What caused the shift of interest in quantum cosmology?

[marcus]

This raises an interesting physics question. What physics reason could there be for the shift of researcher interest in quantum cosmology which this Inspire search illustrates?
Here is the Inspire top ten quantum cosmo list for 1996-1998.

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

If I used it correctly, Inspire search says there were 20 String papers in quantum cosmology during that time period and THREE made it to the top ten. Three out of twenty is doing well, as I see it. They were numbers 5, 7, and 9 in the top ten list. Click on the link to see what the three stringy QC papers were about. There were no Loop papers that made the list.

By contrast, the same list for the period 2009-2011 shows no stringy QC papers but five are LQG. A sixth is part LQG and part some other approach. Again there were 20 String papers classified as QC, but this time none made the top ten.

http://inspirebeta.net/search?ln=en...2y=2011&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

You can see by clicking what sorts of quantum cosmology papers WERE favored by researchers. Horava gravity shows up. Verlinde entropic gravity also. Horava and Verlinde are former string folks whose current ideas do not require extra dimensions.

If you wish you can also enlarge the list to show the topcited 25 instead of the top 10. Same general impression.

So why did the quantum cosmology research community's interest shift in this pronounced way over the course of a dozen years? What physical reasons do you think could underly this change in focus?
Physics considerations might involve factors such as compatibility with inflation (generic in LQG), lack of evidence for supersymmetry, doubt about extra dimensions, the String Landscape, observations confirming a positive cosmological constant: in other words deSitter rather than AdS universe.

 

[marcus]

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.

 

[mitchell porter]

Big discoveries in string theory were made in the period 1995-2000, above all AdS/CFT, and the field is still dominated by the study of their implications. In 2010, the three most cited theory papers (#3, #8, #9 in the list) - not just string theory papers, but any theory - were still the three founding papers of AdS/CFT. So the story is that none of the subsequent developments within the subfield of AdS/CFT (or any of the newer string discoveries, such as the ABJM model) have attained to the same central significance as those three founding papers, which get cited in almost every paper on the subject. But I repeat: those were the most cited theory papers of 2010 (all the others around them are cosmological observations) - not just the most cited string theory papers. So string theory is still dominating theoretical research in general.

 

[atyy]

Maybe string has declined because it has become accepted physics. It's taught to undergraduates nowadays. That doesn't mean there isn't still a lot to be done, it's just become harder and harder.

http://en.wikipedia.org/wiki/Hype_cycle

As you can see, a decline does not indicate that there was not real progress!

 

[ensabah6]

Maybe string has declined because it has become accepted physics. It's taught to undergraduates nowadays.

A scary thought given that string theory remains unverified. Do undergraduates understand that string theory is speculative?

 

[atyy]

A scary thought given that string theory remains unverified. Do undergraduates understand that string theory is speculative?

Well, they already learn false theories like Newtonian mechanics, so what's the harm

 

[ensabah6]

Well, they already learn false theories like Newtonian mechanics, so what's the harm

:)

Would you object to Universities follow Penn state in creating LQG-specific departments within physics, and hiring LQG trained professors, and teaching undergraduates LQG?

 

[atyy]

:)

Would you object to Universities follow Penn state in creating LQG-specific departments within physics, and hiring LQG trained professors, and teaching undergraduates LQG?

Is there really an LQG specific department at Penn State?

 

[ensabah6]

Is there really an LQG specific department at Penn State?

I should say they have several professors who are LQG-focused :)

 

[smoit]

Dear Marcus,
Once again, your analysis is completely misleading. First of all, note that there are only 3 theory papers in that 2010 list, which came out in 2009! By the way, one of them is on AdS/CMT by Sean Hartnoll that uses strings in AdS directly so your zero is wrong. To get the real feel of what's going on it would be more productive to compare citations for the papers which all came out in 2008 and 2009.
The list of the most highly cited papers written in 2008 and 2009:

http://www.slac.stanford.edu/spires/find/hep/www?topcit=50%2B+and+date+2009+or+(topcit+100%2B+and+date+2008)&sequence=citecount(d)"
A theory paper with the highest number of citations on that list is:
N=6 superconformal Chern-Simons-matter theories, M2-branes and their gravity duals
by Ofer Aharony, Oren Bergman, Daniel Louis Jafferis and Juan Maldacena.
http://arxiv.org/abs/0806.1218"

There are plenty of string theory papers in the top 100 papers on the list, mostly on AdS/CFT applications but also on F-theory as well as the M2-branes. Note also that there are exactly zero papers on LQG in the top 100 list of the most highly cited papers written in 2008 and 2009.

 

[marcus]

Is there really an LQG specific department at Penn State?

I should say they have several professors who are LQG-focused :)

Right. Most of the young researchers in Loop are either Ashtekar PhDs (from Penn State) or Rovelli PhDs (from Marseille). Employment opportunities seem currently good because more university physics department now want a stake in Loop. And the new centers (Lyon, Sydney, Beijing, Erlangen, Tours, Morelia, University of Florida, Louisiana State,...) are still getting up to speed.

A typical path would be you get an Ashtekar PhD then go postdoc at Marseille. Or vice versa. Then possibly job. So far it is looking like a fairly good choice of specialty, for a theoretical physics PhD.

Undergrads normally do not get exposure (they have plenty of standard physics they should be learning!). But John Barrett at Nottingham has set up a two-year QG Masters program. I think this may be the first of its kind.

 

[marcus]

I checked my numbers for 2009 (one) and 2010 (zero) and they were correct. The main thing is apply the same measure consistently over time and see what the trend is.

Does anyone have ideas about physics explanation for the decline in interest?

What people have offered so far doesn't come to grips with any of the technical issues I mentioned. We are talking about a broad phenomenon. Top people getting other interests, or getting out of String altogether. Decline in the quality of the annual Strings conference (look at Strings 2010 in College Station, Texas and the advance material on Strings 2011 in Upsalla, Sweden.) Former friends like Weinberg and Gell-Mann expressing disappointment and impatience.

It is possible to make excuses based on socioeconomic circumstances, or quibble with specific pieces of evidence, or hurl epithets at the messenger (like "misleading!" and "obfuscation!" ) but the overall picture of decline is clear. It is the elephant in the room and I'm pretty sure there has to be some physics basis.

Any ideas?

 

[smoit]

Is there really an LQG specific department at Penn State?

In fact, at Penn State a Center for Gravitation & the Cosmos was created recently and they just held the Inaugural workshop last September. Guess what the theme of the workshop at the new center at Penn State was, this will make Mr. Marcus really happy , -
"Black Holes in Supergravity and M/Superstring Theory" !
Here is the link: http://www.gravity.psu.edu/events/blackholes_supergravity/index.shtml"

 

[atyy]

I don't think it is true that Weinberg and Gell-Mann have expressed disappointment in strings.

Weinberg said asymptotic safety is not ruled out and has shown progress, which is true.

Gell-Mann basically said people should work harder on the fundamental symmetries of string theory, which is a call for increased research in string!

http://arxiv.org/abs/gr-qc/9508064 "it seems that any acceptable quantum theory of gravity, whatever its ultimate formulation, is likely to reduce to a perturbative string theory in the appropriate limit."

http://arxiv.org/abs/gr-qc/0210094 "If there is any valid perturbative description of quantum gravity then it is almost certainly string theory."

So Gell-Mann, Smolin '96 and Thiemann '02, Gross etc all agree that a major research direction should be to find out what a non-perturbative definition of string theory is.

 

[marcus]

I don't think it is true that Weinberg and Gell-Mann have expressed disappointment in strings.

Weinberg said asymptotic safety is not ruled out and has shown progress, which is true.

Gell-Mann basically said people should work harder on the fundamental symmetries of string theory, which is a call for increased research in string!

http://arxiv.org/abs/gr-qc/9508064 "it seems that any acceptable quantum theory of gravity, whatever its ultimate formulation, is likely to reduce to a perturbative string theory in the appropriate limit."

http://arxiv.org/abs/gr-qc/0210094 "If there is any valid perturbative description of quantum gravity then it is almost certainly string theory."

So Gell-Mann, Smolin '96 and Thiemann '02, Gross etc all agree that a major research direction should be to find out what a non-perturbative definition of string theory is.

Weinberg used the word "disappointed" talking to the national conference of science writers I think in 2009. That was the theme of what he had to say about String. There is a video. It is interesting. He's usually (talking to physicists) more gentle and careful not to hurt their feelings. Nice guy. Do you want me to hunt for the link?

Gell-Mann, as I recall, expressed impatience (translation: why don't you guys go ahead and tackle the hard roadblock problems, don't just keep diddling around with the same old stuff).

I should have made it clearer. Weinberg expressed the disappointment. Gell-Mann the impatience

Hermann Nicolai, a longterm and influential string theorist also had some sharp words I quote: "this is another way that string theory has failed" and "string phenomenology is increasingly Baroque, if you follow the literature" That was summer 2009.

 

[atyy]

Weinberg is the only one you can make an at least plausible case for.

As you say, Gell-Mann expressed more impatience.

And Nicolai, whatever he may say, has turned out
http://arxiv.org/abs/0906.1987
http://arxiv.org/abs/0907.3048
http://arxiv.org/abs/0912.3491
http://arxiv.org/abs/1007.5472
http://arxiv.org/abs/1010.2212

and even the apparently non-string http://arxiv.org/abs/0907.3298 is motivated by "Our construction is inspired by a recent re-derivation from gauged supergravities in three dimensions [20, 21] of the conformally invariant and globally supersymmetric models thought to describe multiple M2 branes."

 

[marcus]

In fact, at Penn State a Center for Gravitation & the Cosmos was created recently and they just held the Inaugural workshop last September. Guess what the theme of the workshop at the new center at Penn State was, this will make Mr. Marcus really happy , -
"Black Holes in Supergravity and M/Superstring Theory" !
Here is the link: http://www.gravity.psu.edu/events/blackholes_supergravity/index.shtml"

Random collection of misinformation plus failed sarcasm. The Institute for Gravitation and the Cosmos was created in 2007. The Inaugural Conference was not "last September" which would have been September 2010. It was August 9-11, 2007.
I reported on it here at Beyond forum:
https://www.physicsforums.com/showthread.php?t=177711

The complete program (containing Loop and String people together with observational cosmologists, Roger Penrose etc etc) is here:
http://igc.psu.edu/events/conferences/inaugural/program_complete.pdf

I also gave a link here:
https://www.physicsforums.com/showpost.php?p=2904073&postcount=250
to the listing of IGC workshops including the September 9-11 one on Black Holes in Supergravity, M-theory.
http://www.gravity.psu.edu/events/workshops.shtml
That was NOT inaugurating IGC, it was simply the first of a planned series of workshops to be held in several different places. So it was titled "Inaugural" merely because it began that series of workshops.

The IGC mixes theory observation and in the theory department it mixes Loop and String--I approve of this. I like to see institutes, departments, and conferences mix a bunch of different active approaches and get people talking to each other. I think String has to some extent stalled (at least in US institutions) because the community got too securely entrenched and was just talking to itself.

The best QG conference of 2011 will pretty clearly be the June Zurich "Quantum Theory and Gravitation" which is organized by Barrett, Nicolai(string) and Rovelli(loop) and will have about equal Loop and String, plus several other contingents (CDT, NCG, GFT, QEG, NC-QFT...) Thirty plenary speakers. Historical conference. Exciting.

 

[marcus]

To focus on the original question, what physics factors caused the shift in quantum cosmology
from String (to a significant extent) in 1996-1998
to largely Loop in 2009-2011?

I'll unfold the links I gave in the first post:
Here is the Inspire topcited ten quantum cosmo list for 2009-2011.
http://inspirebeta.net/search?ln=en...2y=2011&sf=&so=a&rm=citation&rg=10&sc=0&of=hb
Notice that it is pretty much all Loop, Horava and Verlinde.
==Inspire quantum cosmology top ten 2009-present==
1.
(198)
Cosmology of the Lifgarbagez universe.
Gianluca Calcagni (Penn State U.). IGC-09-4-2. Apr 2009. 21 pp.
Published in JHEP 0909 (2009) 112
e-Print: arXiv:0904.0829 [hep-th]

2.
(54)
Pathological behaviour of the scalar graviton in Horava-Lifgarbagez gravity.
Kazuya Koyama (Portsmouth U., ICG), Frederico Arroja (Kyoto U., Yukawa Inst., Kyoto). Oct 2009. 7 pp.
Published in JHEP 1003 (2010) 061
e-Print: arXiv:0910.1998 [hep-th]

3.
(32)
Loop quantum cosmology of Bianchi I models.
Abhay Ashtekar, Edward Wilson-Ewing (Penn State U.). Mar 2009. 33 pp.
Published in Phys.Rev. D79 (2009) 083535
e-Print: arXiv:0903.3397 [gr-qc]

4.
(28)
On Inflation with Non-minimal Coupling.
Mark P. Hertzberg (MIT & KIPAC, Menlo Park & Stanford U., ITP). Feb 2010. 13 pp.
Published in JHEP 1011 (2010) 023
e-Print: arXiv:1002.2995 [hep-ph]

5.
(25)
Entropy-Corrected Holographic Dark Energy.
Hao Wei (Beijing, Inst. Tech.). Feb 2009. 12 pp.
Published in Commun.Theor.Phys. 52 (2009) 743-749
e-Print: arXiv:0902.0129 [gr-qc]

6.
(24)
Casting Loop Quantum Cosmology in the Spin Foam Paradigm.
Abhay Ashtekar, Miguel Campiglia, Adam Henderson (Penn State U.). IGC-10-1-1. Jan 2010. 36 pp.
Published in Class.Quant.Grav. 27 (2010) 135020
e-Print: arXiv:1001.5147 [gr-qc]

7.
(24)
Loop Quantum Cosmology and Spin Foams.
Abhay Ashtekar, Miguel Campiglia, Adam Henderson (Penn State U.). IGC-09-9-1. Sep 2009. 11 pp.
Published in Phys.Lett. B681 (2009) 347-352
e-Print: arXiv:0909.4221 [gr-qc]

8.
(23)
Entropic cosmology: a unified model of inflation and late-time acceleration.
Yi-Fu Cai, Jie Liu, Hong Li (Beijing, Inst. High Energy Phys. & TPCSF, Beijing). Mar 2010. 9 pp.
Published in Phys.Lett. B690 (2010) 213-219
e-Print: arXiv:1003.4526 [astro-ph.CO]

9.
(22)
Towards Spinfoam Cosmology.
Eugenio Bianchi, Carlo Rovelli (Marseille, CPT), Francesca Vidotto (Marseille, CPT & INFN, Rome & Pavia U. & INFN, Pavia). Mar 2010. 8 pp.
Published in Phys.Rev. D82 (2010) 084035
e-Print: arXiv:1003.3483 [gr-qc]

10.
(20)
Possible observational effects of loop quantum cosmology.
Jakub Mielczarek (Jagiellonian U., Astron. Observ. & LPSC, Grenoble). Aug 2009. 11 pp.
Published in Phys.Rev. D81 (2010) 063503
e-Print: arXiv:0908.4329 [gr-qc]

==endquote==

Numbers 1 and 2 are Horava-Lifgarbagez anistropic.
Numbers 3, 6, 7, 9 and 10 are obviously Loop
Number 5 is less obvious but if you read it you find roughly half is about Loop.
Number 8 is Verlinde entropic force.
Number 4 is just conventional straight QFT, no extra dimensions.
NONE of the top ten quantum cosmo papers here involve extra dimensions in any essential way.

Here are 18 "string model" quantum cosmology papers that appeared in the 1996-2000 period.
http://inspirebeta.net/search?ln=en...2y=2000&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

Eleven of these are 1996-1998:
http://inspirebeta.net/search?ln=en...2y=2000&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

Here is the quantum cosmology top 10 for the earlier period 1996-1998

http://inspirebeta.net/search?ln=en...2y=2011&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

It has three STRING quantum cosmology papers! If you wrote a string QC paper back then there is a very good chance it made the QC top 10 list! So things were different then.

There has to be some explanation for this change.

 

[Fra]

Do undergraduates understand that string theory is speculative?

I think that most would understand that, but to play with the idea:

What exactly would the loss be, if there are those who would not understand?

I mean if you don't understand that, what do you expect this person to contribute with in science? And perhaps these students ultimatley don't become involved in science anyway.

My experience and impression is that it's only a very small fraction of all students that study say physics that is serious. I know from when I studied that a lot of people are "interesting in physics" but those who take this to a level beyond work, profession and making a living and are willing to invest part of their life in it are rare. I think this is the people that is needed, and I can't imagine such a person to not understand such things.

/Fredrik

 

[mitchell porter]

To focus on the original question, what physics factors caused the shift in quantum cosmology
from String (to a significant extent) in 1996-1998
to largely Loop in 2009-2011?
...
There has to be some explanation for this change.

Marcus, if you conduct your search (quantum+cosmology) for almost any period from 1999-2001 forwards, the top 10 list is full of "loop" papers. Meanwhile, "string" hardly ever shows. 2 of your hits for 1996-1998 came from the same research program, the "pre big bang" scenario of Gasperini et al, which appears to be defunct and which was never remotely a dominant idea. And yet if you look at the recent work of someone like Andrei Linde, it's full of supergravity, multiverse, etc. So I conclude that what happened around 2000 is that Martin Bojowald happened. It's not that quantum cosmologists embraced LQG, but LQG researchers started doing cosmology. I also conclude that your keyword search isn't the right one to unearth string cosmology papers, which are mostly about inflation.

 

[marcus]

... I also conclude that your keyword search isn't the right one to unearth string cosmology papers, which are mostly about inflation.

Mitchell, what I want to unearth is a continuation of stringy work in quantum cosmology. Not classical cosmology which has a breakdown at high density (e.g. big bounce or black hole conditions).
String research USED to address the quantum cosmology questions, why does it not continue? QC is an important area. If the String researchers have given up on QC, then what physical obstacles caused them to give up?

Or, if at least a few are still working on quantum cosmology, what caused the loss of interest in their papers so that they are now less cited than before?

You mentioned research on getting inflation in string context--you might like to compare:
http://inspirebeta.net/record/856677?ln=en
http://inspirebeta.net/record/856677?ln=en
From the abstract "The big bang is replaced by a quantum bounce. The 'horizon problem' disappears. immediately after the big bounce, there is a super-inflationary phase with its own phenomenological ramifications, and, in presence of a standard inflaton potential, initial conditions are naturally set for a long, slow roll inflation independently of what happens in the pre-big bang branch."

 

[martinbn]

Well, they already learn false theories like Newtonian mechanics, so what's the harm

Is string theory at the level of Newtonian mechanics? My impression was that there isn't yet a theory. That string theory is still work in progress and there is a long way to go before it reaches the status of say Newtonian mechanics. This is meant as a question.

 

[marcus]

Martin,
I think your impression is correct. If I'm not mistaken, Atyy was joking.
========

Mitchell,
String theorists definitely used to do quantum cosmology, and get their string quantum cosmo papers cited. I just did this search at Inspire and came up with 50 papers:

http://inspirebeta.net/search?ln=en...n_search=Search&sf=&so=d&rm=&rg=25&sc=0&of=hb

Keywords "quantum cosmology" and "string model".

There are a number of recent papers but (if they are research and not merely multitopic reviews) they seem to be cited seldom if at all. Could something have happened to the perceived quality/relevance of string QC research?

Inspire, being beta, can still act unpredictably. If you try the link and don't get 50 papers please let me know. I'm trying to assess how stable and consistent the search is at Inspire.
Yesterday I got 20 recent string QC, 2009-2011 (8 with "membrane model" and 12 with "string model") and today so far I can't duplicate that.

 

[atyy]

Is string theory at the level of Newtonian mechanics? My impression was that there isn't yet a theory. That string theory is still work in progress and there is a long way to go before it reaches the status of say Newtonian mechanics. This is meant as a question.

String theory is certainly something about quantum gravity we need to know. Whether it turns out to be a correct description of nature is a different matter. I would compare it to at least eg. Nordstrom's second theory, the first relativistic theory of gravitation. Experiments ruled Nordstrom's theory out, and favoured Einstein's later general relativity, which learned from Nordstrom's theory (which itself learned from Einstein's even earlier work). Other alternative relativistic theories of gravity that came later were Whitehead's theory, and Brans-Dicke theory. Understanding the similarities and differences between these are essential for understanding general relativity itself.

Secondly, the AdS/CFT correspondence in which a sector of string theory is formulated as a quantum field theory in a lower dimension is an amazing example of of emergent gravity. There's a long history of interest in emergent gauge bosons in condensed matter physics (ie. non-string, non-high-energy, "mainstream" physics) going back to d’Adda et al in 1978, and Baskaran & Anderson in 1988, with the Levin and Wen model of emergent photons being a recent example. The AdS/CFT or gauge/gravity correspondence ties string in with "mainstream" physics.

Edit: BTW, yes, of course I was joking

 

[martinbn]

atyy, my question was not about the correctness of string theory. It was about its completeness. Is the theory developed enough so that it can be thought to undergrads?

 

[atyy]

atyy, my question was not about the correctness of string theory. It was about its completeness. Is the theory developed enough so that it can be thought to undergrads?

There is a course at MIT http://ocw.mit.edu/courses/physics/8-251-string-theory-for-undergraduates-spring-2007/

And a Masters level course (ie. an advanced undergrad could handle it) at Cambridge http://www.damtp.cam.ac.uk/user/tong/string.html

 

[martinbn]

There is a course at MIT http://ocw.mit.edu/courses/physics/8-251-string-theory-for-undergraduates-spring-2007/

And a Masters level course (ie. an advanced undergrad could handle it) at Cambridge http://www.damtp.cam.ac.uk/user/tong/string.html

I know about that, but is it useful or premature?

 

[marcus]

Atyy the question is what physics caused String-ists to stop doing String Quantum Cosmology research after sometime around 2000? Tom Banks has a 1999 paper called "M-Theory and Cosmology" that is tagged string model, membrane model and quantum cosmology. I saw nothing of comparable stature after that.

Here is the Inspire record for the 1999 Tom Banks:
http://inspirebeta.net/record/509927?ln=en

And to the extent that they continued doing string QC papers after that, why were the papers ignored by the research community and seldom cited?

It is a remarkable change in an important field, and demands some real physics reason (not merely some "hype cycle" or fad-cycle explanation although that may be contributory.)

You often have good ideas, what's your idea about this one?

 

[atyy]

Atyy the question is what physics caused String-ists to stop doing String Quantum Cosmology research after sometime around 2000? Tom Banks has a 1999 paper called "M-Theory and Cosmology" that is tagged string model, membrane model and quantum cosmology. I saw nothing of comparable stature after that.

Here is the Inspire record for the 1999 Tom Banks:
http://inspirebeta.net/record/509927?ln=en

And to the extent that they continued doing string QC papers after that, why were the papers ignored by the research community and seldom cited?

It is a remarkable change in an important field, and demands some real physics reason (not merely some "hype cycle" or fad-cycle explanation although that may be contributory.)

You often have good ideas, what's your idea about this one?

I don't have an idea about this. My interest in string has been more focussed on its small scale properties. The large scale ones are important too, of course.

BTW, Hossenfelder has detailed her views about string cosmology in sections 2.4.8 and 3.3 of http://arxiv.org/abs/1010.3420

 

[marcus]

I don't have an idea about this. My interest in string has been more focussed on its small scale properties. The large scale ones are important too, of course.

BTW, Hossenfelder has detailed her views about string cosmology in sections 2.4.8 and 3.3 of http://arxiv.org/abs/1010.3420

Thanks for the reminder about Sabine Hossenfelder's paper. It provides a pretty good overview of the phenomenological (testing) possibilities of various QG.

When the subject is quantum cosmology I am trying to train myself not to automatically think of the world as divided between the small scale (quantum) and the large scale (gen. rel.) but instead to distinguish between high density and low density.

You may think of the universe as "large scale" but who knows? The portion we currently observe was presumably very small scale at the beginning of expansion. But the whole thing could even have been infinite at the start. It seems we don't have much of an idea about the overall scale.

We can estimate is the density at early times. I suppose that it is that which decides where quantum cosmology applies.
So I try to think of QC as the physics of very high density phenomena.

(It's how one imagines the universe, neither especially large or especially small in linear size, perhaps, but dense.)

Aside from that petty quibble about phrasing, I agree with the spirit of your remark that
" The large scale ones are important too, of course."

 

[atyy]

Well, I was thinking of things like the lambda and boundary conditions when I said scale, since I think those are the things string has difficulty handling.

 

[marcus]

Well, I was thinking of things like the lambda and boundary conditions when I said scale, since I think those are the things string has difficulty handling.

Ah! I was thinking of other things like the nature of space and matter at very high density since that seems to be something we all share serious ignorance about regardless what math model of the universe we are using. What do "dimensions" mean at very high density. What is linear scale, what are angles? And so on. In what sense can we measure these things or make inferences about them from what we observe? What could be observed (even in ideal circumstances) about physics at very high density? What laws might apply, or not apply?
It is a really fascinating realm that people are just beginning to access.

 

[atyy]

Ah! I was thinking of other things like the nature of space and matter at very high density since that seems to be something we all share serious ignorance about regardless what math model of the universe we are using. What do "dimensions" mean at very high density. What is linear scale, what are angles? And so on. In what sense can we measure these things or make inferences about them from what we observe? What could be observed (even in ideal circumstances) about physics at very high density? What laws might apply, or not apply?
It is a really fascinating realm that people are just beginning to access.

I think that's where string has the answer (in principle) for some universe (not ours - at least not obviously so in terms of exact matter content and cosmological constant) with Einstein gravity, because of AdS/CFT.

 

[marcus]

I think that's where string has the answer (in principle) for some universe (not ours) with Einstein gravity, because of AdS/CFT.

But doesn't AdS/CFT assume a smooth manifold, with a fixed dimensionality the same at all scales, which can accept a metric geometry at all scales?

Correct me if I am wrong, but I think there are logical/conceptual reasons why a quantum reality cannot have a smooth manifold geometry at very small scale. It is like expecting a quantum particle to move along a smooth trajectory---one well-defined at every point---without anyone interrogating the particle as to where it went.

Absent evidence, I doubt one can suppose spatial relationships have a definite fixed dimensionality all the way down in scale, without means to ask nature what the dimensionality at some scale and in some particular circumstance.

My hunch is that this could be significant at very high densities (e.g. at the start of expansion) even if something one could ignore otherwise.

 

[atyy]

Energy on the boundary is a spatial dimension in the bulk. The bulk theory is supergravity at low energy and perturbative string theory at high energy, but perturbative string theory fails at some point, while the boundary theory exists. I don't know what the correspondence is then. Presumably one of the string experts on this board will know.

 

[marcus]

...but perturbative string theory fails at some point, while the boundary theory exists. I don't know what the correspondence is then. Presumably one of the string experts on this board will know.

Better ask if the correspondence depends on smooth manifolds. (Which may be just a polite mathematical fiction )

And also what happens to the boundary+bulk setup when there is a cosmological bounce. A crunch+rebound.

 

[Physics Monkey]

AdS/CFT doesn't assume a fixed background metric or "physical dimension". The data specified is only the asymptotic form of configurations i.e. one sums over configurations in the path integral that are asymptotically AdS. However, the bulk may be highly fluctuating to the point where classical geometry is essentially meaningless.

Nevertheless, it is true that in a certain limit, the large N limit, the path integral may be approximated by saddle point and the notion of a classical geometry becomes relevant. This is by far the most explored limit of the duality thus giving the impression that the duality requires a smooth geometry. There are a limited but growing number of tests of the duality away from large N, but this is one of the great open directions for the subject.

 

[fzero]

The strong form of the AdS/CFT conjecture is that the CFT sums over all spacetimes which are asymptotic to \text{AdS}\times X, where X is a sphere in the maximally supersymmetric cases. There are no restrictions on the interior and all string and QG physics can occur there. The corner of coupling constant space everyone is familiar with is the one in which the string coupling is small and the radius of curvature of spacetime is large, so that classical supergravity is reliable and the interior physics is just that of AdS supergravity.

In the case where we have IIB on X=S^5 and the dual CFT is \mathcal{N} =4 U(N) SYM, the 10D Newton constant scales like G_N \sim 1/N^2, while the radius of curvature in Planck units is R^4/l_p\sim N. The inverse string tension is \alpha'\sim 1/\sqrt{g^2N}. When g^2N is large, the massive string modes decouple. For large N gravitational corrections are small and classical supergravity dominates. This is region which tends to be the most familiar both in the literature and with nonexperts. One of the reasons for this is, that while the gauge theory side is strongly coupled and hard to calculate, the nature of the CFT means that many observables can be still be defined. Their correlation functions and other dynamics can be computed in the gravitational theory.

Now as we decrease N, quantum gravitational effects become important in the interior. Note that this large g^2, small N region is one of quantum supergravity, since higher order string modes do not contribute at leading order. So the physics there is that described by any consistent theory of quantum supergravity on spacetimes which are asymptotically AdS. The only requirement is that the quantum theory reduce to supergravity at low energies/weak gravitational coupling.

The conjecture implies that this physics is also completely described by \mathcal{N}=4 SYM at strong gauge coupling. Now, this dual theory is probably the best understood nonAbelian gauge theory of all, but as is the case in any gauge theory, we have limited tools for studying physics at strong coupling. Perhaps the most promising approach would be lattice gauge theory, whose application to the \mathcal{N}=4 theory has been seeing steady progress (http://arxiv.org/abs/1102.1725 is one recent paper). I'm not a lattice expert, but I don't think that the problems are likely to be impossible ones to solve.

I'm not aware of any spacetimes that have a bounce and are asymptotic to AdS, so I can't comment on that. There have been discussions what limit is involved trying to extend AdS/CFT to flat space (Polchinski's http://arxiv.org/abs/hep-th/9901076 is an early paper in this direction), as well as of a dS/CFT correspondence (Witten http://arxiv.org/abs/hep-th/0106109 and Strominger http://arxiv.org/abs/hep-th/0106113). More recently Strominger and collaborators have been studying holographic descriptions of black holes via CFTs, see http://arxiv.org/abs/arXiv:1009.5039 for example.

 

[marcus]

AdS/CFT doesn't assume a fixed background metric.

I didn't say it did!

I also did not assume that a "physical dimension" was fixed, whatever that means.

What it does assume is a differential manifold (one that you can put various metrics on) because otherwise you could not get Einstein gravity.

And a manifold has a fixed dimensionality that holds at all scales. That is part of the definition.

Maybe you should read my post more carefully before you start "correcting" it. I would be delighted to get some feedback.

 

[marcus]

There are no restrictions on the interior and all string and QG physics can occur there.

Fzero thanks for commenting! I was gratified to get such good feedback. What I have been trying to say (which I'd like your reaction to) is that there is a BIG restriction on the interior, which is that it is assumed to be a diff. manifold.

Locally diffeomorphic to R^d for some fixed dimensionality d.
===============

If the interior is not a conventional continuum, a manifold with fixed dimensionality d which does not run with scale, then I don't see how "all string physics can occur there".

Indeed I must assume it is a diff. manifold because it always has been in every presentation of AdS/CFT I have seen.

On the other hand in the long run this could be something of a disaster for AdS/CFT, or at least a severe limitation.

There are types of QG that do not use a manifold, at least in the ordinary sense. Some of these can undergo a nonsingular "bounce" during which density (to the extent you can define it) seems to get up near Planckian scale.

So if "all string physics can occur there" it is hard to see how "all QG physics can occur there". I think you see the point I was trying to make.

There are cogent conceptual arguments why a quantum theory of geometry/gravity cannot live on a spacetime manifold. A different mathematical representation of time and spatial geometry would then be required. Several are being worked on currently.

BTW David Gross has repeatedly acknowledged that to move ahead (he means with string/M) we "may need a completely new idea of time and space". But he doesn't specify what that might be.

 

[smoit]

I didn't say it did!

What it does assume is a differential manifold (one that you can put various metrics on) because otherwise you could not get Einstein gravity.

And a manifold has a fixed dimensionality that holds at all scales. That is part of the definition.

Maybe you should read my post more carefully before you start "correcting" it. I would be delighted to get some feedback.

No, that's true only in the large N and small string coupling limit. That's the only regime when the geometric description in terms of a differential manifold makes sense. When the string coupling is large and N is finite it makes no sense to talk about a differential manifold in the bulk, the physically meaningful quantity is the CFT partition function.

 

[fzero]

Fzero thanks for commenting! I was gratified to get such good feedback. What I have been trying to say (which I'd like your reaction to) is that there is a BIG restriction on the interior, which is that it is assumed to be a diff. manifold.

Locally diffeomorphic to R^d for some fixed dimensionality d.

No assumptions are being made about the details of how you are supposed to describe physics in the bulk. As I tried to motivate, on the gravity side, we have a theory that reduces to classical supergravity in a certain limit. There the use of a fixed spacetime manifold is completely justified. In a small neighborhood of this point in coupling space, the proper description is the IIB string on the fixed manifold. Far from this point, there is another point where there are strong QG effects, but no string effects. If we had a complete description of QG, we could presumably describe the physics there. Note that this description must reduce to classical supergravity in the appropriate limit, just as Newtonian gravity should emerge from a nonsupersymmetric theory of gravity.

Now, in the absence of such a description of QG, the claim is that the dual CFT provides a completely well-defined description of the physics. However it is not a theory that we can compute very much in, since it is nonperturbative gauge theory. Nor do we really know the dictionary between boundary and bulk observables in the absence of a more concrete description of the QG theory in the bulk.

It's natural to expect, given the framework that this theory is sitting in, that the QG theory is some nonperturbative version of the IIB string. However, even if this were not the case, it is still plausible that the gauge theory remains a correct dual description, since we can identify the adjustment of the gauge theory corresponding to tuning the gravitational coupling away from the classical limit.

 

[suprised]

marcus, as always, is trying to emphasize apparent shortcomings of string theory, and then compares this whatever the current fashion in LQG is. In the present situation, he claims (or spindoctors to the same effect) that string theory would intrinsically rest on smooth geometries and thus would be unsuited to describe quantum geometry at small distances.

Of course, rather the opposite is true. As has been known for years, and as I was emphazing here repeatedly, classical smooth manifolds are relevant only in a certain regime; let's loosely say, of measure zero in the full parameter space. In general there are non-perturbative quantum corrections to the geometry to the effect that it becomes modified to some kind of stringy geometry, which is very different from ordinary classical theory based on smooth manifolds. Many notions of classical geometry become blurred in such non-geometric phases, or even stop to make sense. Examples are topology changing transitions, disappearence of singularities, appearence of some kind of space-time foam, submanifolds of naively different dimension becoming indistinguishable (so that the notion of a submanifold stops making sense), etc etc. All this has been investigated to great detail and has improved our conceptual understanding of quantum geometry at small distances. So string theory is a very rich and prolific toolbox to address exactly this kind of questions.

 

[marcus]

Fzero,
I was quite excited by your post #42 and tried repeatedly to post a few minutes after you put it up. But the system kept giving error messages and losing what I wrote, so i gave up.
What you say, if I understand you, makes AdS/CFT much more interesting, though seemingly contradictory. So what mathematically represents the interior?

The interior is not a smooth manifold---is that correct?
So is it a topological manifold (locally homeomorphic to R^d)?
On rereading several posts, I think it must be that. A manifold but no differential structure.
Or does it have a fragmented differential structure with lots of singularities?
Or a superposition of piecewise linear structures. I'm still not clear on this.
============
Putting that to one side, there is the issue of the big bang or big bounce. And today's accelerated expansion. I think you indicated the bang/bounce can not (as of now) be represented. No solutions on that boundary that exhibit that.
There was more: here's the quote:

I'm not aware of any spacetimes that have a bounce and are asymptotic to AdS, so I can't comment on that. There have been discussions what limit is involved trying to extend AdS/CFT to flat space (Polchinski's http://arxiv.org/abs/hep-th/9901076 is an early paper in this direction), as well as of a dS/CFT correspondence (Witten http://arxiv.org/abs/hep-th/0106109 and Strominger http://arxiv.org/abs/hep-th/0106113). More recently Strominger and collaborators have been studying holographic descriptions of black holes via CFTs, see http://arxiv.org/abs/arXiv:1009.5039 for example.​

 

[marcus]

... classical smooth manifolds are relevant only in a certain regime; let's loosely say, of measure zero in the full parameter space. In general there are non-perturbative quantum corrections to the geometry to the effect that it becomes modified to some kind of stringy geometry, which is very different from ordinary classical theory based on smooth manifolds...

Good! This could be helpful, Suprised. So there still is a manifold in the interior. It just might not be smooth. That was one of the possibilities that I was considering. A topological manifold, locally homeomorphic to R^d.

You make the interior geometry sound very nice---so I could take a liking to it. But how about the big bang? You may have talked about this in other threads, but I haven't seen them, so please tell me. Late universe accelerated expansion? Early universe inflation? Bounce maybe? Does the richness you speak of already include those riches?
I would be glad to hear of positive results, especially if you have links to papers.

I thought Fzero gave a negative indication on one of those, however. Maybe that's in the "work in progress" department.

 

[suprised]

So there still is a manifold in the interior

This is not what I was saying. It is known that more general concepts start to play a role, like sheaves and more abstractl objects in certain categories; only in classical limits these turn back into things we know from classical geometry. This can be much more drastic than a simple discretization or dimensional reduction.

Here an interesting paper about related matters, about "quantum gravitational foam": http://arXiv.org/pdf/hep-th/0312022
This is of course "just" a topological toy model, but nevertheless it provides a glimpse of how things may work in a more realistic situation. They find what contributes to the path integral are certain coherent sheaves and not just naive geometries:

"...The path-integral space for quantum gravity should include classical topologies and geometries. However the actual space we integrate over may well be bigger than that given strictly by manifolds with arbitrary topology and metric, as happens for topological strings..."

So string theory seems well capable to address this kind of questions, in fact other approaches appear quite naive to me in comparison. In the best of all worlds, other approaches like LQG won't give different or contradicting results, but rather complementary ones; I'd expect this to be akin to lattice QCD capturing some non-perturbative aspects of continuum QCD.

 

[marcus]

Sheaves? That's interesting. We studied sheaves when I was in graduate school long long ago.
What other structures in the interior do you remember hearing about?

I have to go to sleep (I am on the Pacific coast) but I look forward to hearing more about this.

You could be right that partial and complementary answers will be provided by several different methodologies.

Are you aware of the Zurich conference "Quantum Theory and Gravitation" to be held in mid June 2011 at the ETH.
You may know some of the scheduled plenary speakers. They come from a number of different QG approaches.
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:start
Plenary speakers:

Jan Ambjorn (Copenhagen)*
Joakim Arnlind (AEI Potsdam)
Abhay Ashtekar (Penn State)
Costas Bachas (ENS Paris)
John Baez (Riverside)
John Barrett (Nottingham)
Niklas Beisert (AEI Potsdam)
Matthias Blau (Bern)
Ali Chamseddine (Beirut)
Alain Connes (College de France, Paris)
Ben Craps (Bruxelles)
Axel de Goursac (Louvain)
Lance Dixon (SLAC)
Henriette Elvang (Michigan)
Klaus Fredenhagen (Hamburg)
Laurent Freidel (Perimeter)
Stefan Hollands (Cardiff)
Jens Hoppe (Stockholm)
Ted Jacobson (Maryland)
Jerzy Jurkiewicz (Krakow)
Gandalf Lechner (Wien)
Jerzy Lewandowski (Warsaw)
Renate Loll (Utrecht)*
Roberto Longo (Rom)
Viatcheslav Mukhanov (Munich)
Hermann Nicolai (AEI Potsdam)
Martin Reuter (Mainz)
Carlo Rovelli (Marseille)
Misha Shaposhnikov (EPF Lausanne)
Raimar Wulkenhaar (Münster)

 

[Fra]

Suprised, if I may try to ask a simple question regarding the strategy, concerning some fundamentall by still overall points.

This can be much more drastic than a simple discretization or dimensional reduction.

You mention things like theory spaces.

In MY opinion, discretization serves the purpose of allowing a well defined counting, and thus construction of a measure, Even if we then take a continuum limit, the choice of measure depends on the way the limit is taken.

Those who take the discretness most seriously, like causal sets etc, there the discretization is not really an approximation - it's the continuum that is an approximation.

By taking the continuum limit you loose information, namely how hte limit is taken. And you end up with problems of how to defined measures.

classical smooth manifolds are relevant only in a certain regime; let's loosely say, of measure zero in the full parameter space.

How is the measure on this space physically constructed and justified?

I mean, don't you end up with just an even bigger landscape?

Or put differently, how you do gain more flexibility WITHOUT loosing control? (ie the measure, and thus getting lost in a landscape too larg to process)

Is this not a problem?

/Fredrik

 

[Physics Monkey]

I didn't say it did!

I also did not assume that a "physical dimension" was fixed, whatever that means.

What it does assume is a differential manifold (one that you can put various metrics on) because otherwise you could not get Einstein gravity.

And a manifold has a fixed dimensionality that holds at all scales. That is part of the definition.

Maybe you should read my post more carefully before you start "correcting" it. I would be delighted to get some feedback.

Really, marcus? After all this time, is this really the attitude you're going to take?

I didn't say anything about correcting you nor did I address my post to you. My post was a brief informal note about what ads/cft assumes about the structure of "spacetime". I thought I could discuss physics informally here without having to check every word of my post for compatibility with yours. Perhaps I was mistaken.

Also, I see that others have already pointed out the flaws in your statement above, so I'll leave it at that.

 

[suprised]

How is the measure on this space physically constructed and justified?

I am not claiming that I know, that's why I was writing "loosely". But the point is pretty obvious, in that classical geometry or weakly coupled physics just corresponds, again loosely speaking, to the boundary of the full parameter space. Clearly this boundary is much "less" than the full parameter space itself. Away from the boundary, ordinary notions of geometry generically break down.

I mean, don't you end up with just an even bigger landscape?

Or put differently, how you do gain more flexibility WITHOUT loosing control? (ie the measure, and thus getting lost in a landscape too larg to process)

Is this not a problem?

Problem for what? We are not engineers who design a car according to market demands! We talk here about the parameter space of quantum gravity in the string formulation. That's a question in itself and needs to be investigated, irrespective of feelings whether the landscape becomes "too large". It's like complaining that there would be "too many solutions" of GR, etc.