What caused the shift of interest in quantum cosmology?

[marcus]

Sorry Physicsmonkey, I mistook the tone of your post. It seems too obvious to need saying that the schema doesn't require a fixed background metric on the bulk, and I thought you were talking to me. It sounded condescending and I momentarily lost my temper. My bad. The rest of your post is concise and informative. So thanks for that!

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.

This may explain why I was under the mistaken impression that the interior manifold has a differential structure!

So now I'm quite interested. What structure does it have? How do you talk about what is going on there, in the bulk?

If you have no differential structure (generically---"except on a set of measure zero" as someone suggested) then how do you describe things. Curvature? Matter fields? Distances? Volumes? Geometric relations among events?

So far I think we just have a topological manifold, not so? Continuous functions only. I'm intensely curious to know how analysis on the bulk can proceed from here. Please educate me!

(May not be able to respond for a few hours this morning because of appointments but if you reply soon I'll see it and be able to think about it while I'm out.)

 

[martinbn]

I don't want to diverge from the original topic, but it is very interesting to me, so I would like to ask a side question. What exactly are the structures used that are nor diff. manifolds, that were mentioned above? What would be great to hear is the mathematical definition or the name of the object or a reference where one could find them. Mentioning the word sheaf is not enough, surely one can and does use sheaves on differential manifolds.

 

[Physics Monkey]

Perhaps others have a different opinion, but I suspect it's too early in the game to start delineating exactly what mathematical structure exists in the bulk.

Instead, I'll give two useful examples.

1. Topology change in string theory: One can arrange situations where the initial and final states are well described by smooth manifolds but with different topology. This means that the intermediate state, even if we drop all smoothness assumptions and work only with continuous manifolds, must undergo an evolution that is not a homeomorphism. So the "bulk" cannot be described at all times by a continuous space with continuous evolution. String theory can describe this situation in terms of a kind of condensation phase transition on the string worldsheet. See for example: http://arxiv.org/abs/hep-th/0502021

2. Matrix models: String theory or M-theory can be described by replacing continuous coordinates by finite dimensional NxN matrices. As in AdS/CFT, the large N limit of the matrix model recovers in some sense the notion of smooth geometry. But in general there is no precise notion of a continuous space in these descriptions; one recovers instead some kind of "fuzzy" geometry. One nice example is described here: http://arxiv.org/abs/hep-th/0002016

 

[Fra]

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'm with you that the notions of geometry and manifold must break down in more general cases. I have no objection to, on the contrary. I'm rather fishing for what the more generalized structures are (agreeing they aren't manifolds) and from my perspective, beeing able to count/measure them are a key point. In fact my point would be that a constraint is that they have to be measurable, or we are on the wrong track.

Problem for what?

A problem for inference. I think to be able to make inferences/predictions/expectations and to LEARN about nature is what this is all about, I presume we agree. I try to not loose this focus must never be lost in mathematics.

Normally: one theory => one inference (though it can be inductive rather than deductive).

Now if a theory is not known, but rather we have a space of theories, and accordingly a space of inferences, then if there is any physical basis between this space, then there theory space itself should be the result of another inference: ie you have a bigger theory, from which other (more specific) theories follow. And if this theory is a proper inference, there must exists a justified measure on the theory space.

My point beeing that, if some kind of ideas come up with this theory space, without a measure or means of inference and selection I would personally take this as a clear sign that something just isn't right about that reasoning.

Note that I am not picking on the NOTION of theory space or theory of theory; that is somehow the ambition ST has. This is good. What I feel, is that this "theory of theory" may in fact not be a proper inferencial theory.

Of course no one has all these answers, but I was just trying to pick in a constructive way. I think said before but I think that lack of this measure is because the theory space is describe from an external perspective (say the chair of the physicist) rather than from each subsystem of the universe.

This is why this theory space that is Externally described, IS not measureable from the inside. This is also why it's not an intrinsic theory in the first place.

I think curing this in ST therms, means providing a more clever solution to the landscape problem, in terms of some evolution. And I'm not just talking about antrophics I think something more in line with smolins evoluiotary law is neeeded. If ST is generalized, beyond strings and beyond manifolds, (meaning it's not really "strings" anymore) then I do see how the string program might converge in this direction. So it doesn't look totally dark to me. My favoured picture involves a discrete combinatorial approach where strings may be explained as large complexit limits of such discrete structures, in a way where the continuum strings are just limiting cases. And it's when you TAKE the limit, you loose contact with ground. So it seems the historial starting point of ST is responsbile for plenty of confusion. Maybe there is an alternative starting point... that makes more sense also to ignorant people like me.

/Fredrik

 

[marcus]

Perhaps others have a different opinion, but I suspect it's too early in the game to start delineating exactly what mathematical structure exists in the bulk.
...

Wow! thanks for that viewpoint. I can't respond in any substantive way because I have to be out for most of today. Just in briefly now.

I would like to ask you to take a look at this short Loop tutorial
http://arxiv.org/abs/1102.3660
It is a manifoldless math structure possibly able to describe 4D geometry and matter in the bulk.
Putting matter in has really only just got started.

The basic structure is a non-embedded cellular complex---a 2-complex.
With a graph as boundary.

Suprised conjectured that several different languages for describing the geom+matter in the bulk might turn out to be "complementary".

I am interested in the AdS/CFT language (as well as the spinfoam/GFT language) because I think I hear you say it could possibly be manifoldless.
Or at least the structure is in doubt---and at least it is not a smooth manifold.

So that is interesting. One may be able to compare and one might even find unexpected similarities.

What is in this 20-page tutorial 1102.3660 is a new form of LQG which only appeared in 2010 (although I saw hints of it back in fall 2009). It looks like it has been already or is being adopted by a substantial part of the Loop community (which as you know is still comparatively small, so far only 100-200 or so come to the biannual conferences, though this could now be increasing.)

I'd like to know your reaction to this alternative math language. It is a bulk+boundary schema where the boundary contains the initial final or side conditions, and one is calculating an amplitude.

Also, to amplify Martin BN's question, can anyone speculate for us what some possible CANDIDATES might be for the mathematical structure of the bulk in the AdS/CFT picture?

It would be very interesting to hear about any you can think of. Thx.

 

[marcus]

Well we have had some interesting and informative posts from Physicsmonkey, Fzero, and Suprised (among others). Are we any closer to answering the main question posed in post #1 of the thread?

Why the shift of interest (illustrated by research citations but not confined to that) in quantum cosmology?

AdS/CFT was discussed a lot. Could the answer have anything to do with AdS/CFT?

Perhaps others have a different opinion, but I suspect it's too early in the game to start delineating exactly what mathematical structure exists in the bulk...

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...
...

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.

Fzero had a number of interesting points in the next post, here are excerpts with just a few:

I think the lead up to the commissioning of the LHC made pursuit of phenomenologial issues at least slightly more interesting than pursuing fundamental issues...

...The basic issue is that the fixed background in string theory is an important part of perturbation theory. Perturbation theory is valid when we the energy and density of probes is small enough that we can neglect the backreaction on the spacetime geometry. When curvatures become large, perturbation theory fails to be a good technique to describe the physics...

...Finally, I'd like to comment about linking AdS/CFT to cosmological issues. One the one hand, I don't think anyone in the field would try to base a serious model of cosmology on spacetimes that are asymptotically AdS. So there is no real reason to see inflation or bounces in AdS scenarios. However, I do think that most people would hope that there are significant lessons to be learnedi...

 

[fzero]

Well we have had some interesting and informative posts from Physicsmonkey, Fzero, and Suprised (among others). Are we any closer to answering the main question posed in post #1 of the thread?

Why the shift of interest (illustrated by research citations but not confined to that) in quantum cosmology?

AdS/CFT was discussed a lot. Could the answer have anything to do with AdS/CFT?

I think the lead up to the commissioning of the LHC made pursuit of phenomenologial issues at least slightly more interesting than pursuing fundamental issues. Part of this could be related to the corresponding increase in pheno jobs, but also due to the fact that most theorists, given the chance, would like to see their work make contact with experiment. To this end, the twistor formulation of gauge amplitudes might have had more of an influence than AdS/CFT, though application of the latter to heavy ion physics was fairly successful theme. Along the same vein, WMAP stimulated interest in cosmology that could be approached from many directions other than quantum cosmology.

You also have to understand that the deep, difficult problems have a tougher risk-reward ratio than more modest problems. A younger physicist has to select problems with an eye towards producing a record of publications. The use of citation count as a metric of quality also tends to make it safer to work on existing hot topics, since it's usually a given that someone else in the field will cite your work.

In any case, it might be a more fruitful question to ask "what are people working on instead of quantum cosmology?" or "what are the quantum cosmologists working on now?"

As for the AdS/CFT issues, other people have expounded upon the difference between classical geometry in one corner of the correspondence. I believe that a lot of the confusions are due to simplified explanations of things for a popular audience. The basic issue is that the fixed background in string theory is an important part of perturbation theory. Perturbation theory is valid when we the energy and density of probes is small enough that we can neglect the backreaction on the spacetime geometry. When curvatures become large, perturbation theory fails to be a good technique to describe the physics.

In some cases, we can regain some perturbative picture by changing the background, especially by adding nonperturbative, solitonic objects, like D-branes. This is often necessary when representing a breakdown in geometry by studying what happens at singularities of manifolds. For specific classes of singularities, we have learned that the divergence is due to new degrees of freedom, not seen in perturbation theory, becoming light as a submanifold shrinks to zero size. These new degrees of freedom are wrapped D-branes and by properly introducing them one restores the consistency of the string picture.

For other nonperturbative problems, we still don't have a complete picture of what the correct degrees of freedom are. As was also brought up, matrix theory provides a new set of degrees of freedom that exhibit both emergent geometry and emergent perturbative gravity from a quantum mechanical theory that is in many respects simple.

Finally, I'd like to comment about linking AdS/CFT to cosmological issues. One the one hand, I don't think anyone in the field would try to base a serious model of cosmology on spacetimes that are asymptotically AdS. So there is no real reason to see inflation or bounces in AdS scenarios. However, I do think that most people would hope that there are significant lessons to be learned from holography in more general cases. Since the AdS/CFT correspondence is the best example of a holographic theory, it's a place to learn, if not directly apply to those kinds of things. So the presence or absence of cosmologically relevant behavior in AdS spaces is a red herring.

 

[crackjack]

Also, to amplify Martin BN's question, can anyone speculate for us what some possible CANDIDATES might be for the mathematical structure of the bulk in the AdS/CFT picture?

In addition to the structures that http://www.stringwiki.org/wiki/Matrix_theory" .

 

[marcus]

In addition to the structures that http://www.stringwiki.org/wiki/Matrix_theory".

Thanks crackjack! This could give us a clue as to why there has been that shift in quantum cosmology that I asked about in post #1.

Suprised said something earlier about a "rich and prolific toolbox" which your list corroborates:

...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.

I think we're slowly getting a better understanding of the transformation in quantum cosmology over the past dozen years or so. It may have to do with the prolific richness of the toolbox which Surprised told us about.

Also could have to do with what Fzero said here: "... I don't think anyone in the field would try to base a serious model of cosmology on spacetimes that are asymptotically AdS. So there is no real reason to see inflation or bounces in AdS scenario..." Roughly speaking the real universe appears to be the opposite of AdS. This may have dampened the interest of cosmologists somewhat.

There must be a number of factors and it's a slow job to sort them out and see which are the important ones.

 

[marcus]

I think the lead up to the commissioning of the LHC made pursuit of phenomenologial issues at least slightly more interesting than pursuing fundamental issues...

That sounds plausible but I have two reservations:
1. Early universe cosmology has an observational side, and quantum cosmology has some phenom potential. Don't want to bore you but I'll get some links to illustrate.

2. The timing is wrong. It looks to me as if much of the shift occurred roughly around 2002-2004 long before the "lead up to the commissioning of the LHC."

If we just repeat the Inspire searches for "top ten" quantum cosmology papers given in post #1, but for consecutive 3-year intervals, we find that string representation in the list dropped off fairly early:

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

Most of the other points you make in your post strike me as quite plausible and in part convincing, but the timing seems to be wrong for a "LHC effect".

 

[mitchell porter]

3,3,1,2,1 is not trending anywhere. 0,4,7,7,8 goes from nothing to a plateau and stays there. So I repeat that there is no decline in "string quantum cosmology" research here, there is just the rise from nothing of "loop quantum cosmology".

There is no unified "string quantum cosmology" research program. The people who created quantum cosmology - like Guth, Vilenkin, Linde, Hawking - have at times introduced the stringy version of quantum gravity into their models, but it's only a technical modification, e.g. modifying Hawking's no-boundary proposal so it contains supergravity. In my opinion, the most distinctive convergence of thought between cosmology and string theory is the way that eternal inflation allows the whole string landscape to be realized in the one universe (in different inflationary patches), and it is not at all established that this is the right way to think about cosmology in string theory.

Also in my opinion, string theory so far lacks a compelling cosmological vision. As I said, you can plug existing string models into existing cosmological models, e.g. by having something distinctively stringy (like motion of a D-brane) provide the inflaton field of the inflationary model. But it has a rather arbitrary feel. I believe eternal inflation + string landscape is the most natural fit so far proposed, but that's more because you have one great multiplicity (of inflationary domains) being paired up with another great multiplicity (of string vacua). It doesn't arise from a deep theoretical principle.

If a "dS/CFT correspondence" could be made to work, that would be big news, because it might describe the real world. But ideas about how to do string theory in de Sitter space are still preliminary at best. That might be the fundamental reason why string cosmology is still a mishmash of divergent ideas.

The role of nonclassical geometry in the bulk, in the AdS/CFT correspondence, for low numbers of QCD-like "colors" in the CFT gauge group, is a very interesting fact that this thread has turned up. I have started taking an interest in how difficult it would be to get something like the standard model from N=4 super-Yang-Mills, by projecting out particular amplitudes and by distributing D3-branes in the AdS bulk so that there are massive open string states, and of course the standard model doesn't have many colors (at least, not many compared to "infinity", which is the limit usually considered in AdS/CFT). So it's very interesting to hear that the nonclassical bulk geometry should show up for such a theory; it makes a connection to the noncommutative standard model (of Connes et al) conceivable.

 

[marcus]

Mitchell, as I think you realize, there definitely has been a shift or marked change in the makeup of the QC top ten, and it requires explanation. You have suggested here what could very possibly be the explanation, or at least one important factor:

Also in my opinion, string theory so far lacks a compelling cosmological vision. As I said, you can plug existing string models into existing cosmological models, e.g. by having something distinctively stringy (like motion of a D-brane) provide the inflaton field of the inflationary model. But it has a rather arbitrary feel. I believe eternal inflation + string landscape is the most natural fit so far proposed, but that's more because you have one great multiplicity (of inflationary domains) being paired up with another great multiplicity (of string vacua). It doesn't arise from a deep theoretical principle.
...

Recent papers making the top ten is an index of researcher interest in the recent papers, not the gross output. When I gave this a quick look a few days ago I didn't see a decline in string quantum cosmology output, the main change was the string QC papers coming on line were just not getting cited much by the research community.

Representation in top-cited papers in some category is not a linear scale, so "trend" is not quite the appropriate concept. Once you get up near 10 here there is not much room for improvement and once you get down near zero you can't go much lower. "Trend" is a linear-scale thing, so we don't look for trends with an index like this. That's why I spoke of a shift, or marked change.

3,3,1,2,1 is not trending anywhere. 0,4,7,7,8 goes from nothing to a plateau and stays there. So I repeat that there is no decline in "string quantum cosmology" research here, there is just the rise from nothing of "loop quantum cosmology".
...

What interests me, and what I meant the thread to discuss, is not a particular index or way of measuring research interest, but the physics explanation for the shift. That was made clear in the first post.

Your reasons in the "compelling cosmological vision" paragraph impress me as perceptive and cogently expressed. They have to do with physics. (They are not merely excuses having to do with social or historical accidents.)

I'm sure you know that the String program had a "cosmological vision" in the 1990s presented in many papers by Maurizio Gasperini and Gabriele Veneziano (one of the early founders who initiated the String program).Their stringy "Pre-Big-Bang" scenario attracted quite a lot of interest. So String was making a splash in quantum cosmology when it had a convincing vision.

You point out that on the one hand some string gambits "have an arbitrary feel"---and on the other hand the natural match with Multiverse ideas does not "arise from a deep physical principle".

As I read what you say, it seems to me that your hope or constructive suggestion (judging from what you say about the wish for a dS/CFT to replace AdS) would be for the program to get a quantum cosmology vision that addresses the problem of modeling the Big Bang.

I would would add one desideratum to yours. The need for the vision to be testable---it should rivet the attention of early universe phenomenologists. I promised back in post #60 that I would get some links to illustrate the EU phenomenology business, so I better do that.

 

[atyy]

marcus, does your search pick up papers like http://arxiv.org/abs/hep-th/0204168 ? Or a review like http://arxiv.org/abs/0907.2562 ?

 

[marcus]

Atyy, it's fascinating that there was that 2002 paper by Liu Moore Seiberg!

First though I should say that I'm just going with what the new Stanford-SLAC search tool ("Inspire") says.
With the search, I just put "quantum cosmology" in the keyword field and rank by citecount and go.
The main thing is to do the same search consistently year after year and look for change.
I just count how many make the top 10, there are other things. I could count how many make the top 25, or I could add up the number of cites I suppose. But it would be more bother and this is just meant to illustrate what I think must be a widely shared impression.

I will give the links so you can easily see what Inspire gives, and does not give!
This is exactly like in post #1 except I do it for all the successive 3-year intervals, not just 1996-1998

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

1999-2001 http://inspirebeta.net/search?ln=en...2y=2001&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

2002-2004 http://inspirebeta.net/search?ln=en...2y=2004&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

2005-2007 http://inspirebeta.net/search?ln=en...2y=2007&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

2008-2010 http://inspirebeta.net/search?ln=en...2y=2010&sf=&so=a&rm=citation&rg=10&sc=0&of=hb

Here are the Spires keywords for the Liu Moore Seiberg:
http://www.slac.stanford.edu/spires/find/hep/wwwtopics?key=4908902
I see no mention of anything quantum cosmological. I'll have to take a look at the paper: it is interesting that they have a certain kind of space perform a bounce! I don't think however that they have a quantum universe wave function perform a bounce. I'll check.

It is an interesting distinction. Often in the String program one has a fixed background geometry in which the strings can live and vibrate. The geometry is classical. I think that is what is happening here. The classical background geometry undergoes a bounce. Then the question is can some of the strings pass through the bounce point.

Here are the Inspire records for 3 papers by Liu Moore Seiberg that came out in 2002 and 2003:
http://inspirebeta.net/record/585639?ln=en
http://inspirebeta.net/record/588898?ln=en
http://inspirebeta.net/record/605720?ln=en

We'll see if any of the string folks (Supr. Fzero?) comment. I could of course be wrong, but I had a look just now and it seems to be the fixed background geometry situation usual with perturbative string theory. What is quantized is the PARTICLES not the geometry they live in. One ignores possible backreaction of the matter on the geometry.

I only glanced, but here are a couple of indicative passages. See what you think:
Page 1

In section 2 we describe the model and its geometry. In section 3 we study the functions on our spacetime which are the wave functions of the first quantized particles. In sections 4 and 5 we quantize free strings in the light-cone and conformal gauges and compute the torus partition function. Section 6 is devoted to a preliminary analysis of the interactions and backreaction. Our conclusions are presented in section 7.​

Page 23

The issue of backreaction is currently under study, and might prove to be a serious problem with future development of this example. One way backreaction could ruin the orbifold is through the coupling of gravity to the large energy momentum of particles which are blue shifted near the singularity. Indeed, we found that although the tree level...​

 

[atyy]

Another paper that the key words may not have picked up is http://arxiv.org/abs/hep-th/0506180

 

[atyy]

Here are some interesting perspectives from various reviews

http://arxiv.org/abs/gr-qc/0301001
"We now see that classical string theory is also singular in such orbifolds and cannot be trusted. ... There are at least three possibilities for the interpretation of the singularity:
1. The singularity is a beginning or end of time. In this case we need to understand the appropriate initial conditions at the singularity. For some discussion of these issues see [23,24].
2. Time has no beginning or end. Then one needs to understand how to pass through the singularity (for recent discussions see e.g. [9,25]).
3. The most likely possibility, it seems to us,is that in string theory time is a derived concept.
In conclusion, let us elaborate on the third possibility above. In toroidal compactifications of string theory there is a minimal distance, thanks to T-duality: shrinking radii past the string scale does not produce a theory at shorter distances. In more elaborate compactifications (such as Calabi-Yau compactifications) it turns out that there can be smooth topology-changing processes, and “quantum geometry” can lead to many counterintuitive types of behavior. These examples show that, in string theory, standard notions of topology and geometry are not fundamental but are rather emergent concepts in certain physical regimes (e.g. in regions of large complex and Kahler structure parameters, in the Calabi-Yau context). In another line of development, Matrix theory and the related advent of noncommutative field-theoretic limits of string theory further indicate that the notion of distance and space ceases to make sense in certain otherwise sensible regimes of the theory. Given the principle of relativity it seems quite likely, perhaps even inevitable, that similar statements hold for time as well as for space."

http://arxiv.org/abs/0705.2643
"In other words, we are led to the conclusion that space — and thus, upon quantization, also space-time — actually disappears (or ‘de-emerges’) as the singularity is approached. There is no ‘quantum bounce’ bridging the gap between an incoming collapsing and an outgoing expanding quasi-classical universe. Instead ‘life continues’ at the singularity for an infinite affine time, however, with the understanding that (i) dynamics no longer ‘takes place’ in space, and (ii) the infinite affine time interval ... corresponds to a sub-Planckian interval 0 < T < TPlanck of geometrical proper time."

http://arxiv.org/abs/1001.4367
"In principle, one would like to carry out the following program. Start with a state in the bulk theory (with modified boundary conditions) corresponding to a large, asymptotically AdS space-time with some profile for the scalar field. Translate this state, using the AdS/CFT correspondence, to a state in the dual field theory on the boundary (with a steep unbounded potential). In the dual field theory, evolve the state through the singularity using a self-adjoint extension (if a consistent and natural self-adjoint extension exists). Finally, translate the evolved state back to a state in the bulk theory, and ask whether it has a geometric interpretation. If the boundary theory described only homogeneous modes, experience with self-adjoint extensions in quantum mechanics would suggest that the final state would roughly resemble the initial state, which would suggest a cosmological bounce. Inhomogeneous modes can drastically change this picture, though: particle creation can be potentially attractive for cosmology, but one needs to make sure that backreaction is sufficiently small for the computations to be reliable."

 

[marcus]

I see you got a 2007 paper co-authored by Hermann Nicolai! Intriguing notions pursued in that one--completely unfamiliar to me. http://arxiv.org/abs/0705.2643

 

[atyy]

I see you got a 2007 paper co-authored by Hermann Nicolai! Intriguing notions pursued in that one--completely unfamiliar to me. http://arxiv.org/abs/0705.2643

A more extensive review of those ideas can be found at http://relativity.livingreviews.org/Articles/lrr-2008-1/ . I have no idea if these really are the symmetries of M-theory (my vote is with twistors and AdS/CFT at the moment;) but the work is very pretty.

Mitchell Porter in a different thread pointed out David Berman's http://arxiv.org/abs/1008.1763 which may link that work up to mainstream string theory.

A different line of speculation also starting from BKL is Carlip's http://arxiv.org/abs/1009.1136

 

[marcus]

You are finding some interesting papers! However I want to explore the reasons for the change in quantum cosmology and one potential reason could be that QC has become observational. I said earlier I would list some observational-QC papers, to indicate the scope of activity. The following papers are all 2009-2011, quite recent.

Here are two Spires searches using the keyword "power spectrum" with two other keywords (quantum cosmology, loop space and quantum gravty, loop space). It will not exhaust the field, since other tags besides "power spectrum" could be used to get more. But it will give an idea:

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+QUANTUM+gravity%2C+LOOP+SPACE+AND+DK+POWER+SPECTRUM+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

1) Cosmological footprints of loop quantum gravity. 33 cites
J. Grain, (APC, Paris & Paris, Inst. Astrophys.) , A. Barrau, (LPSC, Grenoble & IHES, Bures-sur-Yvette) . Feb 2009. (Published Feb 27, 2009). 7pp.
Published in Phys.Rev.Lett.102:081301,2009.
e-Print: arXiv:0902.0145 [gr-qc]

2) Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation. 13 cites
J. Grain, (APC, Paris & Paris, Inst. Astrophys.) , A. Barrau, (LPSC, Grenoble & IHES, Bures-sur-Yvette) , A. Gorecki, (LPSC, Grenoble) . Apr 2009. (Published Apr 2009). 15pp.
Published in Phys.Rev.D79:084015,2009.
e-Print: arXiv:0902.3605 [gr-qc]

3) Fully LQC-corrected propagation of gravitational waves during slow-roll inflation. 11 cites
J. Grain, (Paris, Inst. Astrophys.) , T. Cailleteau, A. Barrau, A. Gorecki, (LPSC, Grenoble) . Oct 2009. (Published Jan 15, 2010). 9pp.
Published in Phys.Rev.D81:024040,2010.
e-Print: arXiv:0910.2892 [gr-qc]

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+dk+power+spectrum+and++DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

1) Possible observational effects of loop quantum cosmology. 20 cites
Jakub Mielczarek, (Jagiellonian U., Astron. Observ. & LPSC, Grenoble) . Aug 2009. (Published Mar 15, 2010). 11pp.
Published in Phys.Rev.D81:063503,2010.
e-Print: arXiv:0908.4329 [gr-qc]

2) ...duplication from other list...

3) Observational constraints on a power spectrum from super-inflation in Loop Quantum Cosmology. 12 cites
Masahiro Shimano, Tomohiro Harada, (Rikkyo U.) . Sep 2009. (Published Sep 15, 2009). 17pp.
Published in Phys.Rev.D80:063538,2009.
e-Print: arXiv:0909.0334 [gr-qc]

4) Tensor power spectrum with holonomy corrections in LQC. 10 cites
Jakub Mielczarek, (Jagiellonian U.) . Feb 2009. (Published Feb 2009). 13pp.
Published in Phys.Rev.D79:123520,2009.
e-Print: arXiv:0902.2490 [gr-qc]

5) Loop Quantum Cosmology corrections on gravity waves produced during primordial inflation. 8 cites
J. Grain, (Paris, Inst. Astrophys.) . Nov 2009. 9pp.
To appear in the proceedings of INVISIBLE UNIVERSE INTERNATIONAL CONFERENCE: Toward a new cosmological paradigm, Paris, France, 29 Jun - 3 Jul 2009.
Published in AIP Conf.Proc.1241:600-608,2010.
e-Print: arXiv:0911.1625 [gr-qc]

6) Inflation in loop quantum cosmology: dynamics and spectrum of gravitational waves. 8 cites
Jakub Mielczarek, (Jagiellonian U.) , Thomas Cailleteau, (LPSC, Grenoble) , Julien Grain, (Paris, Inst. Astrophys.) , Aurelien Barrau, (LPSC, Grenoble) . Mar 2010. (Published May 15, 2010). 11pp.
Published in Phys.Rev.D81:104049,2010.
e-Print: arXiv:1003.4660 [gr-qc]

7) Chaplygin inflation in loop quantum cosmology. 4 cites
Xin Zhang, (Shenyang, Northeast U. Tech.) , Jing-fei Zhang, (Shenyang, Northeast U. Tech. & Dalian U. Tech.) , Jing-lei Cui, Li Zhang, (Shenyang, Northeast U. Tech.) . Feb 2009. 6pp.
Published in Mod.Phys.Lett.A24:1763-1773,2009.
e-Print: arXiv:0902.0928 [gr-qc]

8) Observational constraints on loop quantum cosmology. 1 cite
Martin Bojowald, Gianluca Calcagni, Shinji Tsujikawa, . IGC-11-1-1, AEI-2011-004, Jan 2011. 4pp. Temporary entry
e-Print: arXiv:1101.5391 [astro-ph.CO]

After excluding the one duplication, we see that the searches yield 10 papers---primarily observational-QC and mostly written by early universe phenomenologists rather than QC folks themselves.

There are also these that using "cosmic background radiation" instread of "power spectrum" would have caught
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+Dk+cosmic+background+radiation+anD+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

1) ...duplicate...
2) ...duplicate...

3) Constraints on standard and non-standard early Universe models from CMB B-mode polarization. 4 cites
Yin-Zhe Ma, (Cambridge U., KICC & Cambridge U., Inst. of Astron.) , Wen Zhao, (Cardiff U.) , Michael L. Brown, (Cambridge U., KICC & Cambridge U., Inst. of Astron. & Cambridge U.) . Jul 2010. 41pp.
Published in JCAP 1010:007,2010.
e-Print: arXiv:1007.2396 [astro-ph.CO]

4) Observational hints on the Big Bounce. 3 cites
Jakub Mielczarek, (Jagiellonian U., Astron. Observ.) , Michal Kamionka, (Wroclaw U., Astro. Inst.) , Aleksandra Kurek, (Jagiellonian U., Astron. Observ.) , Marek Szydlowski, (Jagiellonian U., Astron. Observ. & Jagiellonian U.) . May 2010. 24pp.
Published in JCAP 1007:004,2010.
e-Print: arXiv:1005.0814 [gr-qc]

5) Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters. 2 cites
Julien Grain, (Paris, Inst. Astrophys. & Orsay, LAL) , Aurelien Barrau, Thomas Cailleteau, (LPSC, Grenoble) , Jakub Mielczarek, (Jagiellonian U.) . Nov 2010. (Published Dec 15, 2010). 12pp.
Published in Phys.Rev.D82:123520,2010.
e-Print: arXiv:1011.1811 [astro-ph.CO]

So that makes a total of 13 in all.

 

[marcus]

Here's a proposed list of quantum cosmology criteria

The bulk should have some definite mathematical structure that represents it.
Asymptotically it should look deSitter (accelerated expansion) or at least not the opposite (which is AdS, accelerated contraction).
There should be a quantum state of bulk geometry which exhibits a bounce.
The bounce should either lead to adequate inflation or, if not, the model should produce the usual effects expected from inflation in some other way.
The model should be testable and attract the attention of phenomenologists so they can study means of testing it.

 

[atyy]

There should be a quantum state of bulk geometry which exhibits a bounce.

http://arxiv.org/abs/1101.5592 , p37: While isotropic solvable models of loop quantum cosmology suggest a role of bouncing cosmologies for potential scenarios, no consistent set of equations to evolve inhomogeneities through a bounce has been found.

 

[marcus]

That was the August 2009 talk that Bojo gave at George Ellis's 70th birthday party! I can't find any discussion of some major developments in Loop Cosmology after 2009.
He has added many articles which appeared 2010-2011 (of certain limited types) to the bibliography, so you might think that it could serve as an up-to-date review of the field. In general that would be wrong I think.

Nevertheless I think it is a good article, given that limitations, and I think it serves to support the above list of desiderata. You could say that order for a QC bid to get traction in today's environment it does seem that it must be making progress towards these goals, especially I think two goals: testability and some definite representation of a quantum universe that resolves the classical singularity. And Bojowald shows that he is concerned about those goals.

The whole paragraph from the look-ahead section on page 37, that you quoted from, is worth copying. It is short and it illustrates some of these concerns.
http://arxiv.org/abs/1101.5592
==quote Bojowald "Loop Quantum Gravity and Cosmology" page 37 ==
While isotropic solvable models of loop quantum cosmology suggest a role of bouncing cosmologies for potential scenarios, no consistent set of equations to evolve inhomogeneities through a bounce has been found. The only available options so far make use of gauge (or frame) fixings before quantization, and thus miss crucial aspects of space-time structures. Any mismatch of growing modes in the collapse and expansion phases can easily be enhanced by cosmic evolution, providing opportunities for potential observations but also requiring extreme care in finding fully consistent equations. Inhomogeneous cosmological scenarios remain uncertain, and with it follow-up issues such as the entropy problem.
==endquote==

The context makes it clear that he is talking about a restricted area of Loop cosmology where one works with analytic approximations technically known as solvable equation models. This is one specific type of LQC and it's an area where Bojowald has made a big contribution. I don't see the main action of the field there now. If you look at the recent LQC papers by Ashtekar, Rovelli, Lewandowski, and their post 2009 coauthors, you see them applying spinfoam. In order to keep solvable equation approximations in the game, Bojowald will have to work some degree of inhomogeneity into them---at least as much as people are getting marginally with the current probing of spinfoam LQC. This is not a lot, but it is a competitive field and inching ahead matters.

 

[atyy]

So is there any paper that has a consistent set of equations, inhomogeneities and a bounce?

It's fine if it's simulations, but those must start form a consistent set of equations.

 

[marcus]

So is there any paper that has a consistent set of equations, inhomogeneities and a bounce?

It's fine if it's simulations, but those must start form a consistent set of equations.

You have to realize that HE meant something special by "consistent set of equations"---he would not have included simulations, i.e. the numerical approach with discrete time steps. It is just his technical jargon.

It is clear because at the beginning of the SAME SENTENCE he refers to the solvable equation model.

And he certainly was not including spinfoam cosmology. You know the landmark March 2010 paper of Bianchi Rovelli Vidotto explicitly explains how some marginal degree of inhomog is being included.
But he doesn't even include BRV paper in his bibliography! A dozen people around Ashtekar Lewandowski Rovelli are doing spinfoam cosmology and Bojo does not include ref to even one paper and he does not (as far as I could see) even say "spinfoam" anywhere. It is not his thing so he blanks it out.

His title is right: Loop Quantum Gravity and Cosmology. That is what LQC is. It is not rigidly restricted to some one particular approach.
It used to be. It used to be completely symmetry reduced. Isotropic homogeneous basically down to 2 degrees of freedom. It used to be strictly hamiltonian canonical.
And Bojowald was the main guy in charge until around 2006.

All these restrictions have gradually been being relaxed. Even the "solvable equation model" is not strictly a canonical formulation. It approximates. People show equivalence and they bridge over to new math tools and alternative formulations. Part of the game is to get closer to the FULL THEORY which now I think means combinatorial spinfoam (with connections to GFT) but Bojowald may well see it as some Thiemannish canonical approach.

Ashtekar is sending all his PhDs over to postdoc in Marseille, and not Erlangen. It is clear he sees that LQG cosmology is going in a spinfoam direction.
Look at the lineup at the Zako school where Ashtekar gave the opening talk.

What I'd say is that if you carefully understand what he means by the words he uses, probably everything in Bojowald's paper is correct. After all he is a worldclass expert. But he acts like he has blindspots. Like he doesn't realize how much LQC has developed outside his personal framework.

What I think is, since he gave the paper at August 2009 Ellisfest, in South Africa. He should not have included any bibliography references to papers after August 2009. Then it would be clear that the paper was a snapshot of how he saw the field at that time and was not meant to be about "Loop Quantum Gravity and Cosmology" (= LQC) after mid-2009.
Or he could have the selectively updated biblio and simply SAY that clearly in the Introduction, either way.

It has to be a very valuable paper but in the present version it is too easy to misunderstand it.

As far as your question about inhomogeneity, they are barely scratching the surface. But they are getting some in, as they begin to apply the full theory to cosmology.
I thought the March paper by BRV made that explicit but maybe it was some other 2010 paper. I'll check.

Here again is the Bojowald paper http://arxiv.org/abs/1101.5592
Here is the BRV paper http://arxiv.org/abs/1003.3483
(it was the real beginning of spinfoam LQC. There have been a number of subsequent papers by Penn State and Marseille groups)

 

[marcus]

Page 7 of the March 2010 paper by BRV---it is actually the final paragraph of the paper and is thus given extra emphasis:

==quote http://arxiv.org/abs/1003.3483 ==
Here, however, homogeneous and isotropic states appear naturally as states peaked on homogeneous and isotropic mean values of the quantum states, in the context of a formalism which –we stress– is not a reduction of the dynamics to homogeneous and isotropic degrees of freedom. In physical terms, these states represent a universe where inhomogeneous and anisotropic degrees of freedom are taken into account but fluctuate around zero. This provides also an elegant solution of the problem of having to choose between coordinate or momenta in imposing a symmetry reduction in cosmology [50–52]. Ideally, this formalism could describe inhomogeneous and anisotropic quantum fluctuations of the geometry at the bounce.
==endquote==

They use coherent quantum states that are peaked on homog. isotropic values but are not exclusive of nonuniformity/asymmetry. That's why I called it a marginal relaxation of the earlier restriction. It's a start. They are beginning to apply the full theory to cosmo.

Here's my earlier list of proposed QC criteria. I think of them as goals. This thread has helped get them into clearer focus. I'd like to get anyone's comments. I think that making progress on these could be necessary for any approach to get researcher attention in today's Quantum Cosmology environment. It has in effect become expected of any active QC initiative:

• The bulk should have some definite mathematical structure that represents it.
• Asymptotically it should look deSitter (accelerated expansion) or at least not the opposite (which is AdS, accelerated contraction).
• There should be a quantum state of bulk geometry which exhibits a bounce.
• The bounce should either lead to adequate inflation or, if not, the model should produce the usual effects expected from inflation in some other way.
• The model should be testable and attract the attention of phenomenologists so they can study means of testing it.

 

[marcus]

If those five criteria are correctly chosen then
A) they help explain the shift in interest in QC, the change in the makeup of the topcited papers.

B) we can expect members of the string community to come up with stringy quantum cosmology which meets or appears able to meet all or most of the five.

(There is no longterm reason string program should be stuck with an AdS bulk, or with confusion as to what mathematical structure represents the bulk, or without a quantum bounce. A resourceful theorist should be able to invent a stringy theory that competes with Loop on each of these five counts.)

If you have other ideas---a different set of physics criteria which you think explains the shift in QC---please let us know. Comments have been very helpful so far!

For convenience I consolidated the search terms for Loop phenomenology papers. The combined Spires search now give 14 papers that appeared 2009-2011:

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+POWER+SPECTRUM+or+dk+cosmic+background+radiation%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

FIND (DK QUANTUM GRAVITY, LOOP SPACE OR DK QUANTUM COSMOLOGY, LOOP SPACE) AND (DK POWER SPECTRUM OR DK COSMIC BACKGROUND RADIATION) AND DATE > 2008


1) Cosmological footprints of loop quantum gravity. 33 cites
J. Grain, (APC, Paris & Paris, Inst. Astrophys.) , A. Barrau, (LPSC, Grenoble & IHES, Bures-sur-Yvette) . Feb 2009. (Published Feb 27, 2009). 7pp.
Published in Phys.Rev.Lett.102:081301,2009.
e-Print: arXiv:0902.0145 [gr-qc]

2) Possible observational effects of loop quantum cosmology. 20 cites
Jakub Mielczarek, (Jagiellonian U., Astron. Observ. & LPSC, Grenoble) . Aug 2009. (Published Mar 15, 2010). 11pp.
Published in Phys.Rev.D81:063503,2010.
e-Print: arXiv:0908.4329 [gr-qc]

3) Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation. 13 cites
J. Grain, (APC, Paris & Paris, Inst. Astrophys.) , A. Barrau, (LPSC, Grenoble & IHES, Bures-sur-Yvette) , A. Gorecki, (LPSC, Grenoble) . Apr 2009. (Published Apr 2009). 15pp.
Published in Phys.Rev.D79:084015,2009.
e-Print: arXiv:0902.3605 [gr-qc]

4) Observational constraints on a power spectrum from super-inflation in Loop Quantum Cosmology. 12 cites
Masahiro Shimano, Tomohiro Harada, (Rikkyo U.) . Sep 2009. (Published Sep 15, 2009). 17pp.
Published in Phys.Rev.D80:063538,2009.
e-Print: arXiv:0909.0334 [gr-qc]

5) Fully LQC-corrected propagation of gravitational waves during slow-roll inflation. 11 cites
J. Grain, (Paris, Inst. Astrophys.) , T. Cailleteau, A. Barrau, A. Gorecki, (LPSC, Grenoble) . Oct 2009. (Published Jan 15, 2010). 9pp.
Published in Phys.Rev.D81:024040,2010.
e-Print: arXiv:0910.2892 [gr-qc]

6) Tensor power spectrum with holonomy corrections in LQC. 10 cites
Jakub Mielczarek, (Jagiellonian U.) . Feb 2009. (Published Feb 2009). 13pp.
Published in Phys.Rev.D79:123520,2009.
e-Print: arXiv:0902.2490 [gr-qc]

7) Loop Quantum Cosmology corrections on gravity waves produced during primordial inflation. 8 cites
J. Grain, (Paris, Inst. Astrophys.) . Nov 2009. 9pp.
To appear in the proceedings of INVISIBLE UNIVERSE INTERNATIONAL CONFERENCE: Toward a new cosmological paradigm, Paris, France, 29 Jun - 3 Jul 2009.
Published in AIP Conf.Proc.1241:600-608,2010.
e-Print: arXiv:0911.1625 [gr-qc]

8) Inflation in loop quantum cosmology: dynamics and spectrum of gravitational waves. 8 cites
Jakub Mielczarek, (Jagiellonian U.) , Thomas Cailleteau, (LPSC, Grenoble) , Julien Grain, (Paris, Inst. Astrophys.) , Aurelien Barrau, (LPSC, Grenoble) . Mar 2010. (Published May 15, 2010). 11pp.
Published in Phys.Rev.D81:104049,2010.
e-Print: arXiv:1003.4660 [gr-qc]

9) Chaplygin inflation in loop quantum cosmology. 4 cites
Xin Zhang, (Shenyang, Northeast U. Tech.) , Jing-fei Zhang, (Shenyang, Northeast U. Tech. & Dalian U. Tech.) , Jing-lei Cui, Li Zhang, (Shenyang, Northeast U. Tech.) . Feb 2009. 6pp.
Published in Mod.Phys.Lett.A24:1763-1773,2009.
e-Print: arXiv:0902.0928 [gr-qc]

10) Loop quantum gravity and the CMB: Toward pre-Big Bounce cosmology. 4 cites
Aurelien Barrau, (LPSC, Grenoble) . Nov 2009. 3pp.
To appear in the proceedings of 12th Marcel Grossmann Meeting on General Relativity (MG 12), Paris, France, 12-18 Jul 2009.
e-Print: arXiv:0911.3745 [gr-qc]

11) Constraints on standard and non-standard early Universe models from CMB B-mode polarization. 4 cites
Yin-Zhe Ma, (Cambridge U., KICC & Cambridge U., Inst. of Astron.) , Wen Zhao, (Cardiff U.) , Michael L. Brown, (Cambridge U., KICC & Cambridge U., Inst. of Astron. & Cambridge U.) . Jul 2010. 41pp.
Published in JCAP 1010:007,2010.
e-Print: arXiv:1007.2396 [astro-ph.CO]

12) Observational hints on the Big Bounce. 3 cites
Jakub Mielczarek, (Jagiellonian U., Astron. Observ.) , Michal Kamionka, (Wroclaw U., Astro. Inst.) , Aleksandra Kurek, (Jagiellonian U., Astron. Observ.) , Marek Szydlowski, (Jagiellonian U., Astron. Observ. & Jagiellonian U.) . May 2010. 24pp.
Published in JCAP 1007:004,2010.
e-Print: arXiv:1005.0814 [gr-qc]

13) Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters. 2 cites
Julien Grain, (Paris, Inst. Astrophys. & Orsay, LAL) , Aurelien Barrau, Thomas Cailleteau, (LPSC, Grenoble) , Jakub Mielczarek, (Jagiellonian U.) . Nov 2010. (Published Dec 15, 2010). 12pp.
Published in Phys.Rev.D82:123520,2010.
e-Print: arXiv:1011.1811 [astro-ph.CO]

14) Observational constraints on loop quantum cosmology. 1 cite
Martin Bojowald, Gianluca Calcagni, Shinji Tsujikawa, . IGC-11-1-1, AEI-2011-004, Jan 2011. 4pp. Temporary entry
e-Print: arXiv:1101.5391 [astro-ph.CO]

 

[mitchell porter]

I was wrong to say that Gasperini et al's pre-big-bang model is defunct; there was a http://arxiv.org/abs/1103.2311" is full of resources and review articles.

In my view, the string model of a cosmological bounce with the best pedigree is the http://arxiv.org/abs/hep-th/0204189" , and that is not a realistic model, nor can it easily be made realistic, because exact solutions of string theory on a time-dependent background are hard to come across, and not adjustable.

Robert Brandenberger also has http://arxiv.org/abs/1103.2271" today reviewing two cosmological models, a bounce model, and a "string gas cosmology" where the string gas is just sitting there (indefinitely, timelessly, as the end product of a collapse, I can't tell, and maybe he doesn't know), and then it fluctuates into an expanding state.

I continue to feel that string theory just hasn't found the right way to think about cosmology yet. Maybe some individual string theorist has done so - the seeds of the right approach may already exist in the literature - but what's lacking is the demonstration that this is the right path. It's apparent that no particular approach has swept the field - I think this is the real lesson of Marcus's database searches. Papers are being written, but it's still a cacophony of conflicting ideas. The version of inflation called eternal inflation is probably the favorite of elite opinion, but I'm not sure there's anything like a consensus on how to think about the initial singularity.

The place in string cosmology where contact with empirical data is occurring is in models of inflation. See section 5.2 in http://arxiv.org/abs/0810.3707" of CMB predictions from various string models of inflation, that will be tested as further WMAP data comes in.

Some further opinions:

If we do adopt the view that string theorists should ultimately prefer to go beyond cosmological model-building which involves arbitrary choices and which employs only approximations to the full theory (e.g. such as the ekpyrotic model of a cosmological bounce), and should instead try to find a cosmological idea which is innately inherent to string theory, it might be worth trying to find a cosmological interpretation or cosmological component to the AdS/CFT duality involving N=4 super-Yang-Mills. N=4 SYM is widely regarded as providing the complete, exact specification of IIB string theory on the relevant AdS space; all the string states, brane states, and so on, are hypothesized to emerge from combinations of SYM operators on the boundary. If this is true, then N=4 SYM, remarkably enough (because it's just a plain old field theory), offers the most advanced formulation of string theory that we possesses - and so, with this cosmological purpose in mind, it would be natural to use it as a starting point in this quest for the "true" approach to string cosmology.

On the topic of de Sitter space - the most famous constructions of de Sitter space in string theory were the ones in http://arxiv.org/abs/hep-th/0301240" which introduced the landscape, and they consist of an "uplift" of a vacuum with an AdS ground state into a metastable dS state, by the inclusion of some branes. So it's possible that AdS cosmology is not just a test run for string theory, but actually a ground state, with the observed dS cosmology as a fluctuation.

I'd also like to say that I'm not at all convinced that any form of quantum cosmology is going to be correct. The whole idea of a wavefunction of the universe may be wrong. We don't talk about the wavefunction of a galaxy, or of a star. I know the idea is that in the primordial conditions, there is a structureless simplicity which makes the idea of a universal wavefunction useful, that by the time we have structure formation the universe has decohered into complex classical systems, etc. But it is such a big leap from atom to universe, that it becomes very likely we are generalizing QM in the wrong way when we take it into cosmology.

 

[Fra]

I'd also like to say that I'm not at all convinced that any form of quantum cosmology is going to be correct. The whole idea of a wavefunction of the universe may be wrong
...
it is such a big leap from atom to universe, that it becomes very likely we are generalizing QM in the wrong way when we take it into cosmology.

I fully agree with this! This important concern is annoyingly often ignored.

To speak for my own personal motivation, I am not only "NOT convinced that the mentioned generalisations are right", I am rather very convinced that they are wrong, simply becase the way of abstraction just fails to be an intrinsic view.

/Fredrik

 

[marcus]

I like Mitchell's statement too! I glanced at Robert Brandenberger's recent paper and it looks like he is steering the String program in the direction of the five criteria I mentioned.

Maybe he also thinks that to get researcher attention in today's Quantum Cosmology environment a program should show progress on some of these fronts:

• The bulk should have some definite mathematical structure that represents it.
• Asymptotically it should look deSitter (accelerated expansion) or at least not the opposite (which is AdS, accelerated contraction).
• There should be a quantum state of bulk geometry which exhibits a bounce.
• The bounce should either lead to adequate inflation or, if not, the model should produce the usual effects expected from inflation in some other way.
• The model should be testable and attract the attention of phenomenologists so they can study means of testing it.

Maybe he doesn't address all five, but he certainly touches base on the bounce issue

 

[marcus]

I'd also like to say that I'm not at all convinced that any form of quantum cosmology is going to be correct. The whole idea of a wavefunction of the universe may be wrong...

Loop cosmology very quickly approximates classical Friedmann equation cosmology, almost instantly or within a few tens of Planck times, after the bounce. This is with coherent states peaked on classical, coming into the bounce.

So one can ask "what is the purpose?" Why bother to quantize cosmology if you very quickly get the same old Friedmann equation model that every cosmologist already uses?

There is a pragmatic reason. The whole purpose of QC is to find quantum corrections that apply during the "quantum regime" of extremely high density, that happens during an extremely brief interval, right around the start of expansion.

In fact the Loop program has found a quantum corrected version of the Friedmann equation, and it turns out that it kicks off inflation. So under broad assumptions you get some 60 e-folds as a kind of bonus without fine adjustment.
It remains to see if this quantum-corrected Friedmann eqn is right or not.

You can get a good discussion of this in Ashtekar's review "The Big Bang and the Quantum".
Mathematically the corrections to the model are simple, and lead to the bounce always happening when the density is around 41% of Planck.

So there is a purpose to having a wave function of the U! It can be important for about 100 Planck time units! And that can set things up for a nice conventional inflation. But otherwise ... *shrug*

Same way with the black hole. (Semi) classical analysis goes a long way in, but not all the way. Ultimately you need a quantum model to help guess what happens, and it is always possible there could be some significant surprise there. Or so I think, anyway.

Ashtekar's paper is worth looking at, if you have not already.
The Big Bang and the Quantum
http://arxiv.org/abs/1005.5491

 

[Fra]

So one can ask "what is the purpose?" Why bother to quantize cosmology if you very quickly get the same old Friedmann equation model that every cosmologist already uses?
...
So there is a purpose to having a wave function of the U! It can be important for about 100 Planck time units! And that can set things up for a nice conventional inflation. But otherwise ... *shrug*

I think it's quite clear why we want a "measurement theory" also for cosmological models, but what is not so clear is wether it makese sense to consider the extrinsic measurement theory that is what current QM is - to the universe. I mean, where exactly is the CONTEXT for the wavefunction of the universe?

If we do acknowledge that the context would have to be relative to one of the inside observer (say Planck scale observers), then does it seem reasonable that this type of context allows for stuff like timeless hilbert spaces? and the usually stuff we have in current QM?

So the concern I have, and which I thought was mitchels is wether the new "inside measurement theory that would be suitable for cosmological models" would be cast in terms of a regular "quantum theory of the universe"; such as containing a state space of the entire universe that has the same properties as we expect from a hilbert space in what I think of as the current extrinsic measurement theory.

What exactly does the "wavefunction of the universe" mean, unless we picture a timeless hilbert space? And it's exactly this that doesn't make sense to me. I don't question that we need to turn cosmological modes into measurement theories, I just question that the structure we know from normal QM really makes sense for this task.

In other worlds: A "cosmological measurement theory" cannot be in the form of "vanilla QM". Most probalby (I think at least) the structure of this measurement needs to be different. the basic understanding of the backbones such as hilbert space needs radical reconsiderations.

Edit: The problem is not JUST that there is no background spacetime, the problem is worse; this is not even a background hilbert space. I normal QM, this background hilbert space does exist effectively. But in cosmologicla models, this must fail as far as I see it. So either you resort to realism and consider this hilbert spces to exists in some mathematical realm, or you face that measurement theory needs an intrinsic formulation, and probably away with it goes the timelss hilbert backbone.

/Fredrik

 

[marcus]

Fra I was responding to Mitchell where he said:
==quote Mitchell's post==
I'd also like to say that I'm not at all convinced that any form of quantum cosmology is going to be correct. The whole idea of a wavefunction of the universe may be wrong...
==endquote==

There is a pragmatic reason for developing QC, and the possible correctness is rooted in that. So I explained. The bits you quoted don't convey much meaning taken out of context.

==quote my post==
Loop cosmology very quickly approximates classical Friedmann equation cosmology, almost instantly or within a few tens of Planck times, after the bounce. This is with coherent states peaked on classical, coming into the bounce.

So one can ask "what is the purpose?" Why bother to quantize cosmology if you very quickly get the same old Friedmann equation model that every cosmologist already uses?

There is a pragmatic reason. The whole purpose of QC is to find quantum corrections that apply during the "quantum regime" of extremely high density, that happens during an extremely brief interval, right around the start of expansion.

In fact the Loop program has found a quantum corrected version of the Friedmann equation, and it turns out that it kicks off inflation. So under broad assumptions you get some 60 e-folds as a kind of bonus without fine adjustment.
It remains to see if this quantum-corrected Friedmann eqn is right or not.

You can get a good discussion of this in Ashtekar's review "The Big Bang and the Quantum".
Mathematically the corrections to the model are simple, and lead to the bounce always happening when the density is around 41% of Planck.

So there is a purpose to having a wave function of the U! It can be important for about 100 Planck time units! And that can set things up for a nice conventional inflation. But otherwise ... *shrug*

Same way with the black hole. (Semi) classical analysis goes a long way in, but not all the way. Ultimately you need a quantum model to help guess what happens, and it is always possible there could be some significant surprise there. Or so I think, anyway.

Ashtekar's paper is worth looking at, if you have not already.
The Big Bang and the Quantum
http://arxiv.org/abs/1005.5491
==endquote==

Cosmology is based on the Friedmann model which shows how vital parameters like scale and density evolve with time. To push that model back to the start of expansion you need quantum corrections reflecting non-classically high density. It blows up otherwise. It's a straightforward commonsense strategy. Keep it simple. And it works. You get a theory with predictions that you can test. (If it fails test, throw it out. If it passes then you get bonuses like a good start to inflation.)

Here's Battisti and Marciano's spinfoam version of the bounce. It is rather nice, and leads to the same predictions as the quantum Friedmann version (because the foam used is a simple low order one).

Big Bounce in Dipole Cosmology
http://arxiv.org/abs/1010.1258

We are moving ahead on an empirical track here, I think, and it is time to read and learn about the models. If this gambit fails then we can maybe go back to philosophizing. But right now it is bounce phenomenology that is at the forefront.
Check out post #76 re phenomenology:
https://www.physicsforums.com/showthread.php?p=3186856#post3186856
Or try this literature search:

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+POWER+SPECTRUM+or+dk+cosmic+background+radiation%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29
This is what shows up in the search window. Spires turns everything to all caps
FIND (DK QUANTUM GRAVITY, LOOP SPACE OR DK QUANTUM COSMOLOGY, LOOP SPACE) AND (DK POWER SPECTRUM OR DK COSMIC BACKGROUND RADIATION) AND DATE > 2008

 

[atyy]

What does it mean to have N change in the CFT? Naively I imagine N to be fixed in a particular theory. Then gravity should be classical at large distances bulk distances and quantum at small bulk distances. Is that somehow equivalent to changing N in some sense?

 

[fzero]

What does it mean to have N change in the CFT? Naively I imagine N to be fixed in a particular theory.

It does look strange from the gauge theory point of view, but it is natural on the AdS side, where N corresponds to flux of a certain p-form charge. You might be more comfortable calling these families of gauge theories. The word we use doesn't matter so much as long as the picture of the duality is understood.

Then gravity should be classical at large distances bulk distances and quantum at small bulk distances. Is that somehow equivalent to changing N in some sense?

The energy of the probe is different to what we were talking about. The AdS X S space has a scalar curvature that is inversely proportional to the square of the radius of the sphere. If this radius is small in Planck units, then quantum gravity is important at all distances.

 

[atyy]

The energy of the probe is different to what we were talking about. The AdS X S space has a scalar curvature that is inversely proportional to the square of the radius of the sphere. If this radius is small in Planck units, then quantum gravity is important at all distances.

How about the when the bulk doesn't have constant curvature? I've read that that the duality can be extended to some QFTs which are not CFTs, but have a UV fixed point - in that case can we have the bulk be classical or quantum without changing N?

Also, if the energy of the probe is different from the "quantumness" of gravity, does it mean that probes of arbitrarily high energy can still see a classical spacetime? I guess I've heard the handwavy talk that we expect quantum gravity to be important in a small window of energies around the Planck scale, with big classical black holes formed far above that.

 

[fzero]

How about the when the bulk doesn't have constant curvature? I've read that that the duality can be extended to some QFTs which are not CFTs, but have a UV fixed point - in that case can we have the bulk be classical or quantum without changing N?

We're still dealing with spacetimes which are asymptotically AdS X M, at least in the UV. There will still be a relationship between the radius of curvature of this asymptotic AdS and N. Classical supergravity will be valid in regions where the curvature is small. Within the conjecture, the dual field theory is valid everywhere, even if there are technical details in performing computations.

Also, if the energy of the probe is different from the "quantumness" of gravity, does it mean that probes of arbitrarily high energy can still see a classical spacetime? I guess I've heard the handwavy talk that we expect quantum gravity to be important in a small window of energies around the Planck scale, with big classical black holes formed far above that.

Probes with energies near the Planck scale will change the background geometry, so QG is presumably needed to give a complete description of their processes. The general issue in curved space is whether there is ever a perturbative description of low energy probes. That's where the consideration of curvature comes into play.

 

[atyy]

fzero, thanks a lot!

What would sending in probes of higher and higher energy in the bulk correspond to in the boundary theory?

 

[fzero]

fzero, thanks a lot!

What would sending in probes of higher and higher energy in the bulk correspond to in the boundary theory?

There's a certain sense where the energy of a probe corresponds to the distance r from the boundary at which it's localized. This follows from the wave equation on AdS. These states are obtained by propagating the boundary data \phi_0 into the bulk:

\phi(r,x) = \int dy G(r,x-y) \phi_0(y).

Then correlation functions of probes are roughly related to certain integrated CFT correlators:

\langle \phi_1 \cdots \phi_n \rangle \sim \int G_1 \cdots \int G_n \langle \mathcal{O}_1 \cdots \mathcal{O}_n \rangle.

A more careful treatment can be found in http://arxiv.org/abs/hep-th/9903048

 

[marcus]

The shift of interest in QC seems correlate with increased interest the bounce as something that can be tested and/or has to do with inflation.

Earlier I used a Spires search that got 14 phenomenological papers using keywords "power spectrum" and "cosmic background radiation"---2009 or later.

I recently tried one that got 29 phenom. papers, using "fluctuation, primordial", "inflation", and "cosmic background radiation"---2009 or later.

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+primordial%2C+fluctuation+OR+DK+INFLATION+OR+DK+COSMIC+BACKGROUND+RADIATION%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

I'd like to know the physics grounds for this (recalling the question asked in post #1 of the thread.)

I can't believe it's a mere whim among the researchers.

 

[marcus]

I corrected an error in the link:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+FLUCTUATION%2C+PRIMORDIAL+OR+DK+INFLATION+OR+DK+COSMIC+BACKGROUND+RADIATION%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

I just ran this same search for successive time periods and found an increase in numbers of Loop early universe phenomenology papers:

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

It is difficult to sort out what has happened and to understand the physical grounds.

Loop Cosmology has risen to prominence in Quantum Cosmology---both in terms of the number of papers and in terms of citations.
Can this have to do with the comparative testability of the theory?
Or with the comparative concreteness (it provides a definite model of the cosmos to work with)?
Or with the relative absence of extra physical baggage?
Or with the theory's comparative mathematical simplicity?

I'd welcome other people's ideas about this.

Ashtekar Sloan's March 2011 paper provides some alternative ideas:
1. the bounce provides a platform for laying out initial conditions and probability measures on initial conditions. Thus one can do physics concerning inflation etc that one cannot do in the classical setup with it's singularity.
http://arXiv.org/abs/1103.2475

2. the bounce makes an adequate inflation episode more "natural" and relieves some need for fine tuning.
http://arXiv.org/abs/0912.4093
http://arXiv.org/abs/1011.5516

 

[marcus]

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

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

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

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

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

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

 

[marcus]

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

I was wrong to say that Gasperini et al's pre-big-bang model is defunct; there was a http://arxiv.org/abs/1103.2311" is full of resources and review articles.

In my view, the string model of a cosmological bounce with the best pedigree is the http://arxiv.org/abs/hep-th/0204189" , and that is not a realistic model, nor can it easily be made realistic, because exact solutions of string theory on a time-dependent background are hard to come across, and not adjustable.

Robert Brandenberger also has http://arxiv.org/abs/1103.2271" today reviewing two cosmological models, a bounce model, and a "string gas cosmology" where the string gas is just sitting there (indefinitely, timelessly, as the end product of a collapse, I can't tell, and maybe he doesn't know), and then it fluctuates into an expanding state.

I continue to feel that string theory just hasn't found the right way to think about cosmology yet. Maybe some individual string theorist has done so - the seeds of the right approach may already exist in the literature - but what's lacking is the demonstration that this is the right path. It's apparent that no particular approach has swept the field - I think this is the real lesson of Marcus's database searches. Papers are being written, but it's still a cacophony of conflicting ideas. The version of inflation called eternal inflation is probably the favorite of elite opinion, but I'm not sure there's anything like a consensus on how to think about the initial singularity.

The place in string cosmology where contact with empirical data is occurring is in models of inflation. See section 5.2 in http://arxiv.org/abs/0810.3707" of CMB predictions from various string models of inflation, that will be tested as further WMAP data comes in. ...

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

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

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

• The bulk should have some definite mathematical structure that represents it.
• Asymptotically it should look deSitter (accelerated expansion) or at least not the opposite (which is AdS, accelerated contraction).
• There should be a quantum state of bulk geometry which exhibits a bounce.
• The bounce should either lead to adequate inflation or, if not, the model should produce the usual effects expected from inflation in some other way.
• The model should be testable and attract the attention of phenomenologists so they can study means of testing it.

...

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

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

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+primordial%2C+fluctuation+OR+DK+INFLATION+OR+DK+COSMIC+BACKGROUND+RADIATION%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

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

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

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

 

[cristo]

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

Where's the testability in loop cosmology?

 

[marcus]

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

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

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+primordial%2C+fluctuation+OR+DK+INFLATION+OR+DK+COSMIC+BACKGROUND+RADIATION%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29
===endquote===

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

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

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

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

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

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

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

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

 

[cristo]

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

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

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

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

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

 

[marcus]

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

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

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

 

[atyy]

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

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

 

[atyy]

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

 

[Sardano]

Hi everybody,

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

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

 

[marcus]

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

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