Reformulation of Loop gravity in progress, comment?

[marcus]

Ultimately if you think of Loop gravity as based on a fixed set of discrete points or a smooth manifold "continuum" then you aren't likely to understand the line of future progress I'm talking about.

The Dirac canonical quantization applied to GR leads to a lot of paraphernalia which it is NOT appropriate to assume (Hartle suggests) unless spacetime geometry is behaving in approximately classical manner. It's a picture that only "emerges" under specific tame circumstances.
That goes for approaches using EMBEDDED spin networks as well. They need a manifold--i.e. extra baggage.

Conversely the spinfoam dynamics approach, by now familiar to everybody, does not involve extra baggage--in particular, no manifold. It is based on what Dowker's abstract seems to be talking about: events. Related combinatorially. No infinite sets, just a finite web of facts/predictions, depending entirely on the history!

So we'll see. We'll watch the video of Dowker's talk and see if it fits with and extends what Hartle had to say to the 2005 Solvay Conference.

Dowker may steer the talk in the Causal Sets direction but that's all right. Loop and Causal Sets share foundation roots--to some extent a common rationale. Eventually "Quantum Foundations" considerations are going to influence the development of Loop gravity--indeed they may already have influenced it to a considerable extent.
================================
It may help clarify the issues if I paste in a short summary of how I see Hartle's "Decoherent Histories" (DH) version of quantum mechanics. This was originally a post in the "Loop future" thread:


Hartle and friends propose a reformulation of Quantum theory we can call "Histories" QM which basically says that the machinery of Dirac quantization does not exist--it is merely emergent at low energies, a convenient workable approximation to reality over a limited range. The spacelike 3D manifold does not exist in reality. To formulate QM, you need three things:
A. Histories
B. Partitions of histories (grouping, classifying, "coarsegraining" them)
C. a Decoherence functional that tells you when a given partition is bettable.

Sets in a partition represent things you might like to know or to predict. A given partition is bettable when you can assign fair odds (approximate conventional probabilities) to it, make predictions, settle bets, in other words make honest book on it.
The Decoherence functional tells you when a partition of the histories is sufficiently uncorrelated that the probabilities will be additive---interference is small enough to be considered negligible.

Hartle Histories QM is, I believe gaining acceptance. So it makes sense to me, in that light, that the Erlangen group should be moving away from a strict Dirac quantization and in the direction of DUST.
...[That gets you to a nice effective halfway station. Since it's not fundamental, why not make life easy and assume some dust? Going further down that road brings you to Histories=Foams]

 

[tom.stoer]

The Dirac canonical quantization applied to GR ... That goes for approaches using EMBEDDED spin networks as well. They need a manifold--i.e. extra baggage.

Conversely the spinfoam dynamics approach, by now familiar to everybody, does not involve extra baggage--in particular, no manifold.

I think this is misleading.

I agree that spin networks are constructed from a manifold and that one get's rid of the manifold during quantization, constraint fixing an "projecting" to phys. d.o.f. = spin networks. Therefore in some sense spin networks have (or had) this extra baggage (historically).

But I do not agree that SFs do not have this extra baggage. They are constructed using the same ideas as spin networks; the only difference is that one switched from networks to foames rather late. There is no conceptual difference between spin networks and spins. It's a matter of taste whether you postulate a kinematical Hilbert space and a Hamiltonian or whether you postulate vertex amplitudes and PI measures.

Spin foams and spin networks share the same weakness; historically they are rooted in a picture using a manifold - and their derivation is by no means complete. Not deriving but postulating them has a different weak point, namely guessing ;-)

Nevertheless I agree that the main weak points could be that one is simply quantizing the wrong degrees of freedom (just like QFT applied to Navier-Stokes equations). These are essentially two weak points
1) wrong d.o.f.
2) quantization (which is never unique)

 

[marcus]

...
I agree that spin networks are constructed from a manifold...

But I do not agree that SFs do not have this extra baggage. They are constructed using the same ideas as spin networks; ...

Sorry, you misunderstood. In the modern treatment spin networks are NOT constructed from a manifold. It used to be the case that spin networks were EMBEDDED.
When I say "embedded spin network" I mean to older object.

When I simply say "spin network" it is a combinatorial object, as per the standard Loop source paper. It does not have the extra baggage.

As you say, SF are constructed using the same ideas. Therefore they do NOT have manifolds or other extra baggage.

It seems you understood completely opposite from what I intended.
I must try to write more clearly.

 

[tom.stoer]

marcus, it's not fair to say that non-embeded networks are not constructed from a manifold; yes, they are combinatorial objects, but nevertheless they share many features with the embedded one; they are not completely bagge-free, even uif this baggage may be deeply hidden.

The first baggage I see is SU(2) or SO(3); why not SU(7) ?

 

[marcus]

marcus, it's not fair to say that non-embeded networks are not constructed from a manifold; yes, they are combinatorial objects, but...

I am just talking about the facts. The standard LQG formulation is http://arxiv.org/abs/1102.3660 ("Zakopane lectures") and in that paper the theory is developed using non-embedded networks and foams. No manifold representing spacetime continuum.

We both recognize this.

HISTORICALLY much of this grew out of work with similar structures EMBEDDED in a manifold.

So let's make a clean break. We recognize that the theory is now defined with combinatorial objects that represent geometric information. Measurements, predictions, hypothetical measurements, events of one sort or another.

There is no continuum in the theory, all we have is relationships among geometric info.

Now you ask "What about the Lie groups? What about SU(2)?"

Well I'm no authority--I can only tell you how I personally understand it. The choice of Lie group, for me, says something about the kinds of measurements that are being made at various points in the network.

We are trying to DESCRIBE Nature and how she responds to geometric measurement and how her geometry evolves. We think manifolds are unrealistic so we throw them out. Now we have a web of measurements (areas volumes angles...). We pick the best Lie group that describes the symmetries of measurement as we experience them. We pick the group that works best.

That's just how I personally understand it. So then the graph Hilbert space automatically comes out to be the square integrable ("L₂") functions on a product of as many copies of the group G as there are links in the graph( G^L)
Some redundancy has to be factored out but basically that's the graph Hilbert space.

You're surely familiar with this--I'm sure you've read the Zako lectures paper.

I suppose using SU(2) is a way of noting that our world has 3D rotations. We don't go so far as to assign it a differential manifold structure, that would be adding a lot of extra. Unnecessary extra. But we do observe that a local observer can turn and tip things.
So we put in that detail about Nature---rotation.

We are painting a portrait, and SU(2) is the color of her eyes.

So the Hilbert space turns out to be L₂[SU(2)^L]

 

[tom.stoer]

SU(2) is one critical relict of 3+1 dim. spacetime; you can't explain why to use SU(2) w/o referring to 3+1 dim. spacetime.

It is not clear what happens if you start with SU(7) - as an example; it is not clear to which manifold this reduces in the semiclassical limit - or if there is convergence to a Riemann or Riemann-Cartan manifold at all - classically there is no Riemann-Cartan manifold with SU(7) structure group.

I agree that the algebraic structures of non-embedded spin networks do not contain any directly visible relict of the manifold, but besides the structure group there are others: In the canonical formulation there is the operator algebra G^a, V^a and H; at least H survives! in addition when using H there is the requirement for a global foliation like R³*T (with T being the time direction). In the SF framework there are the simplicity constraints which are understandable only when referring to a manifold structure from which the theory has been created, and of course there are vertex amplitudes which are related to some - unknown - hamiltonian H.

So yes, the relicts are deeply hidden, but they are present even for non-embedded spin networks.

 

[marcus]

SU(2) is one critical relict of 3+1 dim. spacetime; you can't explain why to use SU(2) w/o referring to 3+1 dim. spacetime.

It is not clear what happens if you start with SU(7) - as an example; it is not clear to which manifold this reduces in the semiclassical limit - or if there is convergence to a Riemann or Riemann-Cartan manifold at all - classically there is no Riemann-Cartan manifold with SU(7) structure group...

Nice comment! I think it would be interesting in a theoretical/mathematical sense for someone to explore what happens when you use some different Lie groups in the Loop setup.

I think there HAS to be some way of telling the theory about the dimensionality we live in and giving it SU(2) is a kind of minimal way.

Youi don't give it a whole differential manifold with all that extra machinery, you just tell it the rotational symmetry that belongs to our world.

For me that's very satisfying. It is a minimal way of telling the theory what dimensionality we live in. I don't expect the theory to tell me why there MUST be 3+1 dimensions to the world (although perhaps some day a theory WILL tell us that--it would be exciting, to be sure!)

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

I've been thinking about the *embedded* issue and I wonder if we couldn't find a recent paper analogous to 1102.3660 that presents the embedded approach--so then we could have a DUAL standard. Would you like this? Then there need be no tension. At the beginning of my post I could say I am talking about purely combinatorial networks+foams as in 1102.3660 and at the beginning of your post you could say you are talking about embedded ones as in 11xx.yyyy. Maybe some recent paper by Lewandowski?

 

[marcus]

Talking about things that might figure in a reformulation of Loop gravity (showing up next year at GR20 and Loops 2013), one thing we seem to have completely overlooked is the new, hard, and potentially very important OSN line of development by Lewandowski's group.
http://arxiv.org/abs/1107.5185
This is the systematic way to do spinfoams without spinfoams. But you have to learn it like a new language. Jerzy is a mathematician's mathematician. Check it out.
http://arxiv.org/abs/1107.5185
Feynman diagrammatic approach to spin foams
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 26 Jul 2011)
"The Spin Foams for People Without the 3d/4d Imagination" could be an alternative title of our work. We derive spin foams from operator spin network diagrams} we introduce. Our diagrams are the spin network analogy of the Feynman diagrams. Their framework is compatible with the framework of Loop Quantum Gravity. For every operator spin network diagram we construct a corresponding operator spin foam. Admitting all the spin networks of LQG and all possible diagrams leads to a clearly defined large class of operator spin foams. In this way our framework provides a proposal for a class of 2-cell complexes that should be used in the spin foam theories of LQG. Within this class, our diagrams are just equivalent to the spin foams. The advantage, however, in the diagram framework is, that it is self contained, all the amplitudes can be calculated directly from the diagrams without explicit visualization of the corresponding spin foams. The spin network diagram operators and amplitudes are consistently defined on their own. Each diagram encodes all the combinatorial information. We illustrate applications of our diagrams: we introduce a diagram definition of Rovelli's surface amplitudes as well as of the canonical transition amplitudes. Importantly, our operator spin network diagrams are defined in a sufficiently general way to accommodate all the versions of the EPRL or the FK model, as well as other possible models. The diagrams are also compatible with the structure of the LQG Hamiltonian operators, what is an additional advantage. Finally, a scheme for a complete definition of a spin foam theory by declaring a set of interaction vertices emerges from the examples presented at the end of the paper.
36 pages, 23 figures

And then just recently there was the followup on this, which (of course) is included in the 2nd quarter MIP poll!

http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 7 Mar 2012)
We find all the spin-foams contributing in the first order of the vertex expansion to the transition amplitude of the Bianchi-Rovelli-Vidotto Dipole Cosmology model. Our algorithm is general and provides spin-foams of arbitrarily given, fixed: boundary and, respectively, a number of internal vertices. We use the recently introduced Operator Spin-Network Diagrams framework.
23 pages, 30 figures

Note that Jerzy is bilingual---he can talk and think non-embedded Loop (purely combinatorial structures) and also the physical (non-Dirac, non-constraint) Hamiltonian approach with DUST--using (if he chooses) the older embedded structures in a manifold. Here is the talk he gave today at Stockholm MG13:

Lewandowski, Jerzy
Quantizable canonical LQG
Abstract :The canonical quantization scheme can be completed with the framework of Loop Quantum Gravity for several examples of the gravitational field coupled to matter fields. Explicitly, that has been accomplished for the generic dust, non-rotating dust, and massless scalar field. Those results will be presented and recent progress will be discussed...
================================

EDIT TO RESPOND TO NEXT POST.
Hi Tom, since I can still edit I will reply to your post here. You've got an interesting perspective that I want to think about. I can't say much at the moment. I'm intrigued. I need to think about it some more. I'd also like to understand Lewandowski OSN diagrams better---is his approach really useful? I think it is but am not completely sure yet.

Alesci, who is currently Thiemann's postdoc, has chosen to go to Warsaw this autumn, for his next postdoc fellowship. It is ridiculous for me to imagine myself young, and wonder what I would do in his place. But I think, ridiculous as it is, that I would do the same as Alesci, at this point.

 

[tom.stoer]

marcus: let me state clearly that I do believe that the non-embedded networks are the right way to go, that the manifold emerges in a semiclassical limit but that one should (and can) get rid of the manifold in order to define the theory; construction (over 25 years) and definition (in its final formulation) need not be identical. In addition I strongly believe that non-embedded spin networks are in some sense equivalent to spin foams. I think that LQG (after further reformulations) will solve quantization issues, operator algebra anomalies, second-class and simplicity constraints and Dirac quantization, PI measure etc.

Some time ago I started to think about limitations of the current approach and issues that are not addressed by LQG as of today.

I identified one central issue, namely SU(2)! SU(2) emerges from the complexification SL(2,C) is therefore deeply related with the local symmetry structure SO(3,1) of the spacetime manifold. So SU(2) spin networks and its cousins still 'know' something about the spacetime manifold Ashtekar started with. Now, 25 years later, a second central question is the semiclassical limit and the emergence of a smooth spacetime manifold in a certain regime i.e. described by a certain limit of the theory.

The simple question is this: why should the manifold of the semiclassical limit be the same kind of manifold we started with? and why should the dimensions coincide?

Related questions are: what would happen if we start with a different manifold, e.g. a manifold of different dimension? The mathematical tools are much less developed, but afaik Thiemann has done some work in this direction.
And what would happen if we start with a different group for the spin network construction, e.g. SU(7), for which no manifold with SU(7) as its 'structure group' is known? What would be the semiclassical limit of such a spin network?

Regarding quantization, derivation of theories etc. I think Wittgenstein's 'Tractatus' has something interesting to say: "… finally recognizes [my propositions] as senseless, when he has climbed out through them, on them, over them. He must so to speak throw away the ladder, after he has climbed up on it … then he sees the world rightly"

 

[marcus]

I should keep this around and think about it.
==quote==
marcus: let me state clearly that I do believe that the non-embedded networks are the right way to go, that the manifold emerges in a semiclassical limit but that one should (and can) get rid of the manifold in order to define the theory; construction (over 25 years) and definition (in its final formulation) need not be identical. In addition I strongly believe that non-embedded spin networks are in some sense equivalent to spin foams. I think that LQG (after further reformulations) will solve quantization issues, operator algebra anomalies, second-class and simplicity constraints and Dirac quantization, PI measure etc.

Some time ago I started to think about limitations of the current approach and issues that are not addressed by LQG as of today.

I identified one central issue, namely SU(2)! SU(2) emerges from the complexification SL(2,C) is therefore deeply related with the local symmetry structure SO(3,1) of the spacetime manifold. So SU(2) spin networks and its cousins still 'know' something about the spacetime manifold Ashtekar started with. Now, 25 years later, a second central question is the semiclassical limit and the emergence of a smooth spacetime manifold in a certain regime i.e. described by a certain limit of the theory.

The simple question is this: why should the manifold of the semiclassical limit be the same kind of manifold we started with? and why should the dimensions coincide?

Related questions are: what would happen if we start with a different manifold, e.g. a manifold of different dimension? The mathematical tools are much less developed, but afaik Thiemann has done some work in this direction.
And what would happen if we start with a different group for the spin network construction, e.g. SU(7), for which no manifold with SU(7) as its 'structure group' is known? What would be the semiclassical limit of such a spin network?

Regarding quantization, derivation of theories etc. I think Wittgenstein's 'Tractatus' has something interesting to say: "… finally recognizes [my propositions] as senseless, when he has climbed out through them, on them, over them. He must so to speak throw away the ladder, after he has climbed up on it … then he sees the world rightly"
==endquote==
At the moment I find myself without anything helpful to say! Only that what you are talking about is interesting.
I don't know if the following is relevant--it's been on my mind for some time. A kind of backbone of the combinatorial network+foam approach, the Zakopane dynamics as defined last year, is the "f" map from SU(2) representations to SL(2,C) representations. Do you have some insight or perspective on this map? I do not understand why something like this should turn out to be so important.

If you or someone else wanted to experiment by constructing a Zakopane-like setup but with different groups, would you need a pair of groups, and an analogous mapping between their representations? Or could this, perhaps, be avoided?

 

[tom.stoer]

It's especially the math like the f-map which makes the 3+1 stuff so special. I have to check Thiemann's papers on LQG+SUGRA etc. He states ants to study LQG in arbitary dimensions, so he must have found a way to get rid of these special properties of 3+1 dim Riemannian manifolds and SL(2,C).

 

[marcus]

I want to look now at a different direction (or perhaps it is related) that the reformulation of LQG could go, over the next year or two. This is indicated by the Gielen Wise paper on the current MIP poll.

In this paper the authors work with the concepts of "field of observers" and "space of observers". I see this as part of an historical process of the subjectification of spacetime. It is related to the research at Perimeter Institute concerning "Principle of Relative Locality". To me personally, what Gielen Wise are talking about it more interesting than "Relative Locality", and may contain it. But this may simply be arbitrary preference on my part.

Here's the latest one:
http://arxiv.org/abs/1206.0658
Linking Covariant and Canonical General Relativity via Local Observers
Steffen Gielen, Derek K. Wise

In this, Gielen and Wise say they have in preparation a new paper called
Lifting general relativity to observer space

So we will see how soon that one comes out and what, if any, impact it has. Last year the authors posted a couple of papers on this general subject:

http://arxiv.org/abs/1111.7195/
Spontaneously broken Lorentz symmetry for Hamiltonian gravity
Steffen Gielen, Derek K. Wise
http://arxiv.org/abs/1112.2390
The geometric role of symmetry breaking in gravity
Derek K. Wise

 

[marcus]

Some thoughts in back of mind about this: a projectile doesn't HAVE a continuous trajectory.
You can't monitor it along an infinite number of points. You just have a finite series of places you know it's been. A continuous traj. for the projectile doesn't EXIST in nature.
(you can't tell which slit it went thru unless you monitored there.)

A spacetime is like a trajectory. So spacetime does not exist in nature. The universe doesn't HAVE a continuous spacetime geometry.

There are kind of two responses that I see:
1. Hartle's new standard QM. You can partition the histories according to a finite number of factual questions. Some partitions will be sufficiently uncorrelated that you can assign odds and make bets (predictions) and settle bets. No essential role for any observer.
(Of course there is no fundamental space time either. It does not exist in the theory, except as a low energy approximation. Conventional spacetime geometry "emerges" under appropriately "tame" conditions from the more primitive Q&A of Hartle's decoherent partitions of the set of possible histories.

2. Gielen and Wise's response, which I'm still vague about. Instead of fashioning a mathematical model of spacetime geometry (which doesn't exist, that's the problem) you construct a mathematical model of the space of observers which they claim is 7 dimensional. On the face of it, it sounds strange. But I think it's probably worth having a look. In any case, I posted the links to their papers.

 

[marcus]

We know there is some interest in joining LQG with the Hartle "Histories" approach to Quantum Mechanics--at least one grad student working on this. So it makes sense to keep that in our field of vision.

In that connection, Fay Dowker is giving a talk at Perimeter on Tuesday about the Histories approach and in particular how one can recover the Hilbert state space of older QM (under certain assumptions) starting from a PoV in which Histories, not states, are fundamental.
She seems to be one of the main people developing the approach that Hartle initiated.

The talk will probably be based on two 2010 papers. I will get the links:
http://arxiv.org/abs/1002.0589
Hilbert Spaces from Path Integrals
Fay Dowker, Steven Johnston, Rafael D. Sorkin
(Submitted on 2 Feb 2010)
It is shown that a Hilbert space can be constructed for a quantum system starting from a framework in which histories are fundamental. The Decoherence Functional provides the inner product on this "History Hilbert space". It is also shown that the History Hilbert space is the standard Hilbert space in the case of non-relativistic quantum mechanics.
22 pages.

http://arxiv.org/abs/1007.2725
On extending the Quantum Measure
Fay Dowker, Steven Johnston, Sumati Surya
(Submitted on 16 Jul 2010)
We point out that a quantum system with a strongly positive quantum measure or decoherence functional gives rise to a vector valued measure whose domain is the algebra of events or physical questions. This gives an immediate handle on the question of the extension of the decoherence functional to the sigma algebra generated by this algebra of events. It is on the latter that the physical transition amplitudes directly give the decoherence functional. Since the full sigma algebra contains physically interesting questions, like the return question, extending the decoherence functional to these more general questions is important. We show that the decoherence functional, and hence the quantum measure, extends if and only if the associated vector measure does...
23 pages, 2 figures

And here is the link for the PIRSA video of next week's (10 July) seminar talk:
http://pirsa.org/12070001/
The Path Integral Interpretation of Quantum Mechanics
Speaker(s): Fay Dowker
Abstract: In 1932 Dirac wrote that the lagrangian approach to classical mechanics was probably more fundamental than the hamiltonian approach because the former is relativistically invariant whereas the latter is "essentially nonrelativistic". In quantum theory the hamiltonian approach leads to canonincal quantisation, Hilbert space, operators and the textbook rules for state vector "collapse", which are all indeed more or less divorced from the spacetime nature of the physical world as revealed by relativity. The "essentially relativistic" lagrangian approach on the other hand leads to the path integral, as shown by Dirac in 1932 and developed by Feynman. I will show how the interpretation of quantum mechanics in a path integral framework is based directly on events in spacetime and show that it leads to a second "fork in the road" depending on whether it is necessary for probabilities to play a fundamental role in the theory.
Date: 10/07/2012 - 3:30 pm

I suspect that the "second fork in the road" is where one decides whether or not to make an additional assumption (may allow one to recover effective use of the mathematical utilities of conventional state-space QM.)

 

[atyy]

@marcus, one of the things Rovelli likes to say is that time is emergent. In the histories formulation, isn't time primary?

I think Markopoulou has argued that time is primary, so maybe the histories formulation would be more compatible with her viewpoint?

Here's a Markopoulou paper that mentions "histories", but I'm not sure if it's related to Hartle's "histories": http://arxiv.org/abs/gr-qc/0703097

Ooops, from this review, I see the histories approach is Griffiths's!

 

[marcus]

In the case of Dowker's talk (and the tentative exploration of Hartle's QM that I've seen from Marseille) we are following a specific line of development. It might just be a distraction at this point to talk about Griffith's work in 1975, or Markopoulou (who means something else by "causal histories") or the Hohenberg review you linked, which does not use Hartle's terminology and is not focused on this specific line.

If anyone is interested in understanding the significance of Dowker's talk, I would suggest studying Hartle's 2006 paper, that was presented at the 23rd Solvay Conference---whose theme was "The Quantum Structure of Space and Time". Hartle's paper sets out axioms for the decoherence functional which is basic to his particular Histories approach. Of course he acknowledges Griffiths 1975 work but that's ancient history.
I've looked over a bunch of papers and this is a key one:

http://arxiv.org/abs/gr-qc/0602013
Generalizing Quantum Mechanics for Quantum Spacetime
James B. Hartle (University of California, Santa Barbara)
(Submitted on 2 Feb 2006)
Familiar textbook quantum mechanics assumes a fixed background spacetime to define states on spacelike surfaces and their unitary evolution between them. Quantum theory has changed as our conceptions of space and time have evolved. But quantum mechanics needs to be generalized further for quantum gravity where spacetime geometry is fluctuating and without definite value. This paper reviews a fully four-dimensional, sum-over-histories, generalized quantum mechanics of cosmological spacetime geometry. This generalization is constructed within the framework of generalized quantum theory. This is a minimal set of principles for quantum theory abstracted from the modern quantum mechanics of closed systems, most generally the universe. In this generalization, states of fields on spacelike surfaces and their unitary evolution are emergent properties appropriate when spacetime geometry behaves approximately classically. The principles of generalized quantum theory allow for the further generalization that would be necessary were spacetime not fundamental...
31 pages. 4 figures.

To paraphrase, states and evolution of fields defined on spacelike surfaces are ONLY appropriate as math idealizations when geometry behaves APPROXIMATELY CLASSICALLY. In more general situations such idealizations are NOT appropriate.
They are, as Dowker puts it in her seminar talk abstract, "more or less divorced from the spacetime nature of the physical world".

 

[atyy]

So is time fundamental in Dowker's approach?

The Hartle paper has a beautiful quote: ‘Traveler, there are no paths, paths are made by walking.’ It's also interesting that David Gross delivered Hartle's talk.

 

[marcus]

So is time fundamental in Dowker's approach?

The Hartle paper has a beautiful quote: ‘Traveler, there are no paths, paths are made by walking.’ It's also interesting that David Gross delivered Hartle's talk.

There is no time. Time is made by histories
As I recall, David Gross was the chairman and main organizer of the 23rd Solvay.
He would have decided the theme "The Quantum Structure of Space and Time" and, I guess,
invited Hartle to contribute a paper. For whatever reason, Hartle was unable to make it to the conference and so the paper was presented in his stead.

If anyone is interested in watching, last year Hartle gave a talk at Perimeter on related matters. Here's a video:
http://pirsa.org/11020124/
Quantum Mechanics with Extended Probabilities
Speaker(s): James Hartle
Abstract: We present a new formulation of quantum mechanics for closed systems like the universe using an extension of familiar probability theory that incorporates negative probabilities. Probabilities must be positive for alternative histories that are the basis of settleable bets. However, quantum mechanics describes alternative histories are not the basis for settleable bets as in the two-slit experiment. These alternatives can be assigned extended probabilities that are sometimes negative. We will compare this with the decoherent (consistent) histories formulation of quantum theory. The prospects for using this formulation as a starting point for testable alternatives to quantum theory or further generalizations of it will be briefly discussed.
Date: 07/03/2011 - 11:00 am
Here's another, but not so recent:
http://pirsa.org/07090064/

 

[atyy]

In Dowker's http://arxiv.org/abs/1002.0589 section 3.2, it looks like time is fundamental for defining a history.

 

[atyy]

The Dowker talk abstract says something about a deterministic versus probabilistic formulation. Is that in her published work, or is that new?

 

[marcus]

In Dowker's http://arxiv.org/abs/1002.0589 section 3.2, it looks like time is fundamental for defining a history.

I don't see any evidence of that. At the beginning of section 3 they say plainly that they are considering a special case. And a time variable IS employed in that setup.

Throughout the paper they are building bridges and comparisons between their Histories approach and conventional QM, especially the example of a conventional non-relativistic particle moving in d-dimensional Euclid space according to a conventional clock. They are interested in showing that their theoretical framework can handle that and get the same results as the conventional one.

So in that paper they are always studying examples in which there IS time. But time does not appear in their axioms. So I think you are mistaken about it looking like it's fundamental.

 

[marcus]

...here is the link for the PIRSA video of next week's (10 July) seminar talk:
http://pirsa.org/12070001/
The Path Integral Interpretation of Quantum Mechanics
Speaker(s): Fay Dowker
Abstract: In 1932 Dirac wrote that the lagrangian approach to classical mechanics was probably more fundamental than the hamiltonian approach because the former is relativistically invariant whereas the latter is "essentially nonrelativistic". In quantum theory the hamiltonian approach leads to canonincal quantisation, Hilbert space, operators and the textbook rules for state vector "collapse", which are all indeed more or less divorced from the spacetime nature of the physical world as revealed by relativity. The "essentially relativistic" lagrangian approach on the other hand leads to the path integral, as shown by Dirac in 1932 and developed by Feynman. I will show how the interpretation of quantum mechanics in a path integral framework is based directly on events in spacetime and show that it leads to a second "fork in the road" depending on whether it is necessary for probabilities to play a fundamental role in the theory.
Date: 10/07/2012 - 3:30 pm
...

The Dowker talk abstract says something about a deterministic versus probabilistic formulation. ...

I think you are mistaken. There is no reference to "deterministic" in the abstract. I think you are probably reading too much into the abstract, or putting your own interpretation on it.

When one is constructing a non-deterministic theory one does not automatically get probabilities (numbers between zero and one satisfying certain laws). It may require additional stronger assumptions in order to make probabilities play a fundamental role.

 

[marcus]

Anyway the way I see it we are in an exciting moment for Loop gravity. There are all these developments that could feed into a reformulation that shows up as early as July 2013 with the Warsaw GR20, or at Perimeter's Loops 2013 conference.

A.Stacking Spin Networks (systematically to generate spin foams)
http://arxiv.org/abs/1107.5185
Feynman diagrammatic approach to spin foams
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 26 Jul 2011)

B.Histories
http://arxiv.org/abs/gr-qc/0602013
Generalizing Quantum Mechanics for Quantum Spacetime
James B. Hartle (University of California, Santa Barbara)
(Submitted on 2 Feb 2006)
and currently
http://pirsa.org/12070001/
The Path Integral Interpretation of Quantum Mechanics
Fay Dowker
10 Jul 2013

C. Unclamping the Immirzi
http://arxiv.org/abs/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi
(Submitted on 23 Apr 2012)

D. Using the tetrad's sign
http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing
(Submitted on 3 May 2012)

E. Thermodynamics
http://arxiv.org/abs/1204.6349
Gravitation and vacuum entanglement entropy
Ted Jacobson
(Submitted on 28 Apr 2012)
http://arxiv.org/abs/1205.5529
General relativity as the equation of state of spin foam
Lee Smolin
(Submitted on 24 May 2012)
http://arxiv.org/abs/1207.0505
Emergent perspective of Gravity and Dark Energy
T. Padmanabhan
(Submitted on 2 Jul 2012)

F. Dust. Actual Hamiltonians (instead of constraints.)
http://arxiv.org/abs/1206.3807
Scalar Material Reference Systems and Loop Quantum Gravity
Kristina Giesel, Thomas Thiemann
(Submitted on 17 Jun 2012)
http://arxiv.org/abs/1206.0658
Linking Covariant and Canonical General Relativity via Local Observers
Steffen Gielen, Derek K. Wise
(Submitted on 4 Jun 2012)

 

[marcus]

Fay Dowker has now given her seminar talk at Perimeter and the video version is already posted here:
http://pirsa.org/12070001

As I said, I see some 6 main ideas that could enter into Loop gravity and change how it is formulated. I can't talk probabilities as I'm just an interested observer, not an expert. These ideas are one the scene and some (or none!) might affect the theory. Because there are so many balls in the air, I boiled them down to 6 keywords to make them easier to recall.

Stacking
Histories
Unclamping
Tetrad
Thermo
Dust

Stacking refers to Lewandowski group's way to systematically ENUMERATE and compute spinfoam histories. They stack up successive spin network states of geometry and join them into a single history.

Histories refers primarily to Hartle's treatment of quantum mechanics which de-emphasizes observers and measurement--focusing on things we care about and want to predict or bet on happening. Histories are partitioned according to these concerns and a decoherence functional is defined on the partitions telling when sets are sufficiently independent to have ordinary probabilities.

Unclamping the Immirzi parameter was a consequence of Bianchi's black hole entropy result S=A/4. It appears to me to have exciting and unpredictable implications for the theory.

The Tetrad's sign could be included in the classical theory upon which Loop gravity is based. Papers by Rovelli and others raise the issue: should the sign be included? If so, in which of two possible ways? How would this affect the quantum theory?

Thermodynamics of geometry is the theme of some recent papers by Jacobson, Smolin, Padmanabhan and others. Could the Einstein GR equation be (like PV = NkT) the equation of state describing overall behavior of microscopic variables (like the vast number of gas molecules whose collective behavior is summarized by PV = NkT.) If GR is the equation of state, what are the underlying degrees of freedom? Do spinfoams describe the underlying degrees of freedom for which EFE is the EoS?

Dust is shorthand for the various approaches being used to recover a real physical Hamiltonian. Members of both the Erlangen and Warsaw groups have research along several related lines. This is familiar from cosmology and I think it's of considerable practical value. It's the one thing I feel sure will be prominently featured in Loops 2013 and next year's GR20 conference in Warsaw.

 

[marcus]

Dowker gave an impressive talk.
It helps if you download the slides PDF first (which takes me about 3 minutes):
http://pirsa.org/pdf/loadpdf.php?pirsa_number=12070001
Then scroll thru the slides while watching the video.
http://pirsa.org/12070001
There is a lot on the slides and their video images are not as legible as the PDF.

She presents Sorkin (and her) QUANTUM MEASURE THEORY as a rival alternative to Hartle's DECOHERENT HISTORIES. Both are proposed histories formulations of QM.
As she presents it, QMT is still being worked out. She also points to a drawback in Hartle's DH approach.

This seems compatively mild to me: it is that there are different ways of partitioning the set of all histories so you get approximate decoherence and additive probabilities. She refers to this as something you have to "struggle with" in the Hartle approach.

But the struggle seems more serious if the probability addition rule is relaxed and all you require is "preclusion" (that events with measure subjectively considered to be very small do not occur.) The preclusion approach is what she and Sorkin are working on. She gave two examples showing a grave contradiction in this approach, where you do not require additive probabilities. In these examples no single history could occur because each one was contained in a subset of measure zero. One example, starting at minute 60, was a variant of the GHZ construction--Greenberger, Horne, and Zeilinger--may be familiar to some.
(e-copy of original GHZ 1998 paper: http://arxiv.org/abs/0712.0921 "Going Beyond Bell's Theorem")

 

[marcus]

Prompted by Dowker's talk, I am trying to assess how serious the problems are with Hartle's Decoherent Histories QM.
Here are some papers by Adrian Kent discussing the problem that decoherent partitions are not unique, and (as of 1994 and 1996 according to Kent) can lead to contradictory predictions. I don't know if the problems alleged by Kent are real or if they have been fixed since then.

Hartle's 2006 paper for the 23rd Solvay Conference proceedings does not cite Kent and does not seem to answer his criticisms, which is puzzling.
http://arxiv.org/abs/gr-qc/9809026
Quantum Histories
http://arxiv.org/abs/gr-qc/9808016
Consistent Sets and Contrary Inferences: Reply to Griffiths and Hartle
http://arxiv.org/abs/gr-qc/9607073
Quantum Histories and Their Implications
http://arxiv.org/abs/gr-qc/9604012
Consistent Sets Yield Contrary Inferences in Quantum Theory
http://arxiv.org/abs/gr-qc/9412067
On the Consistent Histories Approach to Quantum Mechanics

However I see that Griffiths and Hartle did reply to Kent's crit here:
http://arXiv.org/abs/gr-qc/9710025
Comment on "Consistent Sets Yield Contrary Inferences in Quantum Theory''
Robert B. Griffiths (Carnegie-Mellon University), James B. Hartle (University of California, Santa Barbara)
(Submitted on 3 Oct 1997)
In a recent paper Kent has pointed out that in consistent histories quantum theory it is possible, given initial and final states, to construct two different consistent families of histories, in each of which there is a proposition that can be inferred with probability one, and such that the projectors representing these two propositions are mutually orthogonal. In this note we stress that, according to the rules of consistent history reasoning two such propositions are not contrary in the usual logical sense namely, that one can infer that if one is true then the other is false, and both could be false. No single consistent family contains both propositions, together with the initial and final states, and hence the propositions cannot be logically compared. Consistent histories quantum theory is logically consistent, consistent with experiment as far as is known, consistent with the usual quantum predictions for measurements, and applicable to the most general physical systems. It may not be the only theory with these properties, but in our opinion, it is the most promising among present possibilities.
2 pages

 

[marcus]

Loops 2013 conference will be held July 22-26 next year at Perimeter Institute and it's interesting to try to identify now what new ideas and developments could enter into the formulation of Loop quantum geometry/gravity/cosmology that we'll see set out a year from now, at conference.
I should probably update my list of ideas I'm guessing could enter significantly into the picture. There are now 7 of them.
Stacking
Histories
Unclamping
Tetrad-sign
Thermo
Dust
Higgsflation

Stacking refers to Lewandowski group's way to systematically ENUMERATE and compute spinfoam histories. They stack up successive spin network states of geometry and join them into a single history.
http://arxiv.org/abs/1107.5185

Histories refers primarily to Hartle's treatment of quantum mechanics which de-emphasizes observers and measurement--focusing on things we care about and want to predict or bet on happening. Histories are partitioned according to these concerns and a decoherence functional is defined on the partitions telling when sets are sufficiently independent to have ordinary probabilities.
http://arxiv.org/abs/gr-qc/0602013

Unclamping the Immirzi parameter was a consequence of Bianchi's black hole entropy result S=A/4. It appears to me to have exciting and unpredictable implications for the theory.
http://arxiv.org/abs/1204.5122

The Tetrad's sign could be included in the classical theory upon which Loop gravity is based. Papers by Rovelli and others raise the issue: should the sign be included? If so, in which of two possible ways? How would this affect the quantum theory?
http://arxiv.org/abs/1205.0733

Thermodynamics of geometry is the theme of some recent papers by Jacobson, Smolin, Padmanabhan and others. Could the Einstein GR equation be (like PV = NkT) the equation of state describing overall behavior of microscopic variables (like the vast number of gas molecules whose collective behavior is summarized by PV = NkT.) If GR is the equation of state, what are the underlying degrees of freedom? Do spinfoams describe the underlying degrees of freedom for which EFE is the EoS?
http://arxiv.org/abs/1204.6349 http://arxiv.org/abs/1205.5529 http://arxiv.org/abs/1207.0505

Dust is shorthand for the various approaches being used to recover a real physical Hamiltonian. Members of both the Erlangen and Warsaw groups have research along several related lines. This is familiar from cosmology and I think it's of considerable practical value.
http://arxiv.org/abs/1206.3807 http://arxiv.org/abs/1206.0658

Higgs inflation in Loop cosmology is the topic of a new paper by three young researchers that just appeared and impressed me as potentially important. It's by Tom Pawlowski, a postdoc at Warsaw, and two PhD students there: Andrea Dapor and Michal Artymowski.
It puts inflation in a new light for me. So I expect some rapid development in this area:
http://arxiv.org/abs/1207.4353
Inflation from non-minimally coupled scalar field in loop quantum cosmology
Michal Artymowski, Andrea Dapor, Tomasz Pawlowski
(Submitted on 18 Jul 2012)
The FRW model with non-minimally coupled massive scalar field has been investigated in LQC framework. Considered form of the potential and coupling allows applications to Higgs driven inflation. The resulting dynamics qualitatively modifies the standard bounce paradigm in LQC in two ways: (i) the bounce point is no longer marked by critical matter energy density, (ii) the Planck scale physics features the "mexican hat" trajectory with two consecutive bounces and rapid expansion and recollapse between them. Furthermore, for physically viable coupling strength and initial data the subsequent inflation exceeds 60 e-foldings.
14 pages, 5 figures
I should give links to earlier papers by Bezrukov and Shaposhnikov
http://arxiv.org/abs/0710.3755 (209 cites)
The Standard Model Higgs boson as the inflaton
http://arxiv.org/abs/0904.1537 (78 cites)
Standard Model Higgs boson mass from inflation: two loop analysis
The latter was cited by ADP.

 

[marcus]

The Loops conference is biennial, every two years. The previous one, Loops 2011 was held in Madrid. Videos of many of the talks, and PDF files of slide presentations are online here:
http://www.iem.csic.es/loops11/ (click on the Scientific Program menu item)
Loops 2013 conference starts just one year from today. It will be held July 22-26 next year at Perimeter Institute.

I've been trying to visualize what main topics and new developments might figure prominently at the next Loops conference. After thinking it over for several weeks and considering various alternatives I've come around, at least for now, to the belief that Loop cosmology will stand out and show the most active development. Particularly the phenomenology side of it where e.g. you model different types of BOUNCE and different mechanisms of INFLATION and you figure out what traces to look for in the ancient light of the Cosmic Background. Among other things gravitational waves would leave their imprint (magnified by expansion) on the microwave sky.

I think other major themes are going to be
DUST
INHOMOGENEITY

The Warsaw and Erlangen people have been working hard on a straightforward Hamiltonian operator-based Loop program and they sound like they think they've succeeded. Lewandowski says we now have Quantum Gravity. So that puts a new light on the situation. It uses a matter field (uniform dust-like) but that is nothing new. Cosmology models have always had that convenience. I'm using "dust" as shorthand for the various approaches being used to recover a real physical Hamiltonian. This is familiar from cosmology and I think it's of considerable practical value.
http://arxiv.org/abs/1206.3807 http://arxiv.org/abs/1206.0658

And I keep seeing Loop cosmology papers with more and more non-uniformity, various "Bianchi" classes of messed up universes---bouncing nevertheless. That's a major trend in the program---generalizing LQC to remove the uniformity restriction and get increasingly realistic.
Inhomogeneity is the focus of this recent paper:
http://arxiv.org/abs/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 5 Apr 2012)
The era of precision cosmology has allowed us to accurately determine many important cosmological parameters, in particular via the CMB. Confronting Loop Quantum Cosmology with these observations provides us with a powerful test of the theory. For this to be possible we need a detailed understanding of the generation and evolution of inhomogeneous perturbations during the early, Quantum Gravity, phase of the universe. Here we describe how Loop Quantum Cosmology provides a completion of the inflationary paradigm, that is consistent with the observed power spectra of the CMB.
4 pages ICGC (2011) Goa Conference proceedings

Higgs inflation in Loop cosmology is the topic of a new paper that just appeared and impressed me as potentially important. It's by Tom Pawlowski, a postdoc at Warsaw, and two PhD students there: Andrea Dapor and Michal Artymowski.
It puts inflation in a new light for me. So I expect some rapid development in this area:
http://arxiv.org/abs/1207.4353
Inflation from non-minimally coupled scalar field in loop quantum cosmology
Michal Artymowski, Andrea Dapor, Tomasz Pawlowski
(Submitted on 18 Jul 2012)
The FRW model with non-minimally coupled massive scalar field has been investigated in LQC framework. Considered form of the potential and coupling allows applications to Higgs driven inflation. The resulting dynamics qualitatively modifies the standard bounce paradigm in LQC in two ways: (i) the bounce point is no longer marked by critical matter energy density, (ii) the Planck scale physics features the "mexican hat" trajectory with two consecutive bounces and rapid expansion and recollapse between them. Furthermore, for physically viable coupling strength and initial data the subsequent inflation exceeds 60 e-foldings.
14 pages, 5 figures
Here are links to earlier papers by Bezrukov and Shaposhnikov
http://arxiv.org/abs/0710.3755 (209 cites)
The Standard Model Higgs boson as the inflaton
http://arxiv.org/abs/0904.1537 (78 cites)
Standard Model Higgs boson mass from inflation: two loop analysis
The latter was cited by ADP.
====================
I should look and see what recent Loop CosmoPheno papers have been highly cited lately and that could give ideas. This post gave some search links for cite-ranked listings:

...
Here's a link that gets (desy key) LQG and LQC papers that mostly have to do with pheno, i.e. with opportunities for TESTING.
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+INFLATION+OR+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+&FORMAT=www&SEQUENCE=citecount%28d%29
This ranks by cites and gets around 108 papers.

Let's restrict to ones that appeared 2009 or later:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29

This gets 59 papers. Again they are ranked by cites.
...
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+(dk+loop+space+and+(quantum+gravity+or+quantum+cosmology)+)+and+(gravitational+radiation+or+primordial+or+inflation+or+power+spectrum+or+cosmic+background)+and+date%3E2008&SEQUENCE=CITECOUNT(D)&SKIP=25
It only gets 57 of the 59 but works more reliably in some cases.

For sure Agullo Ashtekar Nelson is going to figure in conference and also Artymowski Dapor Pawlowski, I would expect. But those haven't been out long enough to be cited much, so they would not necessarily be spotted in this kind of listing. Let's see what Loop CosmoPheno papers are picked out, however:

Links don't always remain active. So I will copy off a few items from the top of the list.
1) Cosmological footprints of loop quantum gravity.
J. Grain, (APC, Paris & Paris, Inst. Astrophys.) , A. Barrau, (LPSC, Grenoble & IHES, Bures-sur-Yvette) . Feb 2009. (Published Feb 27, 2009). 7pp.
Published in Phys.Rev.Lett.102:081301,2009.
e-Print: arXiv:0902.0145 [gr-qc] (46)

3) Loop quantum cosmology and slow roll inflation.
Abhay Ashtekar, David Sloan, (Penn State U.) . Dec 2009. 8pp.
Published in Phys.Lett.B694:108-112,2010.
e-Print: arXiv:0912.4093 [gr-qc] (32)

4) Possible observational effects of loop quantum cosmology.
Jakub Mielczarek, (Jagiellonian U., Astron. Observ. & LPSC, Grenoble) . Aug 2009. (Published Mar 15, 2010). 11pp.
Published in Phys.Rev.D81:063503,2010.
e-Print: arXiv:0908.4329 [gr-qc] (26)

5) Big Bounce and inhomogeneities.
David Brizuela, Guillermo A.D Mena Marugan, Tomasz Pawlowski, (Madrid, Inst. Estructura Materia) . Feb 2009. 4pp.
Published in Class.Quant.Grav.27:052001,2010.
e-Print: arXiv:0902.0697 [gr-qc] (21)

6) Inflation in loop quantum cosmology: dynamics and spectrum of gravitational waves.
Jakub Mielczarek, (Jagiellonian U.) , Thomas Cailleteau, (LPSC, Grenoble) , Julien Grain, (Paris, Inst. Astrophys.) , Aurelien Barrau, (LPSC, Grenoble) . Mar 2010. (Published May 15, 2010). 11pp.
Published in Phys.Rev.D81:104049,2010.
e-Print: arXiv:1003.4660 [gr-qc] (21)

7) Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation.
J. Grain, (APC, Paris & Paris, Inst. Astrophys.) , A. Barrau, (LPSC, Grenoble & IHES, Bures-sur-Yvette) , A. Gorecki, (LPSC, Grenoble) . Apr 2009. (Published Apr 2009). 15pp.
Published in Phys.Rev.D79:084015,2009.
e-Print: arXiv:0902.3605 [gr-qc] (20)

8) Observational constraints on loop quantum cosmology.
Martin Bojowald, (Penn State U.) , Gianluca Calcagni, (Potsdam, Max Planck Inst.) , Shinji Tsujikawa, (Tokyo U. of Sci.) . IGC-11-1-1, AEI-2011-004, Jan 2011. (Published Nov 18, 2011). 4pp.
Published in Phys.Rev.Lett.107:211302,2011.
e-Print: arXiv:1101.5391 [astro-ph.CO] (20)

9) Observational constraints on a power spectrum from super-inflation in Loop Quantum Cosmology.
Masahiro Shimano, Tomohiro Harada, (Rikkyo U.) . Sep 2009. (Published Sep 15, 2009). 17pp.
Published in Phys.Rev.D80:063538,2009.
e-Print: arXiv:0909.0334 [gr-qc] (19)

10) Fully LQC-corrected propagation of gravitational waves during slow-roll inflation.
J. Grain, (Paris, Inst. Astrophys.) , T. Cailleteau, A. Barrau, A. Gorecki, (LPSC, Grenoble) . Oct 2009. (Published Jan 15, 2010). 9pp.
Published in Phys.Rev.D81:024040,2010.
e-Print: arXiv:0910.2892 [gr-qc] (17)

11) Inhomogeneous Loop Quantum Cosmology: Hybrid Quantization of the Gowdy Model.
L.J. Garay, (Madrid U. & Madrid, Inst. Estructura Materia) , M. Martin-Benito, G.A. Mena Marugan, (Madrid, Inst. Estructura Materia) . May 2010. (Published Aug 15, 2010). 16pp.
Published in Phys.Rev.D82:044048,2010.
e-Print: arXiv:1005.5654 [gr-qc] (17)

12) Tensor power spectrum with holonomy corrections in LQC.
Jakub Mielczarek, (Jagiellonian U.) . Feb 2009. (Published Feb 2009). 13pp.
Published in Phys.Rev.D79:123520,2009.
e-Print: arXiv:0902.2490 [gr-qc] (16)

13) Inflationary observables in loop quantum cosmology.
Martin Bojowald, (Penn State U.) , Gianluca Calcagni, (Potsdam, Max Planck Inst.) . AEI-2010-161, IGC-10-11-1, Nov 2010. 40pp.
Published in JCAP 1103:032,2011.
e-Print: arXiv:1011.2779 [gr-qc] (16)

14) Observational test of inflation in loop quantum cosmology.
Martin Bojowald, (Penn State U.) , Gianluca Calcagni, (Potsdam, Max Planck Inst.) , Shinji Tsujikawa, (Tokyo U. of Sci.) . AEI-2011-042, Jul 2011. 37pp.
Published in JCAP 1111:046,2011.
e-Print: arXiv:1107.1540 [gr-qc] (15)

15) Non-singular inflationary universe from polymer matter.
Golam Mortuza Hossain, Viqar Husain, Sanjeev S. Seahra, (New Brunswick U.) . Jun 2009. (Published Jan 15, 2010). 4pp.
Published in Phys.Rev.D81:024005,2010.
e-Print: arXiv:0906.2798 [astro-ph.CO] (13)

16) Constraints on standard and non-standard early Universe models from CMB B-mode polarization.
Yin-Zhe Ma, (Cambridge U., KICC & Cambridge U., Inst. of Astron.) , Wen Zhao, (Cardiff U.) , Michael L. Brown, (Cambridge U., KICC & Cambridge U., Inst. of Astron. & Cambridge U.) . Jul 2010. 41pp.
Published in JCAP 1010:007,2010.
e-Print: arXiv:1007.2396 [astro-ph.CO] (11)

17) Loop Quantum Cosmology corrections on gravity waves produced during primordial inflation.
J. Grain, (Paris, Inst. Astrophys.) . Nov 2009. 9pp.
To appear in the proceedings of INVISIBLE UNIVERSE INTERNATIONAL CONFERENCE: Toward a new cosmological paradigm, Paris, France, 29 Jun - 3 Jul 2009.
Published in AIP Conf.Proc.1241:600-608,2010.
e-Print: arXiv:0911.1625 [gr-qc] (9)

18) Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters.
Julien Grain, (Paris, Inst. Astrophys. & Orsay, LAL) , Aurelien Barrau, Thomas Cailleteau, (LPSC, Grenoble) , Jakub Mielczarek, (Jagiellonian U.) . Nov 2010. (Published Dec 15, 2010). 12pp.
Published in Phys.Rev.D82:123520,2010.
e-Print: arXiv:1011.1811 [astro-ph.CO] (9)

19) Warm inflationary model in loop quantum cosmology.
Ramon Herrera, (Rio de Janeiro, Pont. U. Catol.) . Jun 2010. (Published Jun 15, 2010). 15pp.
Published in Phys.Rev.D81:123511,2010.
e-Print: arXiv:1006.1299 [astro-ph.CO] (8)

20) The Big Bang and the Quantum.
Abhay Ashtekar, (Penn State U.) . May 2010. 18pp.
Plenary talk at INVISIBLE UNIVERSE INTERNATIONAL CONFERENCE: Toward a new cosmological paradigm, Paris, France, 29 Jun - 3 Jul 2009.
Published in AIP Conf.Proc.1241:109-121,2010.
e-Print: arXiv:1005.5491 [gr-qc] (7)

21) On the measure problem in slow roll inflation and loop quantum cosmology.
Alejandro Corichi, (UNAM, Morelia, Inst. Math. & Penn State U.) , Asieh Karami, (IFM-UMSNH, Michoacan & UNAM, Morelia, Inst. Math.) . Nov 2010. (Published May 15, 2011). 12pp.
Published in Phys.Rev.D83:104006,2011.
e-Print: arXiv:1011.4249 [gr-qc] (7)

 

[marcus]

Besides cosmology (eg Higgs inflation in Loop cosmo) the other 6 topics to watch, that I listed in post #87 and don't want to completely forget about, are as follows:

Stacking refers to Lewandowski group's way to systematically ENUMERATE and compute spinfoam histories. They stack up successive spin network states of geometry and join them into a single history.
http://arxiv.org/abs/1107.5185

Histories refers primarily to Hartle's treatment of quantum mechanics which de-emphasizes observers and measurement--focusing on things we care about and want to predict or bet on happening. Histories are partitioned according to these concerns and a decoherence functional is defined on the partitions telling when sets are sufficiently independent to have ordinary probabilities.
http://arxiv.org/abs/gr-qc/0602013

Unclamping the Immirzi parameter was a consequence of Bianchi's black hole entropy result S=A/4. It appears to me to have exciting and unpredictable implications for the theory.
http://arxiv.org/abs/1204.5122

The Tetrad's sign could be included in the classical theory upon which Loop gravity is based. Papers by Rovelli and others raise the issue: should the sign be included? If so, in which of two possible ways? How would this affect the quantum theory?
http://arxiv.org/abs/1205.0733

Thermodynamics of geometry is the theme of some recent papers by Jacobson, Smolin, Padmanabhan and others. Could the Einstein GR equation be (like PV = NkT) the equation of state describing overall behavior of microscopic variables (like the vast number of gas molecules whose collective behavior is summarized by PV = NkT.) If GR is the equation of state, what are the underlying degrees of freedom? Do spinfoams describe the underlying degrees of freedom for which EFE is the EoS?
http://arxiv.org/abs/1204.6349 http://arxiv.org/abs/1205.5529 http://arxiv.org/abs/1207.0505

Dust is shorthand for the various approaches being used to recover a real physical Hamiltonian. Members of both the Erlangen and Warsaw groups have research along several related lines. This is familiar from cosmology and I think it's of considerable practical value.
http://arxiv.org/abs/1206.3807 http://arxiv.org/abs/1206.0658

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

Actually something just came out today that relates to the "Tetrad sign" idea:

http://arxiv.org/abs/1207.5156
Divergences and Orientation in Spinfoams
Marios Christodoulou, Miklos Långvik, Aldo Riello, Christian Röken, Carlo Rovelli
(Submitted on 21 Jul 2012)
We suggest that large radiative corrections appearing in the spinfoam framework might be tied to the implicit sum over orientations. Specifically, we show that in a suitably simplified context the characteristic "spike" divergence of the Ponzano-Regge model disappears when restricting the theory to just one of the two orientations appearing in the asymptotic limit of the vertex amplitude.
10 pages, 5 figures

For example reference [13] is to the original tetrad sign paper by Rovelli&Wilson-Ewing which enters discussion on page 1 here:

==quote page 1 of Christodoulou et al==
We suggest here that the answer lies in the fact that the asymptotic limit of the Ponzano-Regge amplitude is not the exponential of the Regge action, but rather the sum of two exponentials of the Regge action, taken with certain flipped signs. With flipped signs, the invariant contribution comes when P is outside τ. In other words, the divergence is strictly dependent on the existence of the second term in the expansion of the vertex amplitude.

The geometrical origin of this second term can be traced to the fact that the asymptotic limit of the Ponzano-Regge model is not truly 3d general relativity in metric variables, but rather 3d general relativity in triad variables, with an action that flips sign under reversal of the orientation of the triad [13]. In three dimensions, it is this action (and not metric general relativity) which is equivalent to BF theory. In turn, BF theory has an additional gauge symmetry with respect to general relativity: the shift B → B+d_AΦ (where A is the connection variable: F = dA+A∧A), which can be shown to be related to the displacement of P all over the hyperplane [4].

In this paper, we present two arguments that provide some ground for these intuitions...
==endquote==

 

[marcus]

So as not to forget the active lines of research we're following:
PhenoCosmo (bounce early universe cosmology is where pheno enters most strongly)
Stacking
Histories
Unclamping
Tetrad-handedness
Thermo
Dust

I see that someone named David Craig has some potentially Loop-related consistent/decoherent "Histories" papers.
http://arxiv.org/find/gr-qc/1/au:+Craig_D/0/1/0/all/0/1
Somehow I was not aware of his research until now.
He has co-authored two papers with Loop researcher Param Singh, and also co-authored with Jim Hartle, Fay Dowker, Rafael Sorkin. Recently brought out his first explicitly Loop cosmology paper.

To keep track of a few of the authors involved in each of these research lines, for reference purposes:
PhenoCosmo (Barrau, Grain, Pawlowski, Cailleteau, Agullo, Nelson, Vidotto,...)
Stacking (Lew., ... )
Histories (Hartle, Schroeren?, Craig?)
Unclamping (Bianchi, ...)
Tetrad-handedness (Rov., ...)
Thermo (Jac., Smo., Pad., ...)
Dust (Lew., Thiem., Wise, ...)