Naked singularities: evidence against compactified dimensions?

[marcus]

Just searching for "quantum cosmology" is going to miss a lot of papers that don't bother using that term. For example, Kiritsis and Kofinas, Horava-Lifgarbagez Cosmology, arXiv:0904.1334 has 202 citations but doesn't show up on your list. There's no particular reason to include the Calcagni paper above and not this one.

If you want to draw statistical inferences, you're going to have to be much more careful about how to sort through the data.

I think the K and K paper should NOT have appeared on the list because it is not quantum cosmology. No mention of quantum. No quantum theory analysis. Just pure classical analysis using classical form of Horava gravity.

So InSpire keyword assignments may still have flaws and shortcomings (it is only beta ). I'd like to be apprised of any faults! But this KandK case is not one of them.

Thanks for pointing out the K and K paper, and thanks again for pointing us at the beta version of InSpire! I guess 80 or 90 percent of what I do is follow the literature on my own (without guidance from citation counts etc.) and look at the science itself.

I turn to citation counts as a check on my own impressions as an observer. It provides auxilliary guidance and even occasional confirmation.

Other readers can make of it what they will, it is just realworld objective stuff with all the bugs, warts, and flaws that come with that.

InSpire is NEW and I have no idea as yet how they do their keywording. I've noticed they underreport LQG quite a lot. For 2010, for example, their search with that one keyword finds 95 papers but with the collection of DESY keywords I have learned to use over the years then IIRC I get over 140.

So InSpire is not perfect. Who knows how it will develop? Maybe it will improve substantially, maybe not.

In the meantime I will happily turn to it for guidance (as well as to my old DESY keyword Spires search.)

I appreciate any pointers you have to papers that InSpire seems to overlook. I may be able to improve the search by learning new keyword categories, and by Boolean combinations of terms.

BTW no, I don't wish to "draw statistical inference" in any formal sense.

 

[marcus]

There's no particular reason to include the Calcagni paper above and not this one.
...

I believe you will see you are mistaken if you merely look at the first paragraph of Calcagni's paper. It is chock full of QG discussion, comparing with LQG, CDT, QEG (socalled quantum einstein gravity). It cites a whole lot of LQG. And the author himself tagged it quantum gravity.

K and K did not use the word quantum anywhere AFAICS and did no quantum analysis, purely classical paper. Tagging it with a quantum keyword like the Calcagni would have been a mistake.

IsSpire may have things wrong but this is not one yet. But please keep looking!

 

[fzero]

I believe you will see you are mistaken if you merely look at the first paragraph of Calcagni's paper. It is chock full of QG discussion, comparing with LQG, CDT, QEG (socalled quantum einstein gravity). It cites a whole lot of LQG. And the author himself tagged it quantum gravity.

K and K did not use the word quantum anywhere AFAICS and did no quantum analysis, purely classical paper. Tagging it with a quantum keyword like the Calcagni would have been a mistake.

IsSpire may have things wrong but this is not one yet. But please keep looking!

Unfortunately one has to read beyond the first paragraph to figure out exactly what contribution the paper is making. Towards the end of Calcagni's introduction (page 4), he summarizes what he does:

"To summarize, the aims and results of the present work are:
• To introduce scalar matter in Hoˇrava’s z = 3 theory, thus extending the original proposal for pure gravity. We assume detailed balance, which is not an indis- pensable ingredient of the theory but it makes its quantum properties simpler to analyze. Therefore we start from a three-dimensional action with non-local pseudo-differential operators. Under a ‘separate’ detailed balance condition, one obtains a minimally coupled four-dimensional z = 3 Lifgarbagez scalar action.
• Study the cosmology of the model, with and without matter. We find vac- uum solutions and argue that bouncing solutions exist and avoid the big bang singularity.
• Solutions with Euclidean signature are asymptotically de Sitter and in qualita- tive agreement with the CDT scenario. The correspondence with CDT, already noticed in [50], is supported now at the level of mini-superspace.
• On the other hand, inhomogeneous scalar perturbations against a classical background, generated by quantum fluctuations of an inflationary Lifgarbagez field, are unable to yield a scale-invariant spectrum. Abandoning the detailed balance condition one can obtain scale invariance. This result is not obvious in the vacuum theory and suggests to modify the original formulation."

Kiritsis and Kofinas have their summary on their own page 4:

"In this paper we couple the Hoˇrava-Lifgarbagez gravity with matter and derive the classical equations of motion, allowing a more general class of couplings, not restricted by detailed balance. We further make a cosmological ansatz and derive the analogue of the Friedmann equation. As expected, most of the non-relativistic structure does not enter in this equation. There are, however, contributions from the quadratic curvature terms that dominate in the UV (when the universe is small) and provide a bounce. This is in qualitative agreement with our expectations above.
We also describe the qualitative structure of the equations for cosmological per- turbations, and conclude that the theory can lead to an alternative of inflation in the UV regime. It can solve the horizon and flatness problems, can generate scale- invariant superhorizons perturbations and does not need graceful exit nor reheating."

Apart from the detailed balance assumption (which leads to contradiction with observation) and the comparison with CDT, these papers are studying the same physics, the background information in the introduction not withstanding.


In any case, I'm not particularly sure what conclusions you want to draw from keyword searches. What we call quantum cosmology is particularly well-suited to LQG and related approaches, while most of the work on cosmology with extra dimensions aims at producing inflationary scenarios and the phenomenology of the $$\Lambda$$-CDM model. Both approaches have their merits, but obviously aren't turning up in the same searches. For instance, the Steinhardt et al papers are about cosmology of models with extra dimensions, but have nothing to do with quantum cosmology. The fact that they don't appear in a search for "quantum cosmology" says nothing about the relative level of interest in cosmology with extra dimensions.

Citation counts are far from perfect. It's very typical to cite anything even tangentially related to your work somewhere in the introduction to avoid complaints. It's much more complicated to determine if a cited work had a real influence on a given publication. Without formulating a hypothesis about what fact you want to illustrate and what methodology you need to do it, any of these keyword/citation searches are just going to be misleading and easily picked apart.

 

[marcus]

Unfortunately one has to read beyond the first paragraph to figure out exactly what contribution the paper is making.

I don't think it is "unfortunate", just how it is. Of course I read well beyond the first paragraph because I was curious too, as to why InSpire and Calcagni would both tag his paper with that keyword. Is there enough there? Enough comparison with CDT and QEG?
It is a judgment call. Papers typically get a handful of tags, sometimes a halfdozen or more. One tag does not represent the sole focus (which in some cases may not exist).

I thought it was a fair call. One paper never even said the word "quantum". The other pointedly placed Horava in a QG context and compared with other QG results. I am not going to quarrel with InSpire. I just want to watch them and learn their keyword policy so I can use it intelligently. More than that would just be quibbling, I think.

I thought the rest of your post was substantive. You made good points.

I realize that Steinhardt mostly does not turn up in QG searches and that this is because he mostly uses classical GR geometry, not quantum. I certainly realize that citation counts are not a perfect measure of the interest of the research community! We've discussed that a lot. You make several familiar points here:

==quote fzero==
What we call quantum cosmology is particularly well-suited to LQG and related approaches, while most of the work on cosmology with extra dimensions aims at producing inflationary scenarios and the phenomenology of the $$\Lambda$$-CDM model. Both approaches have their merits, but obviously aren't turning up in the same searches. For instance, the Steinhardt et al papers are about cosmology of models with extra dimensions, but have nothing to do with quantum cosmology. The fact that they don't appear in a search for "quantum cosmology" says nothing about the relative level of interest in cosmology with extra dimensions.
==endquote==

I want to make sure you understand. My point there had nothing to do with Steinhardt not appearing in a QC search! I went and got a bunch of Steinhardt papers and examined them (I am kind of a long term fan of his anyway) and what struck me is how in several recent papers he explicitly says ekpyrotic based on a scalar field with chaning EOS. No mention of branes, colliding or otherwise.
This is what surprised me!
He was redefining a key word in his vocabulary. In one essay he brought in braneclash just as an optional way to think about it, an illustrative example. In other papers he completely leaves it out.
And sometimes (though less often now) he seriously recalls it.

This, as I say, was surprising. One used to have an automatic mental association of that part of Steinhardt's work with extra dimensions. From examining the recent papers I sense that he is moving away from that.

This could be his own personal intellectual track, but it could also reflect a shift in the community. That is something to think more about. There certainly has been a marked retreat from String among the top people. Many are finding other research interests. That's clear from the attendance and content of the annual Strings conference, if from nothing else. So that could have something to do with it. But this is not something to draw definite conclusions about now--it's something to watch over time and get to understand better.

BTW in the 1996-1998 Inspire top ten quantum cosmo there were three string cosmo papers that made the list. They were numbers 5, 7, and 9 in the top ten.
http://inspirebeta.net/search?ln=en...2y=1998&sf=&so=a&rm=citation&rg=10&sc=0&of=hb
As I recall, in the corresponding 2009-present top ten there were none. I wouldn't draw any definite conclusion--this does relate to one of the points you made about QC being less string-friendly. The field may have gotten less stringy over time. I stress the may, no definite conclusions at this point!

 

[fzero]

I want to make sure you understand. My point had nothing to do with Steihardt not appearing in a QC search! Give me a break I went and got a bunch of Steinhardt papers and examined them (I am kind of a long term fan of his anyway) and what struck me is how in several recent papers he explicitly says ekpyrotic based on a scalar field with chaning EOS. No mention of branes, colliding or otherwise.
This is what surprised me!
He was redefining a key word in his vocabulary. In one essay he brought in braneclash just as an optional way to think about it, an illustrative example. In other papers he completely leaves it out.
And sometimes (though less often now) he seriously recalls it.

This, as I say, was surprising. One used to have an automatic mental association of that part of Steinhardt's work with extra dimensions. From examining the recent papers I sense that he is moving away from that.

I was specifically referring to the Steinhardt and Wesley papers about cosmological constraints on theories with extra dimensions, but the ekpyrotic papers are another example. My point was only that if we're using papers on loop quantum cosmology and Horava-Lifgarbagez theory to suggest a trend away from studying the cosmology of theories with extra dimensions, one ought to actually compare with papers studying the latter. Whether or not the Steinhardt-Wesley papers are highly cited, the fact was that they would have not turned up in the first place. However, the original ekpyrotic paper, http://arxiv.org/abs/hep-th/0103239, has over 90 citations since early 2009, which is much larger than most of the papers on the last list that you made.

As for "ekpyrotic based on a scalar field," this is related to a confusion over the mechanics of the ekpyrotic model. The scalar field in question is nothing more than the separation between branes, a point which is explained in the discussion following Figure 1 of http://arxiv.org/abs/astro-ph/0404480 . What happens is that one writes down an effective field theory for this scalar field and couples it to gravity, which is equation (19) in
http://arxiv.org/abs/hep-th/0103239. Much of the cosmology of the model then follows from the resulting Friedmann equation, without further reference to the source of the scalar field.

I wouldn't necessarily attach significance to this not being explained in Steinhardt's 10th paper on the subject. It's also quite possible that some recent work involves modifying the effective field theory of the scalar field in such a way that the authors cannot derive from a brane picture (which is not to say that no one else could).

 

[Lievo]

I thought it was a fair call

Well I don't think many will agree. Anyway. You made your point that there was a shift of interest away from string. You presented what you think support this intuition. Could we now let people judge for themself if it's convincing and end this side question?

I'd appreciate if we could go back to naked singularities (I wonder the proportion of papers... oh forget that!).

You seem interested in minimal size of black holes. Current QG (quantum geometry/quantum gravity) has something to say about that. I recall some tentative results have appeared about that.

Sure, do you have some not too technical refs?

 

[marcus]

Much of the cosmology of the model then follows from the resulting Friedmann equation, without further reference to the source of the scalar field.

I wouldn't necessarily attach significance to this not being explained in Steinhardt's 10th paper on the subject. It's also quite possible that some recent work involves modifying the effective field theory of the scalar field in such a way that the authors cannot derive from a brane picture (which is not to say that no one else could).

The brane picture is not necessary. It is one possible way to explain the variation in the scalar field. It is, I would say, pointedly not mentioned in some of the ekpyrotic papers and I don't think you can argue that it is tacitly assumed.

You suggest the braneclash explanation for the varying field is not mentioned because perhaps the authors could not figure out a way for braneclash to produce exactly the behavior in the scalar field they wanted. That sounds like an excuse. I think they did not mention that explanation because they did not care to. Maybe they are tired of invoking that rather picturesque mechanism. It seems to me that there might be several explanations and what matters is the field itself, not the story behind it.

 

[Haelfix]

The Gregory-Laflamme instability is and has been for many years, one of the testing grounds in classical General Relativity for the cosmic censorship hypothesis... A toy model if you will. One of my colleagues who works down the hall has been actively trying to replicate Lehner and Pretorius' work and trying to extend it.

Anyway the paper is significant and indeed it seems more and more likely that naked singularities do exist in the real world (and wouldn't it be interesting if it remained true in 4d, it would mean we could in principle see whatever it is that UV completes gravity). Anyway the jury is still out on the 4dimensional case, but certainly that these black strings solutions exist leads one to believe that it is perhaps more general than meets the eye.

And no, of course that it doesn't preclude extra dimension solutions. They are indeed ubiquitous in the literature, despite all the obfuscation that Marcus likes to throw around for whatever reason!

 

[fzero]

The brane picture is not necessary. It is one possible way to explain the variation in the scalar field. It is, I would say, pointedly not mentioned in some of the ekpyrotic papers and I don't think you can argue that it is tacitly assumed.

You suggest the braneclash explanation for the varying field is not mentioned because perhaps the authors could not figure out a way for braneclash to produce exactly the behavior in the scalar field they wanted. That sounds like an excuse. I think they did not mention that explanation because they did not care to. Maybe they are tired of invoking that rather picturesque mechanism. It seems to me that there might be several explanations and what matters is the field itself, not the story behind it.

I think you're speculating way too much about sociology and not enough into how physics is done. The story behind any new mechanism is very important, since motivations for adding new physics are very important. For inflation, the source of the scalar field was hypothesized to be a mode of the GUT Higgs, for ekpyrosis it is a modulus corresponding to the separation of branes. Neither model would have appeared so attractive if the authors just introduced a scalar field with no other motivations. I'm not saying that decent physics cannot be done by empirical methods, but the source of new physics is important too.

It is true that the specific behavior in the adiabatic ekpyrotic model has not been connected to concrete behavior in any fundamental theory. I see no reason why the authors would be unhappy to learn that it was, even if it were a stringy model. Khoury in particular continues to work on string-inspired modifications of ekpyrotic models.

In any case, in the most recent Khoury and Steinhardt paper, http://arxiv.org/abs/arXiv:1101.3548 , I find the following:

"Barring some higher-dimensional or stringy effects near the bounce that mixes gravitational potential and curvature fluctuations..."

This comment is about the old ekpyrotic scenario, but seems to contradict the idea that the authors are "tired of invoking that rather picturesque mechanism."

 

[Lievo]

Haelfix, thanks for helping this discussion to go back to the topic

it seems more and more likely that naked singularities do exist in the real world (and wouldn't it be interesting if it remained true in 4d, it would mean we could in principle see whatever it is that UV completes gravity).

Interesting? Don't you think magic would be a better description?