If LQG now satisfactory, how to add matter?

[qsa]

Most of this seems pretty much wishful thinking. I see a huge list of "may" , "promising", "future research" in all those papers, with no real concrete demonstration that any of these zillions of vague proposals may actually work. Kicking around ideas is easy, but getting something to work, even approximately, is not a minor detail, but actually the main part of the problem!

I understand that this is work in progress, but to be fair one should note that if string physicists would get much heat for hype of similar caliber; while the theory is much further developed.

I can understand the title of the thread only as ironic - didn't the recent paper of Alexandrov and Roche exhibit that there are serious problems with LQG at a basic level, so before one looses oneself in speculations about how to possibly add matter, shouldn't one first make sure that any of these many different attempts that one may loosely call "LQG" make sense at all?

LQG carries matter in the following way from Smolin's paper. It is 99% similar to my own idea coming from a very different angle. I am very astonished that nobody so far has mentioned this idea which he has been working very hard on it, and he even tied it to Lisi's idea.

http://arxiv.org/PS_cache/arxiv/pdf/0712/0712.0977v2.pdf



Consider a graph as in
Figure (1) which is regular and therefor may occur in the superposition of states making
up a semiclassical state associated with a flat metric. There is in loop quantum gravity,
no apparent energy cost to contaminating that lattice-like graph with non-local links as
shown in the figure. Nor is there an incompatibility with the semiclasicality of the state.
As there are many more ways to add a link to a lattice that connects two far away nodes
than two nearby nodes, there is an instability for the formation of such non-local links
as the universe expands from Planck scales. Moreover, once inserted in a graph, nonlocal
links are trapped, as they can only be eliminated if two of them annihilate by the
coincidence of their ends arriving by local moves at neighboring nodes. The proposal
is then that these act as Planck scale wormholes, carrying quantum numbers associated
with gauge fields carried by the non-local link.

Let us consider observations made by a local observer in the neighborhood of x. From
their point of view the edge exy simply comes to an end, that is it appears to connect
to a one valent node. But ends, or one valent nodes in loop quantum gravity represent
matter degrees of freedom. Thus, the dislocations due to disordered locality appear in the semiclassical limit as matter degrees of freedom.
Let us suppose that the gauge group is SU(2)⊗H, where H is an internal gauge symmetry.
Then the edge exy carries representations of these groups, (j, r). Local observers will describe exy as a particle of spin j and charge r.This leads to a picture in which for every generator of G, the gauge symmetry, the
semiclassical limit has a gauge field plus a set of particle excitations given by the representations of G.

 

[tom.stoer]

But still "most of this seems pretty much wishful thinking".

In a second paper Smolin tried to explain the cosmological constant via these non-local links. In addition Smolin proposes Bilson-Thompson. And then one can add matter by hand ...

The conclusion is that LQG carries a rich and (to a large extend) unexplored structure. Some known effects may be explained via these structures, some new effects may arise. Some unwanted effects may rule out LQG, ..., everything is possible.

 

[suprised]

If I remember correctly, the Bilson-Thompson model is a preon model which is equivalent to Haim Harari's Rishon model. That model had suddendly disappeared many years ago... why? because a smart student of Harari, Nathan Seiberg, had shown that this model is inconsistent due to anomalies. Well, since anomalies seem to be neglected in the LQG community, I am not surprised that they revive the Rishon model. Indeed, back to the 70s...

Incidentally, the question of matter is an interesting one also from the following perspective. If string theory is any right, matter is necessary for internal consistency. Pure gravity would not be consistent. It would be interesting to see whether LQG comes up with a similar consistency constraint. If not, then this would be a clear dividing line between strings and LQG. I guess it is too early to see because LQG seems so far to be plagued by all sorts of problems; but perhaps some day this issue can be sharpened.

 

[marcus]

Haven't heard much about braid matter for the past couple of years. I hope that in this discussion thread we can get back to the main topic---the ways currently being considered to include matter.

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."

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. We first examine the observational features of the three models, and then use current B-mode polarization data from the BICEP and QUaD experiments to constrain their parameters. We also examine the detectability of the primordial B-mode signal predicted by these models and forecast the parameter constraints achievable with future CMB polarization experiments. We find that:
(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;

(b) since B-mode polarization measurements are mostly unaffected by parameter degeneracies, they provide the cleanest probe of these early Universe models;

(c) using the BICEP and QUaD data we obtain the following parameter constraints:
r=0.02^{+0.31}_{-0.26} (1 sigma for the tensor-to-scalar ratio in the single field inflationary model);

m < 1.36\times 10^{-8} \text{M}_{\text{pl}} and k_{*} < 2.43 \times 10^{-4} \text{Mpc}^{-1} (1 sigma for the mass and scale parameters in the loop quantum cosmology model);

G\mu < 5.77 \times 10^{-7} (1 sigma for the cosmic string tension);

(d) future CMB observations (both satellite missions and forthcoming sub-orbital experiments) will provide much more rigorous tests of these early Universe models."

 

[qsa]

But still "most of this seems pretty much wishful thinking".

In a second paper Smolin tried to explain the cosmological constant via these non-local links. In addition Smolin proposes Bilson-Thompson. And then one can add matter by hand ...

The conclusion is that LQG carries a rich and (to a large extend) unexplored structure. Some known effects may be explained via these structures, some new effects may arise. Some unwanted effects may rule out LQG, ..., everything is possible.

1-Consider particles as lines extending from the particle to everywhere in the universe.

2-generate these lines by throwing a random number, make it on a line i.e. 1D as an example
do above for two particles sitting each at the opposite side of a universe of 10^40 in atomic units-size of the proton-(size of our universe). Throw 10^41 times.

3-if you consider gravity as when both lines meet you have a probability of 1 in 10^40

4-if you consider EM force as when these lines cross one another p is close to .99

you can see the ratio, can't you. trust me ,forces are related to these probabilities.

increasing(decreasing) the universe size changes the ratio, EM stays .99. This is Diracs large number hypothesis(Google). Numbers are approximate. I hope I show details soon.

Quantum gravity in four lines.

 

[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==