What caused the shift of interest in quantum cosmology?

[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?

 

[Sardano]

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

On the String Theory side you are not making the right search. For example, virtually, all the supergravity papers are String Theory papers, because they are focused in the interest of Supergravity as a Strig Theory effective field description, and they include a lot of topics, like phenomenology, Black Holes in String Theory...However, you missed them in your searchs.

 

[marcus]

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

Wait just a minute now! There is no taboo against mentioning the decline in the String program if it is relevant (although it may not be the whole cause of what has happened in QC). The decline in string cites that I mentioned includes all aspects of the String program! Don't take the easy denial route and seem to agree with Sardano that I was simply "looking at words in the title". I examined any papers in the Spires top 50 for each of these years that could possibly be stringy. I think if you wanted to take the time to check them out your classification would agree with mine.

Moreover the general decline in interest IS arguably a factor when we look at the specialized area of quantum cosmology. It is not the whole set of causes but it can be argued that it plays a role.

So if you think I could have misclassified, please look at the list and find some examples.
=====quote=====
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.
==endquote==

 

[marcus]

On the String Theory side you are not making the right search. For example, virtually, all the supergravity papers are String Theory papers,...

I think you may be living in a fantasy world where you attribute, say to Lance Dixon (an expert in SUGRA), a stringy motivation which he does not necessarily have.

But you are free to say this or that logically separate research line is "really string" and construct your own searches. If they have some bearing on the recent shift of interest in quantum cosmology (I use the DESY librarians definition of that category) then be my guest! You are welcome to post them.

Keep in mind that quantum cosmology as most people understand it is not primarily about Black Holes . It tends to be about the early universe, and moreover it is quantum rather than classical.

 

[marcus]

...Do you really think the new spin foam cosmology is the reason Bojowald's stuff is so nice?...

Aren't you ignoring the elephant in the room? Ashtekar is the central figure in Loop cosmology and his papers really define where the field is at present.

I'm not sure how you think "Bojowald's stuff" differs from the current mainstream of Loop cosmology. What is different or special about Bojowalds recent (say 2009-present) work?

 

[Sardano]

I think you may be living in a fantasy world where you attribute, say to Lance Dixon (an expert in SUGRA), a stringy motivation which he does not necessarily have.

.

If you are talking about the finitness of N=8 Sugra I agree, it is nowdays basically the only case in which a Supergravity Theory is studied "per se". However, it is related to string theory in the methods used and in the conclusions, because N=8 is the low energy field theory of type II compactified in torus down to 4 dimensiones, and much of the work made in the topic can be used to extract conclusions in String Theory. See the work of Kallosh for more details.

I all the other cases, the study of supergravity and lately, supergravity plus stringy corrections is always related to string theory.

 

[Haelfix]

It has been explained multiple times now, but the keyword 'quantum cosmology' is basically a buzzword for things like the no boundary proposal, loop quantum cosmology, the Wheeler De Witt equations and things like that.

When string theorists study cosmology, it typically involves models like string gas cosmology or membrane cosmology and the idea is to generate suitable conditions to start inflation and/or to explain the inflaton field as well as other questions (like why 4 dimensions). This is just as testable (in fact testable in the exact same way) as anything from LQC, with the caveat that probing the power spectrum suffers from an enormous inverse problem.

As far as quantum cosmology proper, string theorists lack the mathematical technology to really ask questions about how the quantum foam behaves or if it even exists (that is a question in the strong coupling limit of string theory that is still very opaque even in contexts like AdS/CFT). The few papers on the subject that exist, typically go back to the more established textbook treatments like trying to derive a Hawking-Hartle state.

So there has been no 'shift of interest', instead there hasn't been much interest at all in the same way that there wasnt interest in studying classical fluid mechanics with string theory (up until recently) since the mathematical connections weren't there yet.

However if you want to ask 'what does string theory have to say about cosmology' then by all means, write 'cosmology' or 'inflation' or 'compactification' as keywords and let us know what you get.

 

[marcus]

...As far as quantum cosmology proper, string theorists lack the mathematical technology to really ask questions about how the quantum foam behaves or if it even exists (that is a question in the strong coupling limit of string theory that is still very opaque even in contexts like AdS/CFT). The few papers on the subject that exist, typically go back to the more established textbook treatments like trying to derive a Hawking-Hartle state.

So there has been no 'shift of interest', instead there hasn't been much interest at all in the same way that there wasnt interest in studying classical fluid mechanics with string theory (up until recently) since the mathematical connections weren't there yet.
...

Haelfix that is an interesting point of view. Thanks for sharing your ideas!

It could be that the main change I am noticing here is primarily the growth of interest in Loop quantum cosmology.

I am definitely focusing on quantum cosmology proper, as you call it. I think that is primarily the resolution of the classical singularity ("big bang") by quantum means.

 

[fzero]

I think you may be living in a fantasy world where you attribute, say to Lance Dixon (an expert in SUGRA), a stringy motivation which he does not necessarily have.

Have you ever read one of his papers? A huge part of the program that Bern, Dixon and Kosower have championed for the past 15-20 years is based on string inspired relations between open string amplitudes (which give gauge theory amplitudes) and closed string amplitudes (which give (super)gravity amplitudes). These are referred to as Kawai-Lewellen-Tye (KLT) relations in Dixon's most recent papers. Twistor methods (again string inspired) have also been a huge part, but the most recent papers on N=8 sugra amplitudes rely almost entirely on the KLT relations between N=8 sugra and N=4 SYM amplitudes. This is an entire, extremely fruitful, branch of research that would not exist in the absence of string theory.

But you are free to say this or that logically separate research line is "really string" and construct your own searches. If they have some bearing on the recent shift of interest in quantum cosmology (I use the DESY librarians definition of that category) then be my guest! You are welcome to post them.

Again, these are the same librarians that labeled a single paper with the term "quark model"
in the years 1995-1999 (http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+quark+model%29++AND+DATE+%3E+1995+and+date+%3C+2000&FORMAT=www&SEQUENCE=citecount%28d%29 ), which is incidentally the 4 1/2 year period after the discovery of the top quark. They may have well-intentioned motivations behind their labeling system, but it's pretty obvious that it's not useful for any reliable statistics.

 

[atyy]

Aren't you ignoring the elephant in the room? Ashtekar is the central figure in Loop cosmology and his papers really define where the field is at present.

I'm not sure how you think "Bojowald's stuff" differs from the current mainstream of Loop cosmology. What is different or special about Bojowalds recent (say 2009-present) work?

No, I include Ashtekar automatically when I say Bojowald (like I automatically include Wegner if I say Wilson loop). The distinction I was drawing is between the canonical approach and the new spin foam dipole calculations. The former, which includes Bojowald, Ashtekar, Henderson, and others I'm not remembering off the top of my head, seems beautifully controlled, if under somewhat artificial conditions. The latter drops many higher order terms, whose unimportance has not been demonstrated.

Let me expand a bit. I don't believe spin foams is on the right track. I think the formalism has potential, but they are ignoring AdS/CFT as to its correct interpretation. However, there are two nice results from non-string approaches that at present are in isolation. The first is CDT's, the second is Bojowaldian LQC. What is the correct context to place them in? Vidotto and Rovelli are trying to place it within the Rovellian interpretation of spin foams, which I suspect will not work (I could of course be wrong), and which is undeniably kludgey at the moment, so I am wondering if there are any othe ropenings known to you for explaining Bojowaldian LQC.

Incidentally, I wonder what Loll's thinking of the underlying theory for CDT is at the moment. Asymptotic Safety is no longer seen to be so probably the reason. I wonder if she has strings at the back of her mind.

 

[Haelfix]

It could be that the main change I am noticing here is primarily the growth of interest in Loop quantum cosmology.

Yep!

I am definitely focusing on quantum cosmology proper, as you call it. I think that is primarily the resolution of the classical singularity ("big bang") by quantum means.

Maybe, maybe not. Again it depends really what the correct degrees of freedom are in the quantum gravity world.

Historically, the fundamental object of study in quantum cosmology was the space of connections modulo gauge transformations. This was then inserted into a formal generalization of a path integral (with appropriately illdefined measure), and then various approximations or boundary conditions were imposed on the object (mini/midi superspace, etc).

But the key thing to note is that the ensuing wave function of the universe, was still characterized by the starting point, which was still all about the metric (or really the connection symbols).

If string theory is correct, such a thing will probably be only yet another crude approximation to the fundamental underlying physics, which contains much more information than merely the classical geometry.

 

[marcus]

I think Haelfix's point (if valid) would lead to the conclusion that the String numbers in this table are irrelevant. The String contribution to what we would consider quantum cosmology proper (quantum resolution of the classical bb singularity, basically) is not ready to be counted because the math is not yet there.

To get this table, I went to the Stanford-SLAC Inspire search engine and use the keyword "quantum cosmology", ranking by citation count. Then counted the number of papers of each kind which made the top 25. It would be interesting to know if anyone gets a different count.

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

Perhaps I should just erase the first row because, if Haelfix is right, it is not meaningful. There was no significant "shift away" (in that case) but only a "shift towards". This could be true! and some readers may want to look down the "top 25" search result listings to see what kinds of research is actually involved. It could very well corroborate what Haelfix just said:
1996-1998
http://inspirebeta.net/search?ln=en...2y=1998&sf=&so=a&rm=citation&rg=25&sc=0&of=hb
1999-2001
http://inspirebeta.net/search?ln=en...2y=2001&sf=&so=a&rm=citation&rg=25&sc=0&of=hb
2002-2004
http://inspirebeta.net/search?ln=en...2y=2004&sf=&so=a&rm=citation&rg=25&sc=0&of=hb
2005-2007
http://inspirebeta.net/search?ln=en...2y=2007&sf=&so=a&rm=citation&rg=25&sc=0&of=hb
2008-2010
http://inspirebeta.net/search?ln=en...2y=2010&sf=&so=a&rm=citation&rg=25&sc=0&of=hb

In the last interval, 2008-2010, the one String paper I found in the top 25 QC was actually a review of bounce cosmologies some part of which was about stringy bounces. It was not what you normally think of as stringy research but it did discuss string QC as part of a general review. The paper was
Bouncing Cosmologies.
M. Novello, S.E.Perez Bergliaffa
and it has 76 citations.

 

[Sardano]

Anyway, Marcus, what are you traying to say?

Are you traying to say that string theorist are are changing their fields to LQG?

Are you traying to say that the new researchers are choosing LQC in front of String Theory?


Because if there is really a growth in the number of LQC papers is because the people that used to do LQG is now doing LQC, since LQG is obviously a dead end. You should compare your numbers with the number of LQG papers among the most cited ones. You will see that if LQC is going up, LQG is going down in more or less the same amount. I haven't checked, its a prediction ;) .

 

[marcus]

...

Because if there is really a growth in the number of LQC papers is because the people that used to do LQG is now doing LQC, since LQG is obviously a dead end. You should compare your numbers with the number of LQG papers among the most cited ones. You will see that if LQC is going up, LQG is going down in more or less the same amount. I haven't checked, its a prediction ;) .

Thanks for your prediction.

 

[marcus]

BTW after weighing some of the well-considered points in this thread and thinking some more about it, I am leaning toward the view that what we are seeing in quantum cosmology could simply be described as growth of interest in the bounce picture---mainly the Loop cosmology version perhaps (judging from the make-up of the most highly-cited QC papers---but perhaps not even that specific.

Maybe the key thing here is growth of research interest in bounce cosmology. Haelfix may have concurred with a "yep" to something like that, no time to check right now.

The physics features that could explain that might be (you may want to suggest others):

a) testability
b) simplicity (no extra baggage)
c) concreteness (you can calculate and derive observable predictions leading to a.)
d) natural compatibility with inflation.

Part of the concreteness is that bounce provides a platform for stating initial conditions, since it replaces singularity with a describable event. Probability distributons on initial conditions can be defined, and it then offers a mechanism for triggering inflation, and for juicing up an inflaton (if one is included in the picture) to render the desired number of e-folds.

The bounce gives something concrete and testable that fits together with inflation rather well. It does not logically need to be followed by an inflation episode (the bounce has its own brief inflation without any extra assumptions) but if you want inflation it can eliminate some of the fine-tuning.

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

Here I am just trying to guess what might be physical reasons that researcher interest has grown in the direction of bounce cosmology. That may be what we are looking at here, and I'd like some idea why it has happened.

 

[atyy]

The physical reason is that Bojowaldian LQC is the first place anywhere in LQG-related stuff that something like the Einstein equations have appeared in a principled way - I can make a theory with one fairy so it's simple, and have it predict a bounce so it's testable, but it's unprincipled so no one will be interested in it.

Does anyone find it ironic that background independence was discarded for a hint of the Einstein equations to appear?