Loop Quantum Gravity: Zurück vor den Urknall" by Martin Bojowald

[marcus]

Bahr and Dittrich are still working on it.

What I see Bahr and Dittrich working on is a different approach to QG. It is not canonical Hamiltonian LQG (which is a theory on a 3D manifold).

I like the Bahr Dittrich papers very much! But I see them working on a 4D diffeomorphism invariant approach using simplicial tools. I don't see what they are doing as having much real connection with canonical LQG.

Almost no one works on canonical LQG these days, as I think we all know. You might say Thomas Thiemann does (and in the past Dittrich has co-authored with him and helped considerably with his programme). But even in that case it is essentially different from the old canonical LQG---either it is master-constraint, or something else called "algebraic"----Thiemann's own approaches.

So when you say Bahr and Dittrich are working on "it", what does "it" mean exactly. If you mean a mathematical "proof" of the Marseille spinfoam model starting from the incomplete canonical LQG fossil---what Tom Stoer was talking about---then I think you are mistaken. That is not what Bahr and Dittrich are up to!

 

[atyy]

What I see Bahr and Dittrich working on is a different approach to QG. It is not canonical Hamiltonian LQG (which is a theory on a 3D manifold).

I like the Bahr Dittrich papers very much! But I see them working on a 4D diffeomorphism invariant approach using simplicial tools. I don't see what they are doing as having much real connection with canonical LQG.

Almost no one works on canonical LQG these days, as I think we all know. You might say Thomas Thiemann does (and in the past Dittrich has co-authored with him and helped considerably with his programme). But even in that case it is essentially different from the old canonical LQG---either it is master-constraint, or something else called "algebraic"----Thiemann's own approaches.

So when you say Bahr and Dittrich are working on "it", what does "it" mean exactly. If you mean a mathematical "proof" of the Marseille spinfoam model starting from the incomplete canonical LQG fossil---what Tom Stoer was talking about---then I think you are mistaken. That is not what Bahr and Dittrich are up to!

Aren't they trying to solve the canonical LQG Hamiltonian constraint by finding a perfect spinfoam action?

http://arxiv.org/abs/0905.1670

 

[tom.stoer]

I know of no reasonable theory which does not admit a canonical formulation. I worked on QCD and it became clear tome that certain aspects are better addressed in the canonical approach.

Perhaps something went wrong during the "old-fashioned" LQG program, but then it should be investigated what exactly went wrong!

 

[marcus]

Aren't they trying to solve the canonical LQG Hamiltonian constraint by finding a perfect spinfoam action?

http://arxiv.org/abs/0905.1670

In a sense you are right. But notice that they have mainly been working on simplicial models, which are fully 4D, and they wish to preserve full 4D covariance (i.e. diffeo symmetry). But in the paper you cite they also have explored how to derive a canonical model (something based on a 3D slice.)

But I think this would not turn out to be the old canonical LQG that we know! They are proceeding "backwards" from what I think Tom Stoer is imagining. You do not first fix a precise canonical LQG and then derive a 4D QG from it. The Bahr Dittrich strategy, I would say, is to first find a good 4D QG---whether simplicial or spin foam or whatever---and then re-invent the corresponding canonical version.

==quote Bahr Dittrich==
We will examine the issue of diffeomorphism symmetry in simplicial models of (quantum) gravity, in particular for Regge calculus. We find that for a solution with curvature there do not exist exact gauge symmetries on the discrete level. Furthermore we derive a canonical formulation that exactly matches the dynamics and hence symmetries of the covariant picture. In this canonical formulation broken symmetries lead to the replacements of constraints by so--called pseudo constraints. These considerations should be taken into account in attempts to connect spin foam models, based on the Regge action, with canonical loop quantum gravity, which aims at implementing proper constraints. We will argue that the long standing problem of finding a consistent constraint algebra for discretized gravity theories is equivalent to the problem of finding an action with exact diffeomorphism symmetries.
==endquote==

I know of no reasonable theory which does not admit a canonical formulation. I worked on QCD and it became clear tome that certain aspects are better addressed in the canonical approach.

I agree! Let's have a canonical formulation of 4D QG! But I see no reason that this should be identical to the old form of LQG. When you walk you do not always put the same foot forward Today the Loop/Foam people are working on the foam approach and that is what is changing. When that is advanced then they may well advance "on the other foot" and make an entirely new version of the canonical.
Indeed this is how it went historically with Einstein. He provided a fully 4D covariant approach and only much later people discovered how to make a canonical (Arnowitt Deser Misner) that was compatible with it. It took some 47 years between Einstein 1915 and ADM 1962

Perhaps something went wrong during the "old-fashioned" LQG program, but then it should be investigated what exactly went wrong!

I suppose that is a matter of research taste, of what questions you consider fruitful and illuminating to explore. Most physics gambits turn out wrong, and researchers gain insight from working on them, which they carry on and apply to the next version.
I like Bianca Dittrich's taste in what is interesting. Let's see how she spends her time. Perhaps she will come up with a good covariant or "path integral" QG, and then she might work back to a new canonical formulation---QG on a 3D slice, with constraints. And then maybe, as you would like, she may take some time to investigate why the original attempt did not work and remained incomplete. Or once they get the right answer it may be OBVIOUS. Or again, it might still not be obvious but a researcher like Dittrich might not think it worthwhile to look back and investigate the cause of frustration.
I see no compelling reason for us to say now, ahead of time, what should be done.

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

As I see it, each of these half-dozen approaches gains insight with illuminates the rest, and all are changing. There is no one fixed Loop this or Spin that. Each year or so, there will appear some main article that defines what the approach is, at that moment, approximately.

About Dittrich, we should be on the lookout for these papers that have not yet appeared!
[28] B. Bahr, B. Dittrich, P. Höhn, “Exact and approximate constraints in 4d Regge calculus,”
to appear
[30] B. Bahr, B. Dittrich,“Improving the action for Regge calculus with cosmological constant,”
to appear
[31] B. Dittrich and L. Freidel, to appear

 

[atyy]

Curiously, with the Bahr and Dittrich papers, old LQG is starting to make some sense to me. I've never liked LQG because it's motivated by "background independence" (well, I used to like it because of a Scientific American article I read in the gym, then disliked it after "Trouble with Physics", which converted me to strings). I've long thought that the only serious alternative to strings is Asymptotic Safety, which is well motivated from Wilsonian renormalization. If Bahr and Dittrich are right, then LQG can be motivated from Asymptotic Safety (I'm assuming a perfect action requires asymptotic safety, just as in QCD it requires asymptotic freedom), so it finally makes sense.

 

[marcus]

Curiously, with the Bahr and Dittrich papers, old LQG is starting to make some sense to me. I've never liked LQG because it's motivated by "background independence" (well, I used to like it because of a Scientific American article I read in the gym, then disliked it after "Trouble with Physics", which converted me to strings). I've long thought that the only serious alternative to strings is Asymptotic Safety, which is well motivated from Wilsonian renormalization. If Bahr and Dittrich are right, then LQG can be motivated from Asymptotic Safety (I'm assuming a perfect action requires asymptotic safety, just as in QCD it requires asymptotic freedom), so it finally makes sense.

That's a intriguing story. It was Lubos Motl (on usenet sci.physics.research in 2003) that got me interested in Loop Quantum Gravity. That was before I knew of Woit or Smolin, or even had read the book by Rovelli---which I later (curious because of the violent attacks) found in draft version on the web. Sometime I'd like to learn more of your peripatetic evolution.

But right now you brought up the intensely interesting topic of that Bahr Dittrich paper. I don't think they will come up with the canonical LQG as we know it. They have not said yet how it is going. They give references to papers which are "to appear". I think they will come up with a significantly different canonical QG. Here is what they say in the paper you cited:
==quote page 23==
In the case that one starts with an action with broken symmetries and obtains pseudo
constraints, there might nevertheless exist certain limiting cases in which these turn into proper constraints. This was the case for the first order dynamics in the cosmological constant for 3d Regge calculus. An analysis for 4d Regge calculus for such limiting cases will appear in [28].
Starting from these proper constraints it might be possible to extend the constraints, such that in the end one obtains a system with an alternative dynamics with exact gauge symmetries.
==endquote==
It's not a done deal! We can't tell how it will turn out! They could fail and get nowhere, in the 4D case. If they succeed and get a canonical version, we don't yet know how close to the original 1990s ideas of the LQG hamiltonian. They say "alternative" dynamics. That could mean the hamiltonian is only a little different, or conceivably altogether quite different.

And the same could happen with those people working on the spin foam models. It feels like a time of rapid change. I am benefitting from your ability to scan and recall the literature, maybe it's time to say "thanks."

 

[atyy]

It's not a done deal! We can't tell how it will turn out! They could fail and get nowhere, in the 4D case. If they succeed and get a canonical version, we don't yet know how close to the original 1990s ideas of the LQG hamiltonian. They say "alternative" dynamics. That could mean the hamiltonian is only a little different, or conceivably altogether quite different.

Well, at least now if it fails, it will be a grand failure.

BTW, I should say that "convert to strings" is only my statement of intellectual responsibility - as you know, Wen's work is my personal favourite.

And the same could happen with those people working on the spin foam models. It feels like a time of rapid change. I am benefitting from your ability to scan and recall the literature, maybe it's time to say "thanks."

Marcus, it's really thank YOU - I just read https://www.physicsforums.com/showthread.php?t=7245

 

[atyy]

I just realized - Ur doesn't mean "Big" - it means "Ancient" or "Primal".

 

[tom.stoer]

I suppose that is a matter of research taste, of what questions you consider fruitful and illuminating to explore. Most physics gambits turn out wrong, and researchers gain insight from working on them, which they carry on and apply to the next version ...

Not really.

PI (SF) and canonical (LQG) approach are strictly equivalent iff the basic concepts are well-defined. If they are not, both approaches may suffer from the same weakness. As nobody knows today what is exactly wrong in LQG, this MUST be addressed (I agree that you are free to chose the direction, either via LQG or via SF).

Examples: constraints / gauge fixing including diffeo-symm. will show up in the PI formalism as well; the space of states has to be constructed; you have to regularize; you have to find a reasonable "time evolution"; you have to write down boundary conditions and a measure for the PI. You can relate all these to questions in the canonical approach ...

 

[marcus]

... (I agree that you are free to chose the direction, either via LQG or via SF).
...

Well, I'll be satisfied with that. Except that you have made a mistake by leaving the word "canonical" out of your sentence. I think you meant canonical LQG.
It seems clear that LQG includes both spinfoam and canonical approaches, and that the LQG community has chosen to work on SF for now. You could say that the idea is to make sure of that, make it right, or as right as they can--and then eventually, once the spinfoam model is seen as satisfactory, they will probably shift some attention back to the canonical version.

In the meantime we should recognize that LQG has come to mean more than merely canonical LQG.
Anyone who wants to say canonical LQG should say that, explicitly, if they want to avoid being misleading and confusing.

The reason is that people talk about LQG meaning what the LQG research community does, and they do spinfoam models and group field theory, primarily and almost exclusively.

Unless you can magically force everyone to say "EPRLS-FK"

Repeating, just to be very clear.
Loop Quantum Gravity includes both spinfoam and the earlier canonical approach.
Currently almost all the LQG research concerns spinfoam models.
You say it is acceptable to derive in either direction:
Either spinfoam -> canonical
Or canonical -> spinfoam

What it looks like the LQG researchers are doing, the course they are now on, is to first work out the spinfoam models
and then derive the canonical version.
So it appears that they have chosen the direction spinfoam -> canonical.

 

[marcus]

Here's another excerpt from Bojowald's book, which the publisher provides as a sample.
==quote from introduction of ZvU==
...Weitere Beispiele sind, sowohl in der Quantentheorie als auch in der Allgemeinen Relativitätstheorie, die Rolle von Beobachtern in der Welt und die Frage nach dem, was man überhaupt beobachten kann und was möglicherweise nicht. In der Kosmologie bedeutet der Einzug von physikalischen Methoden die Entstehung empirisch überprüfbarer Weltbilder. Das Urknall-Modell des Universums beruht sowohl auf der Allgemeinen Relativitätstheorie in der Beschreibung von Raum, Zeit und der treibenden Gravitationskraft als auch auf der Quantentheorie, die für eine Kenntnis der Eigenschaften von Materie I am frühen Universum wichtig ist. Insgesamt ergibt sich eine spektakuläre Erklärung für die sukzessive Entstehung von Atomkernen, Atomen und weiter zusammengesetzter Materie bis hin zu Galaxien aus einer extrem heißen Anfangsphase.

Gerade an dieser Stelle werden jedoch auch Grenzen des etablierten Weltbildes sichtbar. Trotz aller Erfolge ergibt die Allgemeine Relativitätstheorie zusammen mit der Quantentheorie, wie sie derzeit benutzt wird, keine vollständige Beschreibung des Universums. Löst man die mathematischen Gleichungen der Allgemeinen Relativitätstheorie, um ein Modell des zeitlichen Verlaufes des Universums zu erhalten, so erhält man immer einen Zeitpunkt, die sogenannte Urknall-Singularität, zu dem die Temperatur des Universums unendlich groß war. Dass das Universum in der Urknall-Phase sehr heiß war, ist keine Überraschung; schließlich war das expandierende Universum damals viel kleiner und komprimierter als heute, was einen enormen Temperaturanstieg bedeutet. Aber Unendlich als Resultat einer physikalischen Theorie bedeutet schlicht, dass die Theorie überstrapaziert wurde. Ihre Gleichungen verlieren an solch einem Punkt sämtlichen Sinn. I am Falle des Urknall-Modells sollte dies nicht als eine Vorhersage eines Anfangs der Welt missverstanden werden, obwohl es oftmals so dargestellt wird. Ein Zeitpunkt, an dem eine mathematische Gleichung Unendlich liefert, ist nicht der Anfang (oder das Ende) der Zeit. Es ist einfach ein Punkt, an dem die Theorie ihre Begrenztheit zeigt. Trotz aller Erfolge in anderen Bereichen muss die Theorie, die durch die Allgemeine Relativitätstheorie in Kombination mit der Quantentheorie der Materie geliefert wird, erweitert werden.

Das Problem hat seine Ursache in der Unvollständigkeit der Revolution, die in der physikalischen Forschung des letzten Jahrhunderts stattfand...
==endquote==

A rough translation of the passage highlighted here could be as follows:
"But infinity as the result of a physical model simply means that the theory has been pushed beyond its limits. At such a point its equations lose all meaning. In the case of the Big Bang models this should not be misunderstood as predicting the beginning of the world, although it is often presented that way.

A point in time where a mathematical equation yields infinity is not the Beginning (or the End) of time. It is simply a point where the theory reveals its limitations..."

 

[marcus]

Earlier several questions were raised about the prospects for testable predictions from Loop Cosmology, which at least for now boils down to looking for "footprints in the CMB".

Several papers about this, from 2008 and 2009, were mentioned. The most recent ones were by Jack Mielczarek (I think the name is pronounced *myel-cha-rek*) and by two co-authors Aurelien Barrau and Julien Grain.

The Barrau-Grain "LQG footprint" paper gives an idea of the current status of efforts to work out tests for LQC. This week a follow-up appeared, a solo paper by Julien Grain. Here is a sample of Grain's recent work:

1. arXiv:0911.1625
Loop Quantum Cosmology corrections on gravity waves produced during primordial inflation
J. Grain
Comments: to be published in the AIP Proceedings of the 'Invisible Universe International Conference', UNESCO-Paris, June 29-July 3, 2009; 9 pp., 4 Figs

2. arXiv:0910.2892
Fully LQC-corrected propagation of gravitational waves during slow-roll inflation
J. Grain, T. Cailleteau, A. Barrau, A. Gorecki
Comments: 9 pages, submitted for publication to Phys. Rev. D

3. arXiv:0903.2350
Polarized CMB power spectrum estimation using the pure pseudo-cross-spectrum approach
J. Grain, M. Tristram, R. Stompor
Comments: 31 pages, 24 figures, typos corrected on Eq. 32, Appendix C clarified, published in Physical Review D
Journal-ref: Phys.Rev.D79:123515,2009

4. arXiv:0902.3605
Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation
J. Grain, A. Barrau, A. Gorecki
Comments: 15 pages, 5 figures, published version with minor modifications, results unchanged
Journal-ref: Phys.Rev.D79:084015,2009

5. arXiv:0902.0145 [ps, pdf, other]
Cosmological footprints of loop quantum gravity
J. Grain, A. Barrau
Comments: Accepted by Phys. Rev. Lett., 7 pages, 2 figures
Journal-ref: Phys.Rev.Lett.102:081301,2009

I have highlighted the titles having to do with Loop Quantum Gravity, or with Loop Quantum Cosmology. Basically this has become an active area of CMB phenomenology. Researchers apparently have sensed a possibility of deriving predictions which can be tested by CMB observatories such as the ESO's Planck spacecraft that was launched this year and is recording data. So far I have only seen qualitative predictions, however, not hard numbers.

 

[RUTA]

marcus,

I was at a talk yesterday in which Ashtekar gave a quick (1 hr) explanation of the bounce in LQC. He created a time operator T which he equated to a matter field Phi (a scalar field satisfying (Delambertian)(Phi) = 0). I asked a question on something else, and he was getting lots of questions, so I didn't ask these:

1. If we replace the continuous Phi by a collection of point sources, is the resulting classical spacetime metric discrete?

2. Does T = Phi imply no vacuum solutions to Einstein's equations?

What do you think?

 

[marcus]

RUTA, delighted to hear about Ashtekar's talk. If it was at Penn State, or part of the International LQG Seminar (ILQGS) then I may be able to find it on line.

I'm afraid I can't give you any useful answer, aside from generalities. You are familiar with the "problem of time" which arises already in classical GR (your area of expertise!).

In a nutshell, there is no preferred time in GR---only the time measured by individual observers (and a physically meaningless coordinate time.)

In LQC they typically add some physical element to the model which can serve as clock. A scalar field.

I hunted around for an online version of Ashtekar's talk----hoping to find the one you attended. But nothing came up. Nothing recent on ILQS, or at Penn State, or anywhere in the immediate area.

Many of Ashtekar's papers, anything pedagogical about LQC, will cover the handling of time however.

 

[RUTA]

It was a focus session at The Institute for Gravitation & the Cosmos (Penn State Univ) called "The Nature of Time in Fundamental Science." Here is a link:

http://www.gravity.psu.edu/events/nature_of_time/index.shtml

When Unruh asked why the speakers were wired he was told their talks were being recorded for online distribution. Maybe they haven't gotten around to posting them yet.

I'll have to find a pedagogical paper and hope I don't have to infer the answers. He did say the approach is Parmedian and relational as regards spacetime, so I would infer the answer to both questions is "yes."

I notice a very poor choice of words in question 2, "no vacuum solns to EE's." There are vacuum solns to EE's regardless of any theory of QG, of course. I should rather have written, "Does T = Phi imply no vacuum solns to LQC?" An affirmative answer doesn't mean LQC is incompatible with GR and its vacuum solns, which are only approximations in the view of QG.

 

[atyy]

See the discussion beginning just before p13 of http://arxiv.org/abs/gr-qc/0702030 .

I guess the question should be is there time if there is no matter?

 

[marcus]

RUTA, The pointer that Atyy posted to page 13 of that Ashtekar article is apt to be just the right thing. I can't think of a better.

So the talks you attended were at the Institute for Gravitation and the Cosmos (IGC) Thanks for the link! I will add that to the list of links to be periodically checked for new source material, which I keep handy in the thread called "Introduction to LQG".
Here are past workshops and conferences held at IGC
http://www.gravity.psu.edu/events/workshops.shtml

I was glad to hear that they were recording the speakers for posting online. This has been the practice with other workshops on the above list.

For example if you click on "Abhayfest" you get the program for this summer's celebration of Ashtekar's 60th birthday, and there are links to slides and audio for the speakers. Hopefully the same will happen with the 2-day workshop you attended.

So I will check that list from time to time and see what talks they have online.

 

[marcus]

Here's a brief excerpt from the Introduction to Bojowald's book. It includes part of what I think is a key paragraph where he sets out what he will do in the rest of the book.

==quote from original German edition of Before the Big Bang==
...Was wir aber vor allem in den letzten Jahren gesehen haben, sind zahlreiche vielversprechende Indizien für ihre Eigenschaften, die bereits analysiert werden können. Die Situation, wie so oft in der Forschung, gleicht dem Anfangsstadium eines Puzzle-Spiels, in dem man das endgültige Bild vielleicht teilweise erahnen kann, dennoch aber auch auf einem Irrweg sein könnte. Unser derzeitiges Bild deutet an, was eine Vervollständigung der physikalischen Theorie bewerkstelligen kann: Sie erlaubt uns zu sehen, was während und sogar vor dem Urknall geschehen sein könnte. Wir erhalten Einblick in die früheste Urzeit unseres Universums und können erstmals analysieren, wie es wohl entstand.

In diesem Buch werden sowohl jüngste Resultate der Theorie als auch für die nähere Zukunft geplante Beobachtungen I am Weltraum erläutert, und es wird gezeigt, wie radikal sie unser Weltbild verändern können. Insbesondere mit der Schleifen-Quantengravitation, eine der Varianten, die derzeit für eine Kombination von Allgemeiner Relativitätstheorie und Quantentheorie gehandelt werden, sind Ansätze für eine nichtsinguläre Beschreibung des Urknalls erzielt worden. In diesem Rahmen existierte das Universum schon vor dem Urknall, und es lässt sich grob abschätzen, wie es sich damals in seinen Eigenschaften von den jetzigen unterschieden haben könnte. Durch den Einfluss auf spätere Phasen der kosmischen Expansion, die empfindlichen Beobachtungen offenstehen, kann man diese Urgeschichte des Universums untersuchen.
==endquote==

I'll try to translate parts of this, as a help in case any reader's knowledge is even more rudimentary than mine. Help would be welcome if anyone who knows the language wants to volunteer.

This book will describe not only of the most recent theoretical results but also the observations in space planned for the near future---and it will show how radically they could change our picture of the world. In the case of loop quantum gravity, one of the current approaches combining General Relativity with Quantum Theory, assumptions allow for a nonsingular description of the Big Bang. In this framework, the universe existed before the Big Bang, and one can roughly estimate how its characteristics then may have differed from those at present. Through its influence on subsequent phases of cosmic expansion, as detected by sensitive instruments, one can probe the prehistory of the universe.

 

[atyy]

He is going to describe how the effect of the existence of the universe before the big bang would have consequences that remain observable to this day - and which would distinguish such a theory from our current best.

Predictions! Really?

 

[marcus]

Atyy, I've been discussing this in the present thread, and others. Here's post #47 for example.
Any statement about the past has to be based on a model of time-evolutiuon that one can test (in some way) in the present.
The LQC model happens to go back before the start of expansion but the problem of validating it is like that with other models that evolve back into the past.

One has to derive predictions--e.g. of what future CMB space missions will find by observing the CMB at higher resolution (with polarization).

One has to say "if this reconstruction of the past, which goes back before expansion started, is correct, then Planck spacecraft will see this and this footprint". And then if the predicted features of the CMB are not found, the model is falsified.

This is the only way we have of seeing back into the past---construct a model and then test the model by observations made in the present.

So this is what people like Jack Mielczarek are doing for us---trying to derive predictions from the LQC model which will hopefully distinguish it and allow it to be falsified if it is wrong. And people like Aurelien Barrau and Julien Grain, whose papers I listed in the post here:

Earlier several questions were raised about the prospects for testable predictions from Loop Cosmology, which at least for now boils down to looking for "footprints in the CMB".

Several papers about this, from 2008 and 2009, were mentioned. The most recent ones were by Jack Mielczarek (I think the name is pronounced *myel-cha-rek*) and by two co-authors Aurelien Barrau and Julien Grain.

The Barrau-Grain "LQG footprint" paper gives an idea of the current status of efforts to work out tests for LQC. This week a follow-up appeared, a solo paper by Julien Grain. Here is a sample of Grain's recent work:

1. arXiv:0911.1625
Loop Quantum Cosmology corrections on gravity waves produced during primordial inflation
J. Grain
Comments: to be published in the AIP Proceedings of the 'Invisible Universe International Conference', UNESCO-Paris, June 29-July 3, 2009; 9 pp., 4 Figs

2. arXiv:0910.2892
Fully LQC-corrected propagation of gravitational waves during slow-roll inflation
J. Grain, T. Cailleteau, A. Barrau, A. Gorecki
Comments: 9 pages, submitted for publication to Phys. Rev. D

3. arXiv:0903.2350
Polarized CMB power spectrum estimation using the pure pseudo-cross-spectrum approach
J. Grain, M. Tristram, R. Stompor
Comments: 31 pages, 24 figures, typos corrected on Eq. 32, Appendix C clarified, published in Physical Review D
Journal-ref: Phys.Rev.D79:123515,2009

4. arXiv:0902.3605
Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation
J. Grain, A. Barrau, A. Gorecki
Comments: 15 pages, 5 figures, published version with minor modifications, results unchanged
Journal-ref: Phys.Rev.D79:084015,2009

5. arXiv:0902.0145 [ps, pdf, other]
Cosmological footprints of loop quantum gravity
J. Grain, A. Barrau
Comments: Accepted by Phys. Rev. Lett., 7 pages, 2 figures
Journal-ref: Phys.Rev.Lett.102:081301,2009

I have highlighted the titles having to do with Loop Quantum Gravity, or with Loop Quantum Cosmology. Basically this has become an active area of CMB phenomenology. Researchers apparently have sensed a possibility of deriving predictions which can be tested by CMB observatories such as the ESO's Planck spacecraft that was launched this year and is recording data. So far I have only seen qualitative predictions, however, not hard numbers.

Bojowald has also been working on this problem---how to test LQC by observations of the microwave background---according to some quoted statements I've seen. He also has some papers attempting to put bounds on how much we can actually tell about pre-bang conditions---assuming the LQC model is right. It's not deterministic, after all, so there appear to be severe limits (but there has been some controversy about this---Bojowald has been on the pessimistic side.)

 

[atyy]

Atyy, I've been discussing this in the present thread, and others. Here's post #47 for example.

Yes, I'm aware of that. I was wondering if Bojowald's book itself said the same thing.

 

[marcus]

Yes, I'm aware of that. I was wondering if Bojowald's book itself said the same thing.

Well it's "through a glass, darkly" because German. I think the untranslated passage I just quoted gives some idea of the tone of his statements---a certain degree of caution and qualification. So I'll attempt another snippet of translation:

"Above all, what we've seen in the last few years has been numerous significant indications of its characteristics which can actually be analyzed. The situation, as so often happens in research, is like the beginning stages of solving a jig-saw puzzle---in which one might perhaps guess the final picture, but nevertheless could still be on the wrong track.

Our current picture indicates what a completion of the physical theory can accomplish: It allows us to see what could have happened during and even before the Big Bang. We obtain insight into our universe's earliest prehistory and for the first time are able to analyze how it actually arose."

====That was just a rough translation so I'll repeat the original for comparison====
...Was wir aber vor allem in den letzten Jahren gesehen haben, sind zahlreiche vielversprechende Indizien für ihre Eigenschaften, die bereits analysiert werden können. Die Situation, wie so oft in der Forschung, gleicht dem Anfangsstadium eines Puzzle-Spiels, in dem man das endgültige Bild vielleicht teilweise erahnen kann, dennoch aber auch auf einem Irrweg sein könnte.

Unser derzeitiges Bild deutet an, was eine Vervollständigung der physikalischen Theorie bewerkstelligen kann: Sie erlaubt uns zu sehen, was während und sogar vor dem Urknall geschehen sein könnte. Wir erhalten Einblick in die früheste Urzeit unseres Universums und können erstmals analysieren, wie es wohl entstand.
==endquote==

 

[marcus]

He is going to describe how the effect of the existence of the universe before the big bang would have consequences that remain observable to this day - and which would distinguish such a theory from our current best.

Predictions! Really?

Well in terms of the amount of research being done and the citations to it, objective criteria in other words, I guess you could say that the LQC that Bojowald is talking about IS our current best theory of how our expanding universe got started.

At least if by "current" you mean "since 2005" or so.

Folks have been working on the LQC bounce picture since 2001. It does appear able to make predictions about what we can observe in the present (e.g. with the Planck spacecraft ) which would distinguish it from our current NEXT best. Or next next best, or in any case from various alternatives, if the alternatives made any predictions.

The alternatives are things like Linde's eternal inflation, Steinhardt's brane clash, Hawking's ideas of the 1980s, Veneziano stringy "pre-big-bang" scenario of the 1990s. The alternatives to LQC either are not currently being much worked on, or are rather nebulous and don't make much in the way of prediction.

To recall what Bojowald says about it, I'll put the two snippets of translation together as they appear in the introduction.

==quote introduction==
...Above all, what we've seen in the last few years has been numerous significant indications of its characteristics which can actually be analyzed. The situation, as so often happens in research, is like the beginning stages of solving a jig-saw puzzle---in which one might perhaps guess the final picture, but nevertheless could still be on the wrong track.

Our current picture indicates what a completion of the physical theory can accomplish: It allows us to see what could have happened during and even before the Big Bang. We obtain insight into our universe's earliest prehistory and for the first time are able to analyze how it actually arose.

This book will describe not only of the most recent theoretical results but also the observations in space planned for the near future---and it will show how radically they could change our picture of the world. In the case of loop quantum gravity, one of the current approaches combining General Relativity with Quantum Theory, assumptions allow for a nonsingular description of the Big Bang. In this framework, the universe existed before the Big Bang, and one can roughly estimate how its characteristics then may have differed from those at present. Through its influence on subsequent phases of cosmic expansion, as detected by sensitive instruments, one can probe the prehistory of the universe.
==endquote==
Here's the spires search for "quantum cosmology" papers since 2005:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=dk+quantum+cosmology+and+date%3E2005&FORMAT=WWW&SEQUENCE=citecount%28d%29

If you look at the top 50 or 75 papers they are almost all LQC. There are a couple by Hawking and Hartle that made it into the top 75 (they are numbers 57 and 60). Very few Steinhardt ("ekpyrotic" or "cyclic" brane clash) papers.
You may want to expand the search and see what is going on in more detail.

I think objectively what we have here is arguably "current best". I'll post the German original in case anyone wants to check the translation against it.

==quote from original German edition of Before the Big Bang==
...Was wir aber vor allem in den letzten Jahren gesehen haben, sind zahlreiche vielversprechende Indizien für ihre Eigenschaften, die bereits analysiert werden können. Die Situation, wie so oft in der Forschung, gleicht dem Anfangsstadium eines Puzzle-Spiels, in dem man das endgültige Bild vielleicht teilweise erahnen kann, dennoch aber auch auf einem Irrweg sein könnte. Unser derzeitiges Bild deutet an, was eine Vervollständigung der physikalischen Theorie bewerkstelligen kann: Sie erlaubt uns zu sehen, was während und sogar vor dem Urknall geschehen sein könnte. Wir erhalten Einblick in die früheste Urzeit unseres Universums und können erstmals analysieren, wie es wohl entstand.

In diesem Buch werden sowohl jüngste Resultate der Theorie als auch für die nähere Zukunft geplante Beobachtungen I am Weltraum erläutert, und es wird gezeigt, wie radikal sie unser Weltbild verändern können. Insbesondere mit der Schleifen-Quantengravitation, eine der Varianten, die derzeit für eine Kombination von Allgemeiner Relativitätstheorie und Quantentheorie gehandelt werden, sind Ansätze für eine nichtsinguläre Beschreibung des Urknalls erzielt worden. In diesem Rahmen existierte das Universum schon vor dem Urknall, und es lässt sich grob abschätzen, wie es sich damals in seinen Eigenschaften von den jetzigen unterschieden haben könnte. Durch den Einfluss auf spätere Phasen der kosmischen Expansion, die empfindlichen Beobachtungen offenstehen, kann man diese Urgeschichte des Universums untersuchen.
==endquote==