tsujikawa
Wen Zhao

I want to update that list and add Mairi Sakellariadou---she has some LQG papers (likewise string and brane) but her main interest now seems to revolve around Spectral Geometry (Connes-NCG) and how it might say things about the early universe that you could check by looking at the ancient light. It is always possible that LQG and the NCG model of matter could come together in the early universe, where both theories mean something real. LQG means bounce and inflation (the inflation made natural by the bounce). And energy is high enough that NCG-style matter comes into its own.

So I want to add Sakellariadou to the "EUP honor roll" (early universe phenomenology). And maybe put some web pages in that introduce these people.

Wen Zhao (Cardiff)
http://www.astro.cardiff.ac.uk/contactsandpeople/?page=full&id=455

Mairi (King's College London)
http://www.kcl.ac.uk/schools/nms/physics/people/academic/sakellariadou/

Aurelien Barrau (Grenoble)
http://en.wikipedia.org/wiki/Aurélien_Barrau
http://lpsc.in2p3.fr/ams/aurelien/index_eng.html

Sabine Hossenfelder (NORDITA-Stockholm)
http://www.nordita.org/people/index.php?variant=single&u=sabineh

Julien Grain (Paris-Sud, CNRS-Orsay)
http://www.ora.nsysu.edu.tw/FT-FoS/downloads/CV/F_PH_Speaker_CV_Grain.pdf

Shinji Tsujiikawa (Tokyo)
http://www.rs.kagu.tus.ac.jp/shinji/Tsujikawae.html
http://relativity.livingreviews.org/About/authors.html#anchor-T (scroll down 5, in the T section)

Also you can expand the iist by looking up the co-authors that work with these six.

 

[marcus]

As samples of the kind of work being done by some of the phenomenologists I just mentioned, here are excerpts from a few earlier posts:
==quote==

... LQG has matured to the point of being testable with the next generation of CMB spacecraft . The proposed NASA B-Pol mission for example--how soon such steps are taken depends mainly economic and political conditions, there are no technical barriers.
http://www.b-pol.org/index.php

...
...

http://arxiv.org/abs/1011.1811
Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters
Julien Grain, A. Barrau, T. Cailleteau, J. Mielczarek
12 pages, 5 figures
(Submitted on 8 Nov 2010)
"Cosmological models where the standard Big Bang is replaced by a bounce have been studied for decades. The situation has however dramatically changed in the last years for two reasons. First, because new ways to probe the early Universe have emerged, in particular thanks to the Cosmic Microwave Background (CMB). Second, because some well grounded theories -- especially Loop Quantum Cosmology -- unambiguously predict a bounce, at least for homogeneous models. In this article, we investigate into the details the phenomenological parameters that could be constrained or measured by next-generation B-mode CMB experiments. We point out that an important observational window could be opened. We then show that those constraints can be converted into very meaningful limits on the fundamental Loop Quantum Cosmology (LQC) parameters. This establishes the early universe as an invaluable quantum gravity laboratory."

http://arxiv.org/abs/1007.2396
Constraints on standard and non-standard early Universe models from CMB B-mode polarization
Yin-Zhe Ma, Wen Zhao, Michael L. Brown
(Submitted on 14 Jul 2010)
"We investigate the observational signatures of three models of the early Universe in the B-mode polarization of the Cosmic Microwave Background (CMB) radiation. In addition to the standard single field inflationary model, we also consider the constraints obtainable on the loop quantum cosmology model (from Loop Quantum Gravity) and on cosmic strings, expected to be copiously produced during the latter stages of Brane inflation. ...

(a) these three models of the early Universe predict different features in the CMB B-mode polarization power spectrum, which are potentially distinguishable from the CMB experiments;
...
...
(d) future CMB observations (both satellite missions and forthcoming sub-orbital experiments) will provide much more rigorous tests of these early Universe models."... a recent paper by Barrau. It is a write-up of the talk he delivered at the ICHEP in Paris (International Conference on High Energy Physics):

http://arxiv.org/abs/1011.5516
Inflation and Loop Quantum Cosmology
Aurelien Barrau
5 pages, Proceedings of the 35th International Conference on High Energy Physics, Paris, 2010 (ICHEP 2010)
(Submitted on 24 Nov 2010)
"On the one hand, inflation is an extremely convincing scenario: it solves most cosmological paradoxes and generates fluctuations that became the seeds for the growth of structures. It, however, suffers from a 'naturalness' problem: generating initial conditions for inflation is far from easy. On the other hand, loop quantum cosmology is very successful: it solves the Big Bang singularity through a non-perturbative and background-independent quantization of general relativity. It, however, suffers from a key drawback: it is extremely difficult to test. Recent results can let us hope that inflation and LQC could mutually cure those pathologies: LQC seems to naturally generate inflation and inflation could allow us to test LQC."

==endquote==

 

[marcus]

A question always in the background in this thread is "how to add matter".
http://owpdb.mfo.de/show_workshop?id=783
I will quote excerpts from the organizers of the February 2010 Oberwolfach workshop:

==quote MFO document http://www.mfo.de/programme/schedule/2010/06b/OWR_2010_09.pdf ==
Noncommutative Geometry and Loop Quantum Gravity: Loops, Algebras and Spectral Triples
Organised by Christian Fleischhack (Paderborn) Matilde Marcolli (Pasadena) Ryszard Nest (Copenhagen)
February 7th – February 13th, 2010
Abstract. Spectral triples have recently turned out to be relevant for different approaches that aim at quantizing gravity and the other fundamental forces of nature in a mathematically rigorous way. The purpose of this workshop was to bring together researchers mainly from noncommutative geometry and loop quantum gravity –-two major fields that have used spectral triples independently so far–- in order to share their results and open issues.Introduction by the Organisers
The workshop “Noncommutative Geometry and Loop Quantum Gravity: Loops, Algebras and Spectral Triples” has been organized by Christian Fleischhack (Paderborn), Matilde Marcolli (Pasadena), and Ryszard Nest (Copenhagen). This meeting was attended by 23 researchers from 8 countries, including several younger postdocs and two PhD students. We enjoyed 16 talks lasting about 50 to 75 minutes plus discussions. As there were no “official” talks after lunch until 4 pm and also no talks in the evening, there was a large amount of time left for informal discussions.

The task of defining both a consistent and mathematically rigorous theory of quantum gravity is one of most challenging undertakings in modern theoretical physics. It is widely expected that at Planck scale the usual notions of smooth geometries have to be replaced by something different. Various arguments point towards geometric notions becoming noncommutative, so that geometric measurements should correspond to noncommuting operators.

In fact, noncommutative geometry (NCG) provides a remarkably successful framework for unification of all known fundamental forces. Mathematically, it mainly grounds on the pioneering work of Connes, who related Riemannian spin geometries to a certain class of spectral triples over commutative C∗-algebras. Extending this formalism, Chamseddine and Connes demonstrated that the standard model coupled to gravitation naturally emerges from a spectral triple over an almost commutative C∗-algebra together with a spectral action. This way they even entailed experimentally falsifiable predictions in elementary particle physics. However, although fully implementing the idea of unification, this approach has remained essentially classical. Moreover, as the theory of spectral triples has only been developed for Riemannian manifolds, full general relativity needing Lorentzian geometries has not been tackled.

Loop quantum gravity (LQG), on the other hand, is one of the most successful theories to quantize canonical gravity. Resting on a generalization of Dirac quantization by Ashtekar and Lewandowski, its decisive idea is to break down the quantization to finite-dimensional problems on graphs and then to reconstruct the continuum theory using projective/inductive limits over all graphs. Although the kinematical part of LQG is nicely understood, the dynamical part is vastly open territory – both mathematically and conceptually. This concerns mainly three, related issues: First of all, the spectral analysis of the quantum Hamiltonian constraint, responsible for time evolution, is very immature. Secondly, it is completely unknown how to reconstruct classical general relativity as a semiclassical limit of loop quantum gravity. And, instead of an emergent unification, matter has to be included by hand.

Although NCG and LQG use very similar mathematical techniques –- e. g., operator algebras in general, or spectral encoding of geometry to be more specific -–, their conceptual problems are rather complementary. Nevertheless, only recently, first steps to join the strengths of both approaches have been made. In several papers since 2005, Aastrup and Grimstrup, later with one of the organizers (RN), have outlined how to construct a semifinite spectral triple for the full theory out of spectral triples based on a restricted system of nested graphs.

One of the main tasks of the meeting was to bring together researchers from different fields – first of all, noncommutative geometry and loop quantum gravity, but also other fields like spectral triples on its own and axiomatic quantum field theory. For this, there were several introductory talks:

• Hanno Sahlmann and Thomas Thiemann gave an overview on the origins and the current status of loop quantum gravity. Sahlmann focused on physical and kinematical issues, Thiemann on open issues concerning dynamics.

• Giovanni Landi and Walter van Suijlekom presented introductions into noncommutative geometry. Whereas Landi spoke on general issues, Walter van Suijlekom showed how one can encode the standard model of particle physics within the language of spectral triples.

• Johannes Aastrup and Jesper Grimstrup demonstrated how spectral triples can fruitfully transfer ideas from noncommutative geometry into loop quantum gravity.

• Klaus Fredenhagen and Rainer Verch introduced axiomatic quantum field theories as functors from the category of globally hyperbolic spacetimes into that of C∗-algebras. Fredenhagen concentrated on perturbation theory, i.e., such functors that are formal power series in . Verch used this framework to extend the notion of spectral triples to the Lorentzian case.

Beyond these talks there have been more specialized ones:
• Alan Carey described a generalization of spectral triples, so-called semifinite spectral triples. They arise naturally in the Aastrup-Grimstrup-Nest approach.

• Matilde Marcolli and Jerzy Lewandowski studied further noncommutative structures arising in loop quantum gravity. Marcolli described how extended spin foams define noncommutative coordinate algebras; Lewandowski replaced the underlying structure group SU(2) of LQG by the quantum group SUq(2).

• Victor Gayral and Thomas Krajewski spoke on quantum groups as well: Gayral from a more generalized perspective, Krajewski inspired by string theory.

• Fedele Lizzi described noncommutative lattices that may lead to emerging spacetime.

• Varghese Mathai and Raimar Wulkenhaar explained different types of deformation quantization. Mathai constructed noncommutative principal bundles and Wulkenhaar outlined why there should be non-perturbative quantum field theories over Moyal deformed R4.

The atmosphere within the workshop benefited very much from the liveliness of the discussions and questions, which occurred frequently before, during, and after the talks. From this point of view the meeting was very successful, on the one hand for enabling a significant exchange of ideas between researchers in the two major fields, and on the other side for presenting the results of the few scientists that work in the intersection of LQG and NCG. In particular the fact that for every talk usually at least half the audience was no specialist in the field covered in it, resulted in a very effective exchange of knowledge, from which both sides gained profit.
==endquote==

If you scroll down further you will find descriptive summaries of many of the talks. In the group photo here:
http://owpdb.mfo.de/detail?photo_id=12390
Fleischhack, one of the organizers, is on the far left, and I believe it is Thiemann third from the left. Richard Nest, another of the organizers, is on the far right. I don't recognized Sahlmann--perhaps he is third from the right with jeans and a black pullover.

 

[MTd2]

What is the largest GUT that Conne`s NCG predicts?

 

[marcus]

You could start a thread called "Connes GUT?" and get various people's opinions on that.

Connes' recent paper allows for the finite space F to change at very high energies. I gather that his predictions are about what one can eventually see with LHC and conceivable extensions along the same lines. In that range, where prediction is practical and meaningful, he has already determined what the finite algebra F must be. So the predictions which he lists are based on that.

I would not advise anyone to suppose that Spectral Geometry simply consists of Connes version of it. I don't think that the question in this thread is addressed by focusing on Connes version NCG and imagining that one simply layers that (in its 2010 form) on top of LQG. So it's not clear how talking about Connes NCG specifically is relevant to the topic. But I'm happy to do so!

The current version is defined by three 2010 papers:

http://arxiv.org/abs/1008.3980
Noncommutative Geometric Spaces with Boundary: Spectral Action
Ali H. Chamseddine, Alain Connes
26 pages, J.Geom.Phys.61:317-332,2011

http://arxiv.org/abs/1008.0985
Space-Time from the spectral point of view
Ali H. Chamseddine, Alain Connes
19 pages. To appear in the Proceedings of the 12th Marcel Grossmann meeting

http://arxiv.org/abs/1004.0464
Noncommutative Geometry as a Framework for Unification of all Fundamental Interactions including Gravity. Part I
Ali H. Chamseddine, Alain Connes
56 pages, Fortschritte der Physik,58:553-600, 2010

Here are the predictions/postdictions listed in 1004.0464:

==quote Ali and Alain==
...We re-derive the leading order terms in the spectral action. The geometrical action yields unification of all fundamental interactions including gravity at very high energies. We make the following predictions:

(i) The number of fermions per family is 16.

(ii) The symmetry group is U(1)xSU(2)xSU(3).

(iii) There are quarks and leptons in the correct representations.

(iv) There is a doublet Higgs that breaks the electroweak symmetry to U(1).

(v) Top quark mass of 170-175 Gev.

(v) There is a right-handed neutrino with a see-saw mechanism. Moreover, the zeroth order spectral action obtained with a cut-off function is consistent with experimental data up to few percent.

We discuss a number of open issues. We prepare the ground for computing higher order corrections since the predicted mass of the Higgs field is quite sensitive to the higher order corrections. We speculate on the nature of the noncommutative space at Planckian energies and the possible role of the fundamental group for the problem of generations.
==endquote==

The Connes model is what they call "almost commutative" where the relevant object is the product of a conventional commutative algebra C(M) with a small finite noncommutative F.
The blue highlight suggests that F can change at Planckian energies! This leaves the model open to new physics. It says that the geometry of spacetime can change radically as you increase the magnification.

The red highlight is how Connes recovers from his pre-2008 bad estimate of Higgs mass. He prepares the ground for higher order corrections, but at this time he does not calculate those corrections.

If you think of Connes "almost commutative" space as a sandwich of |F| different colored copies of ordinary 4D space---a finite sandwich of layers determined by F---then as you zoom into Planckian magnification the number of layers and the coloring can change.

The basic object, as I see it, is still an ordinary 4D manifold M, which we treat via the algebra of continuous functions C(M) defined on M. And then drink a little Connes kool-aid and we see that the right algebra is not simply C(M) but is, in fact, C(M) x F,

the cartesian product of the functions on the manifold M, with a little finite matrix algebra.

Pictorially it is as if M has changed to a sandwich of layers each of which looks like M but has an "F-color".

This is a radical oversimplification of course. If you don't like it then make up your own radical oversimplification.

Now Connes, in the next paper, the one presented at the 2009 Paris Marcel Grossmann, takes the bold step of speculating that if you go to REALLY high energies then even C(M) which you thought was the conventional algebra of functions on a classical 4D manifold becomes, itself, a large but finite algebra of matrices! This is something they didn't tell you when you bought your ticket and walked into the crystal palace.
http://www.icra.it/MG/mg12/en/
http://www.icra.it/MG/mg12/en/invited_speakers_details.htm#connes

αβγδεζηθικλμνξοπρσςτυφχψω...ΓΔΘΛΞΠΣΦΨΩ∏∑∫∂√∧± ÷←↓→↑↔~≈≠≡≤≥½∞⇐⇑⇒⇓⇔∴∃ℝℤℕℂ⋅

 

[marcus]

As you know I consider that LQG-with-simple-matter has reached a satisfactory level of development because it is now TESTABLE. The theory's consequences for the geometry of the early universe have been investigated for over 10 years and the bounce prediction is generic and robust. The visible consequences have been worked out by experts in early universe phenomenology, whose interest is in testing, not in promoting this or that QG.
(see papers by Wen Zhao et al or by Aurelien Barrau et al.)

So it is high time to think about adding a richer variety of matter to LQG. And it's fairly clear that the community is doing just that. So what clues do we see about how that is going?

One clue is the makeup of the 2011 Zakopane QG school. This is a two week school around the beginning of March 2011, when the ski is good at Zakopane. The signs are that people think of including matter by NCG and maybe also GFT. There is to be a series of lectures by Steinacker and also possibly by Krajewski (not yet confirmed) and also possibly by Connes.
Let's look at this in context by reviewing the list of lecture series.
https://www.physicsforums.com/showthread.php?t=457381

 

[marcus]

Here is the list of lecture series around which the two-week school is structured. What can we learn from it about how people in the community think matter might be incorporated?
==quote==

Core lectures

The heart of the program will be a series of core lectures by leading researchers which will give students a solid introduction to the topics of the school.

Hanno Sahlmann/Kristina Giesel - Loop quantum gravity
The field of loop quantum gravity is the technically highest developed construction in quantum gravity. As in the last two schools there will be a thorough introduction into the underlying ideas and mathematical methods. The lectures will cover the basic construction of the kinematical hilbert space and some simple operators, working up to the dynamical Hilbert spaces and physical Hamiltonians following from the deparametrization models.

Carlo Rovelli - Spin foams
The most active field in the network in the last years has been spin foam models, starting with the development of the graviton propagator and the new models, to coherent state techniques and recent asymptotic results, the generalisation to arbitrary 2-complexes and cosmological applications. The lectures will present the current perspective on the construction of these models in terms of 2-complexes.

Harold Steinacker - Non-commutative geometry and matrix models
Non-commutative geometry is a natural extension of geometry in the context of quantum theories that potentially, may also include gravity.. NCG naturally occurs in particle physics, as shown by Alain Connes, and also appears naturally in the context of three-dimensional quantum gravity via Chern-Simons theory. It is also used as a technical tool in state sum models, particularly via quantum groups, which provide deformations of the usual spin network calculus which can be used to construct quantum gravity models. The lectures will cover the definition and construction of non-commutative spaces as well as the construction of QFTs on them. Another theme will be the relationship to matrix models.

Thomas Krajewski (to be confirmed) - Group field theories
Recently group field theories, generalisations of matrix models to higher dimensions, have received renewed attention. In the last year work has begun to take them serious as quantum field theories and analyse their properties using the tools of QFT. The lectures will cover the general structure of GFTs and introduce the QFT tools used to study their renormalisation theory.

Stefan Hollands - Exact QFT in curved backgrounds
QFT on curved backgrounds is the formulation of QFT which does not require the symmetries of Minkowski or (Anti) de Sitter space times. From the mathematical point of view this is the highest development of QFT. It is also an important intermediate step between the standard QFT and quantum gravity. As an approximation to quantum gravity it supplies some of the most potent intuitions of the field (holography, black hole entropy). The lecture will cover the recent results and successes in the exact construction of these quantum field theories.

Alain Connes (to be confirmed) - Non-commutative geometry
===endquote===

There will also be individual auxilliary lectures to follow up on the introductory core lecture series in most cases by presenting some more advanced or specialized ideas suitable for research. Here are a few of these, as a sample (not the full list.)

Singh - Loop quantum cosmology
Jurkiewicz (an Ambjorn Loll co-author) on CDT
Barrett on the large j limit of spinfoam amplitudes
Rivasseau on EPRL-GFT
Noui on SL(2,C)_q the quantum deformation of the Lorentz group
http://www.fuw.edu.pl/~kostecki/school3/

Basically it looks to me as if the whole Loop contingent has wheeled around to confront the MATTER issue. Because in the introductory lecture series that in a sense define the field for an entry-level researcher, there is a big representation of NCG and GFT and curved background QFT. This is what somebody guesses you likely need to move ahead on the matter issue.

 

[marcus]

More on the including matter front. This was posted today:

http://arxiv.org/abs/1012.4719
Spinfoam fermions
Eugenio Bianchi, Muxin Han, Elena Magliaro, Claudio Perini, Carlo Rovelli, Wolfgang Wieland
8 pages
(Submitted on 21 Dec 2010)
"We describe a minimal coupling of fermions and Yang Mills fields to the loop quantum gravity dynamics. The coupling takes a very simple form."

My first take, on reading portions and glancing through the rest: this paper is great.

 

[MTd2]

Are these YM already quantized? Where are the bosons, then?

 

[marcus]

Are these YM already quantized? Where are the bosons, then?

At this point I can only suggest read section VII. It is short.

BTW I think in a dynamically curved spacetime the idea of a particle is non-essential and poorly defined. Particles are more at home in flat, or other prearranged geometries.

 

[genneth]

At this point I can only suggest read section VII. It is short.

BTW I think in a dynamically curved spacetime the idea of a particle is non-essential and poorly defined. Particles are more at home in flat, or other prearranged geometries.

Actually, reading the paper, it seems the concept is not so hard --- after all, the current spinfoam incarnation is conceptually a quantum (i.e. linearly superposable) discretised geometry. As has been known for a while, on the classical level fermions + YM can be written as a theory of gauge strings connected by fermions; this paper I believe simply (!) implements that idea. Thus, particle states are localised --- but at the same time slightly delocalised --- to spacetime vertices, which means that at each vertex you get a set of fermion states (0, +/- or 2).

I'm not entirely sure at the moment what they mean by using the gravitational radiative corrections to generate the YM action, but I suspect they mean by a Einstein-KK-esque argument, on the quantum level.

I find all this development to be massively exciting, though in the end the proof will be in the form of concrete calculations (and of course, experimental verification of said calculations).

 

[MTd2]

BTW I think in a dynamically curved spacetime the idea of a particle is non-essential and poorly defined. Particles are more at home in flat, or other prearranged geometries.

So what is responsible for the transmission of forces? There is no gauge particles now?