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What caused the shift of interest in quantum cosmology?

by marcus
Tags: caused, cosmology, quantum, shift
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Fra
#19
Mar10-11, 03:24 AM
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Quote Quote by ensabah6 View Post
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 alot 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
#20
Mar10-11, 07:18 AM
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Quote Quote by marcus View Post
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
#21
Mar10-11, 08:31 AM
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Quote Quote by mitchell porter View Post
... 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
#22
Mar10-11, 10:03 AM
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Quote Quote by atyy View Post
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
#23
Mar10-11, 10:26 AM
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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&...=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
#24
Mar10-11, 10:38 AM
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Quote Quote by martinbn View Post
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 learnt from Nordstrom's theory (which itself learnt 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
#25
Mar10-11, 10:43 AM
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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
#26
Mar10-11, 10:55 AM
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Quote Quote by martinbn View Post
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...s-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
#27
Mar10-11, 10:56 AM
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Quote Quote by atyy View Post
There is a course at MIT http://ocw.mit.edu/courses/physics/8...s-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
#28
Mar10-11, 11:12 AM
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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
#29
Mar10-11, 11:36 AM
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Quote Quote by marcus View Post
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
#30
Mar10-11, 01:32 PM
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Quote Quote by atyy View Post
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
#31
Mar10-11, 02:24 PM
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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
#32
Mar10-11, 02:55 PM
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Quote Quote by atyy View Post
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
#33
Mar10-11, 04:33 PM
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Quote Quote by marcus View Post
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
#34
Mar10-11, 04:42 PM
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Quote Quote by atyy View Post
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
#35
Mar10-11, 06:23 PM
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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
#36
Mar10-11, 07:05 PM
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Quote Quote by atyy View Post
...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.


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