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.