What caused the shift of interest in quantum cosmology?

[mitchell porter]

3,3,1,2,1 is not trending anywhere. 0,4,7,7,8 goes from nothing to a plateau and stays there. So I repeat that there is no decline in "string quantum cosmology" research here, there is just the rise from nothing of "loop quantum cosmology".

There is no unified "string quantum cosmology" research program. The people who created quantum cosmology - like Guth, Vilenkin, Linde, Hawking - have at times introduced the stringy version of quantum gravity into their models, but it's only a technical modification, e.g. modifying Hawking's no-boundary proposal so it contains supergravity. In my opinion, the most distinctive convergence of thought between cosmology and string theory is the way that eternal inflation allows the whole string landscape to be realized in the one universe (in different inflationary patches), and it is not at all established that this is the right way to think about cosmology in string theory.

Also in my opinion, string theory so far lacks a compelling cosmological vision. As I said, you can plug existing string models into existing cosmological models, e.g. by having something distinctively stringy (like motion of a D-brane) provide the inflaton field of the inflationary model. But it has a rather arbitrary feel. I believe eternal inflation + string landscape is the most natural fit so far proposed, but that's more because you have one great multiplicity (of inflationary domains) being paired up with another great multiplicity (of string vacua). It doesn't arise from a deep theoretical principle.

If a "dS/CFT correspondence" could be made to work, that would be big news, because it might describe the real world. But ideas about how to do string theory in de Sitter space are still preliminary at best. That might be the fundamental reason why string cosmology is still a mishmash of divergent ideas.

The role of nonclassical geometry in the bulk, in the AdS/CFT correspondence, for low numbers of QCD-like "colors" in the CFT gauge group, is a very interesting fact that this thread has turned up. I have started taking an interest in how difficult it would be to get something like the standard model from N=4 super-Yang-Mills, by projecting out particular amplitudes and by distributing D3-branes in the AdS bulk so that there are massive open string states, and of course the standard model doesn't have many colors (at least, not many compared to "infinity", which is the limit usually considered in AdS/CFT). So it's very interesting to hear that the nonclassical bulk geometry should show up for such a theory; it makes a connection to the noncommutative standard model (of Connes et al) conceivable.

 

[marcus]

Mitchell, as I think you realize, there definitely has been a shift or marked change in the makeup of the QC top ten, and it requires explanation. You have suggested here what could very possibly be the explanation, or at least one important factor:

Also in my opinion, string theory so far lacks a compelling cosmological vision. As I said, you can plug existing string models into existing cosmological models, e.g. by having something distinctively stringy (like motion of a D-brane) provide the inflaton field of the inflationary model. But it has a rather arbitrary feel. I believe eternal inflation + string landscape is the most natural fit so far proposed, but that's more because you have one great multiplicity (of inflationary domains) being paired up with another great multiplicity (of string vacua). It doesn't arise from a deep theoretical principle.
...

Recent papers making the top ten is an index of researcher interest in the recent papers, not the gross output. When I gave this a quick look a few days ago I didn't see a decline in string quantum cosmology output, the main change was the string QC papers coming on line were just not getting cited much by the research community.

Representation in top-cited papers in some category is not a linear scale, so "trend" is not quite the appropriate concept. Once you get up near 10 here there is not much room for improvement and once you get down near zero you can't go much lower. "Trend" is a linear-scale thing, so we don't look for trends with an index like this. That's why I spoke of a shift, or marked change.

3,3,1,2,1 is not trending anywhere. 0,4,7,7,8 goes from nothing to a plateau and stays there. So I repeat that there is no decline in "string quantum cosmology" research here, there is just the rise from nothing of "loop quantum cosmology".
...

What interests me, and what I meant the thread to discuss, is not a particular index or way of measuring research interest, but the physics explanation for the shift. That was made clear in the first post.

Your reasons in the "compelling cosmological vision" paragraph impress me as perceptive and cogently expressed. They have to do with physics. (They are not merely excuses having to do with social or historical accidents.)

I'm sure you know that the String program had a "cosmological vision" in the 1990s presented in many papers by Maurizio Gasperini and Gabriele Veneziano (one of the early founders who initiated the String program).Their stringy "Pre-Big-Bang" scenario attracted quite a lot of interest. So String was making a splash in quantum cosmology when it had a convincing vision.

You point out that on the one hand some string gambits "have an arbitrary feel"---and on the other hand the natural match with Multiverse ideas does not "arise from a deep physical principle".

As I read what you say, it seems to me that your hope or constructive suggestion (judging from what you say about the wish for a dS/CFT to replace AdS) would be for the program to get a quantum cosmology vision that addresses the problem of modeling the Big Bang.

I would would add one desideratum to yours. The need for the vision to be testable---it should rivet the attention of early universe phenomenologists. I promised back in post #60 that I would get some links to illustrate the EU phenomenology business, so I better do that.

 

[atyy]

marcus, does your search pick up papers like http://arxiv.org/abs/hep-th/0204168 ? Or a review like http://arxiv.org/abs/0907.2562 ?

 

[marcus]

Atyy, it's fascinating that there was that 2002 paper by Liu Moore Seiberg!

First though I should say that I'm just going with what the new Stanford-SLAC search tool ("Inspire") says.
With the search, I just put "quantum cosmology" in the keyword field and rank by citecount and go.
The main thing is to do the same search consistently year after year and look for change.
I just count how many make the top 10, there are other things. I could count how many make the top 25, or I could add up the number of cites I suppose. But it would be more bother and this is just meant to illustrate what I think must be a widely shared impression.

I will give the links so you can easily see what Inspire gives, and does not give!
This is exactly like in post #1 except I do it for all the successive 3-year intervals, not just 1996-1998

1996-1998 http://inspirebeta.net/search?ln=en...2y=1998&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

1999-2001 http://inspirebeta.net/search?ln=en...2y=2001&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

2002-2004 http://inspirebeta.net/search?ln=en...2y=2004&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

2005-2007 http://inspirebeta.net/search?ln=en...2y=2007&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

2008-2010 http://inspirebeta.net/search?ln=en...2y=2010&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

Here are the Spires keywords for the Liu Moore Seiberg:
http://www.slac.stanford.edu/spires/find/hep/wwwtopics?key=4908902
I see no mention of anything quantum cosmological. I'll have to take a look at the paper: it is interesting that they have a certain kind of space perform a bounce! I don't think however that they have a quantum universe wave function perform a bounce. I'll check.

It is an interesting distinction. Often in the String program one has a fixed background geometry in which the strings can live and vibrate. The geometry is classical. I think that is what is happening here. The classical background geometry undergoes a bounce. Then the question is can some of the strings pass through the bounce point.

Here are the Inspire records for 3 papers by Liu Moore Seiberg that came out in 2002 and 2003:
http://inspirebeta.net/record/585639?ln=en
http://inspirebeta.net/record/588898?ln=en
http://inspirebeta.net/record/605720?ln=en

We'll see if any of the string folks (Supr. Fzero?) comment. I could of course be wrong, but I had a look just now and it seems to be the fixed background geometry situation usual with perturbative string theory. What is quantized is the PARTICLES not the geometry they live in. One ignores possible backreaction of the matter on the geometry.

I only glanced, but here are a couple of indicative passages. See what you think:
Page 1

In section 2 we describe the model and its geometry. In section 3 we study the functions on our spacetime which are the wave functions of the first quantized particles. In sections 4 and 5 we quantize free strings in the light-cone and conformal gauges and compute the torus partition function. Section 6 is devoted to a preliminary analysis of the interactions and backreaction. Our conclusions are presented in section 7.​

Page 23

The issue of backreaction is currently under study, and might prove to be a serious problem with future development of this example. One way backreaction could ruin the orbifold is through the coupling of gravity to the large energy momentum of particles which are blue shifted near the singularity. Indeed, we found that although the tree level...​

 

[atyy]

Another paper that the key words may not have picked up is http://arxiv.org/abs/hep-th/0506180

 

[atyy]

Here are some interesting perspectives from various reviews

http://arxiv.org/abs/gr-qc/0301001
"We now see that classical string theory is also singular in such orbifolds and cannot be trusted. ... There are at least three possibilities for the interpretation of the singularity:
1. The singularity is a beginning or end of time. In this case we need to understand the appropriate initial conditions at the singularity. For some discussion of these issues see [23,24].
2. Time has no beginning or end. Then one needs to understand how to pass through the singularity (for recent discussions see e.g. [9,25]).
3. The most likely possibility, it seems to us,is that in string theory time is a derived concept.
In conclusion, let us elaborate on the third possibility above. In toroidal compactifications of string theory there is a minimal distance, thanks to T-duality: shrinking radii past the string scale does not produce a theory at shorter distances. In more elaborate compactifications (such as Calabi-Yau compactifications) it turns out that there can be smooth topology-changing processes, and “quantum geometry” can lead to many counterintuitive types of behavior. These examples show that, in string theory, standard notions of topology and geometry are not fundamental but are rather emergent concepts in certain physical regimes (e.g. in regions of large complex and Kahler structure parameters, in the Calabi-Yau context). In another line of development, Matrix theory and the related advent of noncommutative field-theoretic limits of string theory further indicate that the notion of distance and space ceases to make sense in certain otherwise sensible regimes of the theory. Given the principle of relativity it seems quite likely, perhaps even inevitable, that similar statements hold for time as well as for space."

http://arxiv.org/abs/0705.2643
"In other words, we are led to the conclusion that space — and thus, upon quantization, also space-time — actually disappears (or ‘de-emerges’) as the singularity is approached. There is no ‘quantum bounce’ bridging the gap between an incoming collapsing and an outgoing expanding quasi-classical universe. Instead ‘life continues’ at the singularity for an infinite affine time, however, with the understanding that (i) dynamics no longer ‘takes place’ in space, and (ii) the infinite affine time interval ... corresponds to a sub-Planckian interval 0 < T < TPlanck of geometrical proper time."

http://arxiv.org/abs/1001.4367
"In principle, one would like to carry out the following program. Start with a state in the bulk theory (with modified boundary conditions) corresponding to a large, asymptotically AdS space-time with some profile for the scalar field. Translate this state, using the AdS/CFT correspondence, to a state in the dual field theory on the boundary (with a steep unbounded potential). In the dual field theory, evolve the state through the singularity using a self-adjoint extension (if a consistent and natural self-adjoint extension exists). Finally, translate the evolved state back to a state in the bulk theory, and ask whether it has a geometric interpretation. If the boundary theory described only homogeneous modes, experience with self-adjoint extensions in quantum mechanics would suggest that the final state would roughly resemble the initial state, which would suggest a cosmological bounce. Inhomogeneous modes can drastically change this picture, though: particle creation can be potentially attractive for cosmology, but one needs to make sure that backreaction is sufficiently small for the computations to be reliable."

 

[marcus]

I see you got a 2007 paper co-authored by Hermann Nicolai! Intriguing notions pursued in that one--completely unfamiliar to me. http://arxiv.org/abs/0705.2643

 

[atyy]

I see you got a 2007 paper co-authored by Hermann Nicolai! Intriguing notions pursued in that one--completely unfamiliar to me. http://arxiv.org/abs/0705.2643

A more extensive review of those ideas can be found at http://relativity.livingreviews.org/Articles/lrr-2008-1/ . I have no idea if these really are the symmetries of M-theory (my vote is with twistors and AdS/CFT at the moment;) but the work is very pretty.

Mitchell Porter in a different thread pointed out David Berman's http://arxiv.org/abs/1008.1763 which may link that work up to mainstream string theory.

A different line of speculation also starting from BKL is Carlip's http://arxiv.org/abs/1009.1136

 

[marcus]

You are finding some interesting papers! However I want to explore the reasons for the change in quantum cosmology and one potential reason could be that QC has become observational. I said earlier I would list some observational-QC papers, to indicate the scope of activity. The following papers are all 2009-2011, quite recent.

Here are two Spires searches using the keyword "power spectrum" with two other keywords (quantum cosmology, loop space and quantum gravty, loop space). It will not exhaust the field, since other tags besides "power spectrum" could be used to get more. But it will give an idea:

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+QUANTUM+gravity%2C+LOOP+SPACE+AND+DK+POWER+SPECTRUM+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

1) Cosmological footprints of loop quantum gravity. 33 cites
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]

2) Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation. 13 cites
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]

3) Fully LQC-corrected propagation of gravitational waves during slow-roll inflation. 11 cites
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]

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+dk+power+spectrum+and++DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

1) Possible observational effects of loop quantum cosmology. 20 cites
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]

2) ...duplication from other list...

3) Observational constraints on a power spectrum from super-inflation in Loop Quantum Cosmology. 12 cites
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]

4) Tensor power spectrum with holonomy corrections in LQC. 10 cites
Jakub Mielczarek, (Jagiellonian U.) . Feb 2009. (Published Feb 2009). 13pp.
Published in Phys.Rev.D79:123520,2009.
e-Print: arXiv:0902.2490 [gr-qc]

5) Loop Quantum Cosmology corrections on gravity waves produced during primordial inflation. 8 cites
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]

6) Inflation in loop quantum cosmology: dynamics and spectrum of gravitational waves. 8 cites
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]

7) Chaplygin inflation in loop quantum cosmology. 4 cites
Xin Zhang, (Shenyang, Northeast U. Tech.) , Jing-fei Zhang, (Shenyang, Northeast U. Tech. & Dalian U. Tech.) , Jing-lei Cui, Li Zhang, (Shenyang, Northeast U. Tech.) . Feb 2009. 6pp.
Published in Mod.Phys.Lett.A24:1763-1773,2009.
e-Print: arXiv:0902.0928 [gr-qc]

8) Observational constraints on loop quantum cosmology. 1 cite
Martin Bojowald, Gianluca Calcagni, Shinji Tsujikawa, . IGC-11-1-1, AEI-2011-004, Jan 2011. 4pp. Temporary entry
e-Print: arXiv:1101.5391 [astro-ph.CO]

After excluding the one duplication, we see that the searches yield 10 papers---primarily observational-QC and mostly written by early universe phenomenologists rather than QC folks themselves.

There are also these that using "cosmic background radiation" instread of "power spectrum" would have caught
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+Dk+cosmic+background+radiation+anD+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

1) ...duplicate...
2) ...duplicate...

3) Constraints on standard and non-standard early Universe models from CMB B-mode polarization. 4 cites
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]

4) Observational hints on the Big Bounce. 3 cites
Jakub Mielczarek, (Jagiellonian U., Astron. Observ.) , Michal Kamionka, (Wroclaw U., Astro. Inst.) , Aleksandra Kurek, (Jagiellonian U., Astron. Observ.) , Marek Szydlowski, (Jagiellonian U., Astron. Observ. & Jagiellonian U.) . May 2010. 24pp.
Published in JCAP 1007:004,2010.
e-Print: arXiv:1005.0814 [gr-qc]

5) Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters. 2 cites
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]

So that makes a total of 13 in all.

 

[marcus]

Here's a proposed list of quantum cosmology criteria

The bulk should have some definite mathematical structure that represents it.
Asymptotically it should look deSitter (accelerated expansion) or at least not the opposite (which is AdS, accelerated contraction).
There should be a quantum state of bulk geometry which exhibits a bounce.
The bounce should either lead to adequate inflation or, if not, the model should produce the usual effects expected from inflation in some other way.
The model should be testable and attract the attention of phenomenologists so they can study means of testing it.

 

[atyy]

There should be a quantum state of bulk geometry which exhibits a bounce.

http://arxiv.org/abs/1101.5592 , p37: While isotropic solvable models of loop quantum cosmology suggest a role of bouncing cosmologies for potential scenarios, no consistent set of equations to evolve inhomogeneities through a bounce has been found.

 

[marcus]

That was the August 2009 talk that Bojo gave at George Ellis's 70th birthday party! I can't find any discussion of some major developments in Loop Cosmology after 2009.
He has added many articles which appeared 2010-2011 (of certain limited types) to the bibliography, so you might think that it could serve as an up-to-date review of the field. In general that would be wrong I think.

Nevertheless I think it is a good article, given that limitations, and I think it serves to support the above list of desiderata. You could say that order for a QC bid to get traction in today's environment it does seem that it must be making progress towards these goals, especially I think two goals: testability and some definite representation of a quantum universe that resolves the classical singularity. And Bojowald shows that he is concerned about those goals.

The whole paragraph from the look-ahead section on page 37, that you quoted from, is worth copying. It is short and it illustrates some of these concerns.
http://arxiv.org/abs/1101.5592
==quote Bojowald "Loop Quantum Gravity and Cosmology" page 37 ==
While isotropic solvable models of loop quantum cosmology suggest a role of bouncing cosmologies for potential scenarios, no consistent set of equations to evolve inhomogeneities through a bounce has been found. The only available options so far make use of gauge (or frame) fixings before quantization, and thus miss crucial aspects of space-time structures. Any mismatch of growing modes in the collapse and expansion phases can easily be enhanced by cosmic evolution, providing opportunities for potential observations but also requiring extreme care in finding fully consistent equations. Inhomogeneous cosmological scenarios remain uncertain, and with it follow-up issues such as the entropy problem.
==endquote==

The context makes it clear that he is talking about a restricted area of Loop cosmology where one works with analytic approximations technically known as solvable equation models. This is one specific type of LQC and it's an area where Bojowald has made a big contribution. I don't see the main action of the field there now. If you look at the recent LQC papers by Ashtekar, Rovelli, Lewandowski, and their post 2009 coauthors, you see them applying spinfoam. In order to keep solvable equation approximations in the game, Bojowald will have to work some degree of inhomogeneity into them---at least as much as people are getting marginally with the current probing of spinfoam LQC. This is not a lot, but it is a competitive field and inching ahead matters.

 

[atyy]

So is there any paper that has a consistent set of equations, inhomogeneities and a bounce?

It's fine if it's simulations, but those must start form a consistent set of equations.

 

[marcus]

So is there any paper that has a consistent set of equations, inhomogeneities and a bounce?

It's fine if it's simulations, but those must start form a consistent set of equations.

You have to realize that HE meant something special by "consistent set of equations"---he would not have included simulations, i.e. the numerical approach with discrete time steps. It is just his technical jargon.

It is clear because at the beginning of the SAME SENTENCE he refers to the solvable equation model.

And he certainly was not including spinfoam cosmology. You know the landmark March 2010 paper of Bianchi Rovelli Vidotto explicitly explains how some marginal degree of inhomog is being included.
But he doesn't even include BRV paper in his bibliography! A dozen people around Ashtekar Lewandowski Rovelli are doing spinfoam cosmology and Bojo does not include ref to even one paper and he does not (as far as I could see) even say "spinfoam" anywhere. It is not his thing so he blanks it out.

His title is right: Loop Quantum Gravity and Cosmology. That is what LQC is. It is not rigidly restricted to some one particular approach.
It used to be. It used to be completely symmetry reduced. Isotropic homogeneous basically down to 2 degrees of freedom. It used to be strictly hamiltonian canonical.
And Bojowald was the main guy in charge until around 2006.

All these restrictions have gradually been being relaxed. Even the "solvable equation model" is not strictly a canonical formulation. It approximates. People show equivalence and they bridge over to new math tools and alternative formulations. Part of the game is to get closer to the FULL THEORY which now I think means combinatorial spinfoam (with connections to GFT) but Bojowald may well see it as some Thiemannish canonical approach.

Ashtekar is sending all his PhDs over to postdoc in Marseille, and not Erlangen. It is clear he sees that LQG cosmology is going in a spinfoam direction.
Look at the lineup at the Zako school where Ashtekar gave the opening talk.

What I'd say is that if you carefully understand what he means by the words he uses, probably everything in Bojowald's paper is correct. After all he is a worldclass expert. But he acts like he has blindspots. Like he doesn't realize how much LQC has developed outside his personal framework.

What I think is, since he gave the paper at August 2009 Ellisfest, in South Africa. He should not have included any bibliography references to papers after August 2009. Then it would be clear that the paper was a snapshot of how he saw the field at that time and was not meant to be about "Loop Quantum Gravity and Cosmology" (= LQC) after mid-2009.
Or he could have the selectively updated biblio and simply SAY that clearly in the Introduction, either way.

It has to be a very valuable paper but in the present version it is too easy to misunderstand it.

As far as your question about inhomogeneity, they are barely scratching the surface. But they are getting some in, as they begin to apply the full theory to cosmology.
I thought the March paper by BRV made that explicit but maybe it was some other 2010 paper. I'll check.

Here again is the Bojowald paper http://arxiv.org/abs/1101.5592
Here is the BRV paper http://arxiv.org/abs/1003.3483
(it was the real beginning of spinfoam LQC. There have been a number of subsequent papers by Penn State and Marseille groups)

 

[marcus]

Page 7 of the March 2010 paper by BRV---it is actually the final paragraph of the paper and is thus given extra emphasis:

==quote http://arxiv.org/abs/1003.3483 ==
Here, however, homogeneous and isotropic states appear naturally as states peaked on homogeneous and isotropic mean values of the quantum states, in the context of a formalism which –we stress– is not a reduction of the dynamics to homogeneous and isotropic degrees of freedom. In physical terms, these states represent a universe where inhomogeneous and anisotropic degrees of freedom are taken into account but fluctuate around zero. This provides also an elegant solution of the problem of having to choose between coordinate or momenta in imposing a symmetry reduction in cosmology [50–52]. Ideally, this formalism could describe inhomogeneous and anisotropic quantum fluctuations of the geometry at the bounce.
==endquote==

They use coherent quantum states that are peaked on homog. isotropic values but are not exclusive of nonuniformity/asymmetry. That's why I called it a marginal relaxation of the earlier restriction. It's a start. They are beginning to apply the full theory to cosmo.

Here's my earlier list of proposed QC criteria. I think of them as goals. This thread has helped get them into clearer focus. I'd like to get anyone's comments. I think that making progress on these could be necessary for any approach to get researcher attention in today's Quantum Cosmology environment. It has in effect become expected of any active QC initiative:

• The bulk should have some definite mathematical structure that represents it.
• Asymptotically it should look deSitter (accelerated expansion) or at least not the opposite (which is AdS, accelerated contraction).
• There should be a quantum state of bulk geometry which exhibits a bounce.
• The bounce should either lead to adequate inflation or, if not, the model should produce the usual effects expected from inflation in some other way.
• The model should be testable and attract the attention of phenomenologists so they can study means of testing it.

 

[marcus]

If those five criteria are correctly chosen then
A) they help explain the shift in interest in QC, the change in the makeup of the topcited papers.

B) we can expect members of the string community to come up with stringy quantum cosmology which meets or appears able to meet all or most of the five.

(There is no longterm reason string program should be stuck with an AdS bulk, or with confusion as to what mathematical structure represents the bulk, or without a quantum bounce. A resourceful theorist should be able to invent a stringy theory that competes with Loop on each of these five counts.)

If you have other ideas---a different set of physics criteria which you think explains the shift in QC---please let us know. Comments have been very helpful so far!

For convenience I consolidated the search terms for Loop phenomenology papers. The combined Spires search now give 14 papers that appeared 2009-2011:

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+POWER+SPECTRUM+or+dk+cosmic+background+radiation%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

FIND (DK QUANTUM GRAVITY, LOOP SPACE OR DK QUANTUM COSMOLOGY, LOOP SPACE) AND (DK POWER SPECTRUM OR DK COSMIC BACKGROUND RADIATION) AND DATE > 2008


1) Cosmological footprints of loop quantum gravity. 33 cites
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]

2) Possible observational effects of loop quantum cosmology. 20 cites
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]

3) Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation. 13 cites
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]

4) Observational constraints on a power spectrum from super-inflation in Loop Quantum Cosmology. 12 cites
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]

5) Fully LQC-corrected propagation of gravitational waves during slow-roll inflation. 11 cites
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]

6) Tensor power spectrum with holonomy corrections in LQC. 10 cites
Jakub Mielczarek, (Jagiellonian U.) . Feb 2009. (Published Feb 2009). 13pp.
Published in Phys.Rev.D79:123520,2009.
e-Print: arXiv:0902.2490 [gr-qc]

7) Loop Quantum Cosmology corrections on gravity waves produced during primordial inflation. 8 cites
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]

8) Inflation in loop quantum cosmology: dynamics and spectrum of gravitational waves. 8 cites
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]

9) Chaplygin inflation in loop quantum cosmology. 4 cites
Xin Zhang, (Shenyang, Northeast U. Tech.) , Jing-fei Zhang, (Shenyang, Northeast U. Tech. & Dalian U. Tech.) , Jing-lei Cui, Li Zhang, (Shenyang, Northeast U. Tech.) . Feb 2009. 6pp.
Published in Mod.Phys.Lett.A24:1763-1773,2009.
e-Print: arXiv:0902.0928 [gr-qc]

10) Loop quantum gravity and the CMB: Toward pre-Big Bounce cosmology. 4 cites
Aurelien Barrau, (LPSC, Grenoble) . Nov 2009. 3pp.
To appear in the proceedings of 12th Marcel Grossmann Meeting on General Relativity (MG 12), Paris, France, 12-18 Jul 2009.
e-Print: arXiv:0911.3745 [gr-qc]

11) Constraints on standard and non-standard early Universe models from CMB B-mode polarization. 4 cites
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]

12) Observational hints on the Big Bounce. 3 cites
Jakub Mielczarek, (Jagiellonian U., Astron. Observ.) , Michal Kamionka, (Wroclaw U., Astro. Inst.) , Aleksandra Kurek, (Jagiellonian U., Astron. Observ.) , Marek Szydlowski, (Jagiellonian U., Astron. Observ. & Jagiellonian U.) . May 2010. 24pp.
Published in JCAP 1007:004,2010.
e-Print: arXiv:1005.0814 [gr-qc]

13) Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters. 2 cites
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]

14) Observational constraints on loop quantum cosmology. 1 cite
Martin Bojowald, Gianluca Calcagni, Shinji Tsujikawa, . IGC-11-1-1, AEI-2011-004, Jan 2011. 4pp. Temporary entry
e-Print: arXiv:1101.5391 [astro-ph.CO]

 

[mitchell porter]

I was wrong to say that Gasperini et al's pre-big-bang model is defunct; there was a http://arxiv.org/abs/1103.2311" is full of resources and review articles.

In my view, the string model of a cosmological bounce with the best pedigree is the http://arxiv.org/abs/hep-th/0204189" , and that is not a realistic model, nor can it easily be made realistic, because exact solutions of string theory on a time-dependent background are hard to come across, and not adjustable.

Robert Brandenberger also has http://arxiv.org/abs/1103.2271" today reviewing two cosmological models, a bounce model, and a "string gas cosmology" where the string gas is just sitting there (indefinitely, timelessly, as the end product of a collapse, I can't tell, and maybe he doesn't know), and then it fluctuates into an expanding state.

I continue to feel that string theory just hasn't found the right way to think about cosmology yet. Maybe some individual string theorist has done so - the seeds of the right approach may already exist in the literature - but what's lacking is the demonstration that this is the right path. It's apparent that no particular approach has swept the field - I think this is the real lesson of Marcus's database searches. Papers are being written, but it's still a cacophony of conflicting ideas. The version of inflation called eternal inflation is probably the favorite of elite opinion, but I'm not sure there's anything like a consensus on how to think about the initial singularity.

The place in string cosmology where contact with empirical data is occurring is in models of inflation. See section 5.2 in http://arxiv.org/abs/0810.3707" of CMB predictions from various string models of inflation, that will be tested as further WMAP data comes in.

Some further opinions:

If we do adopt the view that string theorists should ultimately prefer to go beyond cosmological model-building which involves arbitrary choices and which employs only approximations to the full theory (e.g. such as the ekpyrotic model of a cosmological bounce), and should instead try to find a cosmological idea which is innately inherent to string theory, it might be worth trying to find a cosmological interpretation or cosmological component to the AdS/CFT duality involving N=4 super-Yang-Mills. N=4 SYM is widely regarded as providing the complete, exact specification of IIB string theory on the relevant AdS space; all the string states, brane states, and so on, are hypothesized to emerge from combinations of SYM operators on the boundary. If this is true, then N=4 SYM, remarkably enough (because it's just a plain old field theory), offers the most advanced formulation of string theory that we possesses - and so, with this cosmological purpose in mind, it would be natural to use it as a starting point in this quest for the "true" approach to string cosmology.

On the topic of de Sitter space - the most famous constructions of de Sitter space in string theory were the ones in http://arxiv.org/abs/hep-th/0301240" which introduced the landscape, and they consist of an "uplift" of a vacuum with an AdS ground state into a metastable dS state, by the inclusion of some branes. So it's possible that AdS cosmology is not just a test run for string theory, but actually a ground state, with the observed dS cosmology as a fluctuation.

I'd also like to say that I'm not at all convinced that any form of quantum cosmology is going to be correct. The whole idea of a wavefunction of the universe may be wrong. We don't talk about the wavefunction of a galaxy, or of a star. I know the idea is that in the primordial conditions, there is a structureless simplicity which makes the idea of a universal wavefunction useful, that by the time we have structure formation the universe has decohered into complex classical systems, etc. But it is such a big leap from atom to universe, that it becomes very likely we are generalizing QM in the wrong way when we take it into cosmology.

 

[Fra]

I'd also like to say that I'm not at all convinced that any form of quantum cosmology is going to be correct. The whole idea of a wavefunction of the universe may be wrong
...
it is such a big leap from atom to universe, that it becomes very likely we are generalizing QM in the wrong way when we take it into cosmology.

I fully agree with this! This important concern is annoyingly often ignored.

To speak for my own personal motivation, I am not only "NOT convinced that the mentioned generalisations are right", I am rather very convinced that they are wrong, simply becase the way of abstraction just fails to be an intrinsic view.

/Fredrik

 

[marcus]

I like Mitchell's statement too! I glanced at Robert Brandenberger's recent paper and it looks like he is steering the String program in the direction of the five criteria I mentioned.

Maybe he also thinks that to get researcher attention in today's Quantum Cosmology environment a program should show progress on some of these fronts:

• The bulk should have some definite mathematical structure that represents it.
• Asymptotically it should look deSitter (accelerated expansion) or at least not the opposite (which is AdS, accelerated contraction).
• There should be a quantum state of bulk geometry which exhibits a bounce.
• The bounce should either lead to adequate inflation or, if not, the model should produce the usual effects expected from inflation in some other way.
• The model should be testable and attract the attention of phenomenologists so they can study means of testing it.

Maybe he doesn't address all five, but he certainly touches base on the bounce issue

 

[marcus]

I'd also like to say that I'm not at all convinced that any form of quantum cosmology is going to be correct. The whole idea of a wavefunction of the universe may be wrong...

Loop cosmology very quickly approximates classical Friedmann equation cosmology, almost instantly or within a few tens of Planck times, after the bounce. This is with coherent states peaked on classical, coming into the bounce.

So one can ask "what is the purpose?" Why bother to quantize cosmology if you very quickly get the same old Friedmann equation model that every cosmologist already uses?

There is a pragmatic reason. The whole purpose of QC is to find quantum corrections that apply during the "quantum regime" of extremely high density, that happens during an extremely brief interval, right around the start of expansion.

In fact the Loop program has found a quantum corrected version of the Friedmann equation, and it turns out that it kicks off inflation. So under broad assumptions you get some 60 e-folds as a kind of bonus without fine adjustment.
It remains to see if this quantum-corrected Friedmann eqn is right or not.

You can get a good discussion of this in Ashtekar's review "The Big Bang and the Quantum".
Mathematically the corrections to the model are simple, and lead to the bounce always happening when the density is around 41% of Planck.

So there is a purpose to having a wave function of the U! It can be important for about 100 Planck time units! And that can set things up for a nice conventional inflation. But otherwise ... *shrug*

Same way with the black hole. (Semi) classical analysis goes a long way in, but not all the way. Ultimately you need a quantum model to help guess what happens, and it is always possible there could be some significant surprise there. Or so I think, anyway.

Ashtekar's paper is worth looking at, if you have not already.
The Big Bang and the Quantum
http://arxiv.org/abs/1005.5491

 

[Fra]

So one can ask "what is the purpose?" Why bother to quantize cosmology if you very quickly get the same old Friedmann equation model that every cosmologist already uses?
...
So there is a purpose to having a wave function of the U! It can be important for about 100 Planck time units! And that can set things up for a nice conventional inflation. But otherwise ... *shrug*

I think it's quite clear why we want a "measurement theory" also for cosmological models, but what is not so clear is wether it makese sense to consider the extrinsic measurement theory that is what current QM is - to the universe. I mean, where exactly is the CONTEXT for the wavefunction of the universe?

If we do acknowledge that the context would have to be relative to one of the inside observer (say Planck scale observers), then does it seem reasonable that this type of context allows for stuff like timeless hilbert spaces? and the usually stuff we have in current QM?

So the concern I have, and which I thought was mitchels is wether the new "inside measurement theory that would be suitable for cosmological models" would be cast in terms of a regular "quantum theory of the universe"; such as containing a state space of the entire universe that has the same properties as we expect from a hilbert space in what I think of as the current extrinsic measurement theory.

What exactly does the "wavefunction of the universe" mean, unless we picture a timeless hilbert space? And it's exactly this that doesn't make sense to me. I don't question that we need to turn cosmological modes into measurement theories, I just question that the structure we know from normal QM really makes sense for this task.

In other worlds: A "cosmological measurement theory" cannot be in the form of "vanilla QM". Most probalby (I think at least) the structure of this measurement needs to be different. the basic understanding of the backbones such as hilbert space needs radical reconsiderations.

Edit: The problem is not JUST that there is no background spacetime, the problem is worse; this is not even a background hilbert space. I normal QM, this background hilbert space does exist effectively. But in cosmologicla models, this must fail as far as I see it. So either you resort to realism and consider this hilbert spces to exists in some mathematical realm, or you face that measurement theory needs an intrinsic formulation, and probably away with it goes the timelss hilbert backbone.

/Fredrik

 

[marcus]

Fra I was responding to Mitchell where he said:
==quote Mitchell's post==
I'd also like to say that I'm not at all convinced that any form of quantum cosmology is going to be correct. The whole idea of a wavefunction of the universe may be wrong...
==endquote==

There is a pragmatic reason for developing QC, and the possible correctness is rooted in that. So I explained. The bits you quoted don't convey much meaning taken out of context.

==quote my post==
Loop cosmology very quickly approximates classical Friedmann equation cosmology, almost instantly or within a few tens of Planck times, after the bounce. This is with coherent states peaked on classical, coming into the bounce.

So one can ask "what is the purpose?" Why bother to quantize cosmology if you very quickly get the same old Friedmann equation model that every cosmologist already uses?

There is a pragmatic reason. The whole purpose of QC is to find quantum corrections that apply during the "quantum regime" of extremely high density, that happens during an extremely brief interval, right around the start of expansion.

In fact the Loop program has found a quantum corrected version of the Friedmann equation, and it turns out that it kicks off inflation. So under broad assumptions you get some 60 e-folds as a kind of bonus without fine adjustment.
It remains to see if this quantum-corrected Friedmann eqn is right or not.

You can get a good discussion of this in Ashtekar's review "The Big Bang and the Quantum".
Mathematically the corrections to the model are simple, and lead to the bounce always happening when the density is around 41% of Planck.

So there is a purpose to having a wave function of the U! It can be important for about 100 Planck time units! And that can set things up for a nice conventional inflation. But otherwise ... *shrug*

Same way with the black hole. (Semi) classical analysis goes a long way in, but not all the way. Ultimately you need a quantum model to help guess what happens, and it is always possible there could be some significant surprise there. Or so I think, anyway.

Ashtekar's paper is worth looking at, if you have not already.
The Big Bang and the Quantum
http://arxiv.org/abs/1005.5491
==endquote==

Cosmology is based on the Friedmann model which shows how vital parameters like scale and density evolve with time. To push that model back to the start of expansion you need quantum corrections reflecting non-classically high density. It blows up otherwise. It's a straightforward commonsense strategy. Keep it simple. And it works. You get a theory with predictions that you can test. (If it fails test, throw it out. If it passes then you get bonuses like a good start to inflation.)

Here's Battisti and Marciano's spinfoam version of the bounce. It is rather nice, and leads to the same predictions as the quantum Friedmann version (because the foam used is a simple low order one).

Big Bounce in Dipole Cosmology
http://arxiv.org/abs/1010.1258

We are moving ahead on an empirical track here, I think, and it is time to read and learn about the models. If this gambit fails then we can maybe go back to philosophizing. But right now it is bounce phenomenology that is at the forefront.
Check out post #76 re phenomenology:
https://www.physicsforums.com/showthread.php?p=3186856#post3186856
Or try this literature search:

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+POWER+SPECTRUM+or+dk+cosmic+background+radiation%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29
This is what shows up in the search window. Spires turns everything to all caps
FIND (DK QUANTUM GRAVITY, LOOP SPACE OR DK QUANTUM COSMOLOGY, LOOP SPACE) AND (DK POWER SPECTRUM OR DK COSMIC BACKGROUND RADIATION) AND DATE > 2008

 

[atyy]

What does it mean to have N change in the CFT? Naively I imagine N to be fixed in a particular theory. Then gravity should be classical at large distances bulk distances and quantum at small bulk distances. Is that somehow equivalent to changing N in some sense?

 

[fzero]

What does it mean to have N change in the CFT? Naively I imagine N to be fixed in a particular theory.

It does look strange from the gauge theory point of view, but it is natural on the AdS side, where N corresponds to flux of a certain p-form charge. You might be more comfortable calling these families of gauge theories. The word we use doesn't matter so much as long as the picture of the duality is understood.

Then gravity should be classical at large distances bulk distances and quantum at small bulk distances. Is that somehow equivalent to changing N in some sense?

The energy of the probe is different to what we were talking about. The AdS X S space has a scalar curvature that is inversely proportional to the square of the radius of the sphere. If this radius is small in Planck units, then quantum gravity is important at all distances.

 

[atyy]

The energy of the probe is different to what we were talking about. The AdS X S space has a scalar curvature that is inversely proportional to the square of the radius of the sphere. If this radius is small in Planck units, then quantum gravity is important at all distances.

How about the when the bulk doesn't have constant curvature? I've read that that the duality can be extended to some QFTs which are not CFTs, but have a UV fixed point - in that case can we have the bulk be classical or quantum without changing N?

Also, if the energy of the probe is different from the "quantumness" of gravity, does it mean that probes of arbitrarily high energy can still see a classical spacetime? I guess I've heard the handwavy talk that we expect quantum gravity to be important in a small window of energies around the Planck scale, with big classical black holes formed far above that.

 

[fzero]

How about the when the bulk doesn't have constant curvature? I've read that that the duality can be extended to some QFTs which are not CFTs, but have a UV fixed point - in that case can we have the bulk be classical or quantum without changing N?

We're still dealing with spacetimes which are asymptotically AdS X M, at least in the UV. There will still be a relationship between the radius of curvature of this asymptotic AdS and N. Classical supergravity will be valid in regions where the curvature is small. Within the conjecture, the dual field theory is valid everywhere, even if there are technical details in performing computations.

Also, if the energy of the probe is different from the "quantumness" of gravity, does it mean that probes of arbitrarily high energy can still see a classical spacetime? I guess I've heard the handwavy talk that we expect quantum gravity to be important in a small window of energies around the Planck scale, with big classical black holes formed far above that.

Probes with energies near the Planck scale will change the background geometry, so QG is presumably needed to give a complete description of their processes. The general issue in curved space is whether there is ever a perturbative description of low energy probes. That's where the consideration of curvature comes into play.

 

[atyy]

fzero, thanks a lot!

What would sending in probes of higher and higher energy in the bulk correspond to in the boundary theory?

 

[fzero]

fzero, thanks a lot!

What would sending in probes of higher and higher energy in the bulk correspond to in the boundary theory?

There's a certain sense where the energy of a probe corresponds to the distance r from the boundary at which it's localized. This follows from the wave equation on AdS. These states are obtained by propagating the boundary data \phi_0 into the bulk:

\phi(r,x) = \int dy G(r,x-y) \phi_0(y).

Then correlation functions of probes are roughly related to certain integrated CFT correlators:

\langle \phi_1 \cdots \phi_n \rangle \sim \int G_1 \cdots \int G_n \langle \mathcal{O}_1 \cdots \mathcal{O}_n \rangle.

A more careful treatment can be found in http://arxiv.org/abs/hep-th/9903048

 

[marcus]

The shift of interest in QC seems correlate with increased interest the bounce as something that can be tested and/or has to do with inflation.

Earlier I used a Spires search that got 14 phenomenological papers using keywords "power spectrum" and "cosmic background radiation"---2009 or later.

I recently tried one that got 29 phenom. papers, using "fluctuation, primordial", "inflation", and "cosmic background radiation"---2009 or later.

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+primordial%2C+fluctuation+OR+DK+INFLATION+OR+DK+COSMIC+BACKGROUND+RADIATION%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

I'd like to know the physics grounds for this (recalling the question asked in post #1 of the thread.)

I can't believe it's a mere whim among the researchers.

 

[marcus]

I corrected an error in the link:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+FLUCTUATION%2C+PRIMORDIAL+OR+DK+INFLATION+OR+DK+COSMIC+BACKGROUND+RADIATION%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

I just ran this same search for successive time periods and found an increase in numbers of Loop early universe phenomenology papers:

2003-2004 5
2005-2006 4
2007-2008 18
2009-2010 26

It is difficult to sort out what has happened and to understand the physical grounds.

Loop Cosmology has risen to prominence in Quantum Cosmology---both in terms of the number of papers and in terms of citations.
Can this have to do with the comparative testability of the theory?
Or with the comparative concreteness (it provides a definite model of the cosmos to work with)?
Or with the relative absence of extra physical baggage?
Or with the theory's comparative mathematical simplicity?

I'd welcome other people's ideas about this.

Ashtekar Sloan's March 2011 paper provides some alternative ideas:
1. the bounce provides a platform for laying out initial conditions and probability measures on initial conditions. Thus one can do physics concerning inflation etc that one cannot do in the classical setup with it's singularity.
http://arXiv.org/abs/1103.2475

2. the bounce makes an adequate inflation episode more "natural" and relieves some need for fine tuning.
http://arXiv.org/abs/0912.4093
http://arXiv.org/abs/1011.5516