What caused the shift of interest in quantum cosmology?

[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

 

[marcus]

One reaction might be denial---to say there is nothing to explain because no shift in research emphasis occurred. As an observer I have the clear impression that one has. This is partly a matter of perception, but I try to supplement personal judgment with minor bits of evidence. Earlier I tabulated the Loop and String representation in the QC "top ten", over the years since 1995.
To get a little larger sample size I now want to re-tabulate using the "top 25".

Papers in the QC top ten
Years   1996-1998  1999-2001  2002-2004  2005-2007  2008-2010
String       3          3          1          2          1
Loop         0          4          7          7          8

All we do here is just go to the Stanford-SLAC Inspire search engine and use the keyword "quantum cosmology", ranking by citation count. Then count the number of papers of each kind which made the top 25.

Papers in the QC top 25
Years   1996-1998  1999-2001  2002-2004  2005-2007  2008-2010
String       5          6          3          5          1
Loop         0          7         16         16         16

I still have to double check some of the earlier numbers ( the title and abstract of some papers aren't clear enough to classify the article and I have to examine the text, which takes time.) But I think they are nearly correct.

The most startling are those for 2008-2010, which I did just now doublecheck, because the imbalance is so stark. I'll put the Inspire search link in case anyone would like to make their own count.
http://inspirebeta.net/search?ln=en...2y=2010&sf=&so=a&rm=citation&rg=25&sc=0&of=hb

 

[marcus]

Mitchell Porter brought up some interesting thoughts earlier. I will highlight some parts:

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. ...

There is much more food for thought if you go back to Mitchell's original post. For brevity I merely excerpt from it here. Unfortunately AFAIK no further WMAP is expected. Polchinski was writing in 2008 and WMAP is over.

Rightly or not, the Robert Brandenberger paper prompted me to recall some criteria that I suspect might tend to shift quantum cosmology interest back in the direction of the string program...

... it looks like he is steering the String program in the direction of the five criteria I mentioned.
...

• 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.

...

As long as we are looking for reasons, part of what caused the shift might have to do with the concreteness, simplicity, and testability of Loop cosmology. That is, instead of negative aspects in one program it might have to do with positives in an alternate.
The shift of QC interest does appear to correlate with increased interest Loop bounce as something that can be tested and/or has to do with inflation.

The following search now gets 30 Loop phenomenology papers--2009 or later--using "fluctuation, primordial", "inflation", and "cosmic background radiation".

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

The shift also parallels a recent decline in research the DESY librarians classify as "string model" and/or "membrane model". I noticed this in another thread. Each name was added "blind" to the list without checking ahead to see how the numbers turned out. PAllen kindly provided some of the names.

          1995-1998      1999-2002      2003-2006      2007-2010
Witten         38             29              9              5
Strominger     23             14             22              4
Maldacena      27             33             24              9 
Polchinski     21             17             11              4
Harvey,J       16             15              9              2
Duff,M         24             17              8              5
Gibbons,G      17             29             11              2
Dijkgraaf      18             11              9              7
Ooguri         31             18             13              8
Silverstein,E  16             15             16             10
Seiberg,N      19             16             22              1

=======================
If you would like to check Spires keyword string or membrane publication numbers for anyone else, simply substitute another name instead of "Silverstein, E" in the following links and repeat the search.
1995-1998
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=a+Silverstein, E +and+%28dk+string+model+OR+dk+membrane+model%29+and+date+%3E+1994+and+date+%3C+1999&FORMAT=WWW&SEQUENCE=
1999-2002
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=find+a+Silverstein, E+and+%28dk+string+model+or+dk+membrane+model%29+and+date+%3E+1998+and+date+%3C+2003&FORMAT=WWW&SEQUENCE=
2003-2006
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=find+a+Silverstein, E+and+%28dk+string+model+or+dk+membrane+model%29+and+date+%3E+2002+and+date+%3C+2007&FORMAT=WWW&SEQUENCE=
2007-2010
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=find+a+Silverstein, E+and+%28dk+string+model+or+dk+membrane+model%29+and+date+%3E+2006+and+date+%3C+2011&FORMAT=WWW&SEQUENCE=

 

[cristo]

As long as we are looking for reasons, part of what caused the shift might have to do with the concreteness, simplicity, and testability of Loop cosmology.

Where's the testability in loop cosmology?

 

[marcus]

"Where's the testability?" That is an excellent question! See this part of the immediately preceding post:
=====quote=====

The following Spires search now gets 30 Loop phenomenology papers--2009 or later--using "fluctuation, primordial", "inflation", and "cosmic background radiation".

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
===endquote===

One point to notice is that most of the papers are by phenomenologists (professional theory testers) rather than the Loop theorists themselves. The pheno people have only recently gotten interested in Loop cosmology (since 2007 or 2008 ) and this has led to a significant increase in the number and type of Loop early universe pheno papers.

Another point is that several offer definite ideas of observable consequences of the LQG bounce, which could potentially falsify or rule out the bounce.

Another point to notice is that the LQG bounce is robust in the sense that they try all kinds of variations (including some inhomogeneity and anisotropy, and varying parameters and open/closed etc) and they always get the bounce.

Preliminary calculations by Benedetti&Marciano also get the bounce using spin foam dynamics.

So if you can rule out or falsify the LQG bounce you essentially rule out all or much of the current LQG theory.
=======================

Here is a sample of the papers which that Spires link turns up:1) Cosmological footprints of loop quantum gravity.
J. Grain, (APC, Paris & Paris, Inst. Astrophys.) , A. Barrau, (LPSC, Grenoble & IHES, Bures-sur-Yvette) . Feb 2009. (Published Feb 27, 2009). 7pp.
Published in Phys.Rev.Lett.102:081301,2009.
e-Print: arXiv:0902.0145 [gr-qc] 34 cites
...
...
9) Inflation in loop quantum cosmology: dynamics and spectrum of gravitational waves
Jakub Mielczarek, (Jagiellonian U.) , Thomas Cailleteau, (LPSC, Grenoble) , Julien Grain, (Paris, Inst. Astrophys.) , Aurelien Barrau, (LPSC, Grenoble) . Mar 2010. (Published May 15, 2010). 11pp.
Published in Phys.Rev.D81:104049,2010.
e-Print: arXiv:1003.4660 [gr-qc] 10 cites
...
...
15) Constraints on standard and non-standard early Universe models from CMB B-mode polarization.
Yin-Zhe Ma, (Cambridge U., KICC & Cambridge U., Inst. of Astron.) , Wen Zhao, (Cardiff U.) , Michael L. Brown, (Cambridge U., KICC & Cambridge U., Inst. of Astron. & Cambridge U.) . Jul 2010. 41pp.
Published in JCAP 1010:007,2010.
e-Print: arXiv:1007.2396 [astro-ph.CO] 4 cites
...
...
20) Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters.
Julien Grain, (Paris, Inst. Astrophys. & Orsay, LAL) , Aurelien Barrau, Thomas Cailleteau, (LPSC, Grenoble) , Jakub Mielczarek, (Jagiellonian U.) . Nov 2010. (Published Dec 15, 2010). 12pp.
Published in Phys.Rev.D82:123520,2010.
e-Print: arXiv:1011.1811 [astro-ph.CO] 3 cites
...
...
25) Inflation and Loop Quantum Cosmology.
Aurelien Barrau, . Nov 2010. 5pp.
e-Print: arXiv:1011.5516 [gr-qc] 2 cites

Cristo, thanks for asking! I'm glad to have an opportunity to go into a little deeper into the the recent testability developments. Basically the pheno people are talking about missions like "B-pol", if a CMB-polarization spacecraft gets funded---kind of a "next thing after Planck".

Thanks to Sabine Hossenfelder for pointing out the Wen Zhao paper. She has a review of quantum gravity testing. I think it was Atyy who gave the link to Sabine's review paper. The Wen Zhao paper is #15 above, I made it one of my sample from the Spires search.
I should get the link to Sabine's paper, to provide a broad context.

 

[cristo]

Thanks for the references, especially the Ma et al paper, which is a good review.

So if you can rule out or falsify the LQG bounce you essentially rule out all or much of the current LQG theory.

This is an interesting comment, and one that I've been wondering about for a while. If I've understood things correctly, one can essentially think of loop cosmology as inflation preceded by a bounce. The major observational prediction by this model is that the tensor spectrum is not near scale invariant, but is very blue on large scales, has a peak and then returns to near scale invariant on small scales.

This is a good prediction, but would one be able to rule out loop cosmology by seeing a GW spectrum that was not like this (from, e.g., something like CMBPol)? That is, is this a prediction that would be made by all bouncing cosmologies and therefore enough to rule out the loop quantum gravity? If not, then loop gravity is not testable in this way.

Surely there are many ways to change loop cosmology (e.g. not assuming Friedmann-Robertson-Walker) that could feasibly change this prediction?

 

[marcus]

What you say sounds reasonable. Observations from a mission like CMBPol (one of those referred to in the paper you mentioned) might be expected merely to constrain, and to rule out at most some versions.

I don't know enough to try to say how much wiggle room Loop cosmology has, if observation of GW spectrum by a mission like CMBPol would go against the current version.

One direction of research that I think likely to clarify this is exemplified by the Battisti Marciano paper about the spinfoam bounce. As I understand it, this is just a preliminary first order calculation but does not assume FRW. It is not modeled on the earlier LQC or on the homongeneous isotropic Friedmann picture. In a rudimentary way it actually uses the full theory! But peaked on homog and isotropic, and using only the very simplest spin foams.
And apparently Battisti Marciano still get a bounce. I hope to see that confirmed and I also hope to see more work along those lines: using the full LQG (spinfoam) theory, not just the quantized FRW simplification. It is more challenging to calculate but it deepens the extent of testability.

 

[atyy]

Hmmm, the full theory is not known to give Einstein's equations or even to exist mathematically, so I doubt it would be better than Bojowaldian LQC (which seems to be nicely controlled).

I would accept the prediction of Bojowald's LQC alone, with the caveats that it assumes symmetry, does not proceed from a full theory of QG and cannot be ruled out by observation. There is really nothing wrong with saying, if we see this, then this theory can explain it easily, even if not seeing it doesn't mean the theory can't be tweaked.

 

[atyy]

http://backreaction.blogspot.com/2010/10/experimental-search-for-quantum-gravity.html points to http://agenda.albanova.se/conferenceDisplay.py?confId=1124

 

[Sardano]

Hi everybody,

I think the main problem here its what a string theory paper is. By the analysis performed by marcus, it seems that he thinks that a string theory paper is a paper whose title contains the word "string" and otherwise it is not a string theory paper. Of course, this is not true: there are plenty of papers about string theory nowdays, but they cover a lot of related areas that have appeared with the development of String Theory, and that if you are not a String Theorist you may not recognize them as String Theory related papers.

The fundamental investigation of String Theory like it was done in the 90's is not used anymore due to the intrisinc difficulty of the calculations: alternative approaches have been pursued that may not develop the String Theory itself but its connections with other areas of physics: of course, this is still string theory research and this kind of papers should be taken into account when we speak about string theory papers.

 

[marcus]

By the analysis performed by marcus, it seems that he thinks that a string theory paper is a paper whose title contains the word "string" and otherwise it is not a string theory paper...

No. I don't think that. It is certainly not assumed in any of the analysis or discussion here!

 

[marcus]

Here's an example of what we are talking about in this thread, and the very simple analysis that comes up.
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
This depends on the classification of research papers by DESY librarians. The particular search here picks out Loop early universe phenomenology papers. It may not get them all, and it may occasionally pick up false positives, but I can testify that it works pretty well (having watched the literature myself for a considerable time) and it is repeatable.

If you click on the link you will see how to change the search, and in particular how to set it for different time periods. The link gets papers that appeared 2009 and later, but you can set it to get 2003-2004 papers, for instance. I did this and came up with the following little table.

Here are Loop early universe pheno papers for some successive time periods, using that Spires search.

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

If you check out the recent ones you see they are mostly not by Loop theorists but instead are by phenomenologists--professional theory testers interested in confronting various theories with observation.

The obvious growth in this kind of thing is a large part of what I mean by the shift in quantum cosmology.

Nothing in this this thread depends on merely looking for the presence or absence of particular words in the titles of research papers.

What is interesting, I think, is to try to figure out what feature of the physics have caused these various shifts in the field of quantum cosmology.
I suggested a few back in post #91 of this thread.

 

[atyy]

Hi everybody,

I think the main problem here its what a string theory paper is. By the analysis performed by marcus, it seems that he thinks that a string theory paper is a paper whose title contains the word "string" and otherwise it is not a string theory paper. Of course, this is not true: there are plenty of papers about string theory nowdays, but they cover a lot of related areas that have appeared with the development of String Theory, and that if you are not a String Theorist you may not recognize them as String Theory related papers.

The fundamental investigation of String Theory like it was done in the 90's is not used anymore due to the intrisinc difficulty of the calculations: alternative approaches have been pursued that may not develop the String Theory itself but its connections with other areas of physics: of course, this is still string theory research and this kind of papers should be taken into account when we speak about string theory papers.

That's pretty much what everyone has said.

 

[marcus]

That's pretty much what everyone has said.

I don't think you comment fairly, Atyy. Some of what Sardano said about the change of focus in the String program (e.g. towards AdS/CFT) is quite true. But this is not central to the discussion of the shift of interest in QC which we are trying to understand in this thread.

Or if you think internal changes in String program are important to where the field of quantum cosmology is developing, and why, then please explain why you think that

 

[atyy]

I don't think you comment fairly, Atyy. Some of what Sardano said about the change of focus in the String program (e.g. towards AdS/CFT) is quite true. But this is not central to the discussion of the shift of interest in QC which we are trying to understand in this thread.

Or if you think internal changes in String program are important to where the field of quantum cosmology is developing, and why, then please explain why you think that

I think you were the first one to bring in strings.

I also think we can learn some physics by asking what caused the sizeable drop in researcher interest in string over the past 10 years. Those who remember the confidence and excitement back around 2001-2003 must realize there has been a huge decline. We don't need statistics to prove this, it's frankly obvious. But I'll give an illustration--one of quite a few available.

It used to be that as many as twelve recent string papers would make the annual Spires top 50 list---the most cited papers during a particular year.

Here are top 50 lists for some past years with number of recent string papers making the list shown in parentheses.

http://www.slac.stanford.edu/spires/topcites/2001/annual.shtml (twelve)
http://www.slac.stanford.edu/spires/topcites/2003/annual.shtml (six)
http://www.slac.stanford.edu/spires/topcites/2005/annual.shtml (two)
http://www.slac.stanford.edu/spires/topcites/2007/annual.shtml (one)
http://www.slac.stanford.edu/spires/topcites/2009/annual.shtml (one)
http://www.slac.stanford.edu/spires/topcites/2010/annual.shtml (zero)

In this tally, papers are counted as recent if they appeared during the past five years. For instance in 2001 (recent meaning 1997-2001) twelve of the most highly cited fifty were recent string . Their ranks were 2,3,4,5,6,13,14,17,22,39,49, and 50.

By contrast in 2009 (recent being 2005-2009) only one of the fifty top-cited papers was recent string . It was number 33 on the list.

There are many kinds of evidence all pointing to the same disappointing fact. Recent string papers simply are valued less by other researchers and attract less attention (and citations) than they used to.

What is of interest is not this or that piece of evidence, most of us probably realize this has happened and do not require proof at this point. The interesting thing is the concrete physics reasons. What theoretical features and results correlate with this decline and may have contributed to it?

What do you think are the most important reasons?

Here are some possible physics causes you might wish to consider, I would be glad to have other possibilities suggested.

Supersymmetry not confirmed.
The String Landscape (the KKLT paper of 2003, so far no way to choose among 10⁵⁰⁰ versions of physics)
Positive cosmological constant (universe is not AdS) measured in 1998 but took a while to sink in
Seeming awkwardness accommodating cosmic inflation (search for alternatives to it)
Many parts of program dependent on a "fixed prior geometry" (Wheeler's term)

Any other ideas of physics circumstances that contributed? Which causes do you think are the most important?

I don't think we're interested in social, or political/economic, explanations in this thread---mainly because they don't appear to be very important in this case. The decline in string citations began by 2003, long before any public news or discussion (at least that I recall.) And I think the physics reasons are in any case much stronger and more decisive than any social ones could be. So hopefully we can focus on physics explanations. Potentially far more instructive.

 

[atyy]

Anyway, I think we shall never agree on this.

Let's talk about physics. Do you really think the new spin foam cosmology is the reason Bojowald's stuff is so nice? I mean, the new stuff is completely kludged in comparison. I understand pioneering often has to be that way, but are there any other known openings for generalizing Bojowald's work?