Naked singularities: evidence against compactified dimensions?

[Lievo]

Hi,

Here's a http://blogs.discovermagazine.com/cosmicvariance/2011/03/04/fractal-black-holes-on-strings/" ).

Basically, they showed that if http://en.wikipedia.org/wiki/Black_string" is to be expected.

However naked singularities would be such a strange thing that it seems safe to postulate it can’t exist. Of course who knows, but is it correct it would be really surprising?

If you agree, then can we interpret the result of Lehner and Pretorius as an evidence that black strings cannot exists, thus (too much) compactified dimensions?

Thx for your input

 

[marcus]

I can't comment on the particular argument against extra dimensions which you present here, but one of the top people at Princeton, Paul Steinhardt, has written about ways to constrain or even rule out extra dimensions based on cosmology observations---parameters of cosmological models determined by data-fitting.

You might be interested in looking at these papers because they have the same research goal, ruling out XD's.

http://arxiv.org/abs/0811.1614
Dark Energy, Inflation and Extra Dimensions
Paul J. Steinhardt, Daniel Wesley
26 pages, 1 figure.
(Submitted on 11 Nov 2008)
"We consider how accelerated expansion, whether due to inflation or dark energy, imposes strong constraints on fundamental theories obtained by compactification from higher dimensions. For theories that obey the null energy condition (NEC), we find that inflationary cosmology is impossible for a wide range of compactifications; and a dark energy phase consistent with observations is only possible if both Newton's gravitational constant and the dark energy equation-of-state vary with time. If the theory violates the NEC, inflation and dark energy are only possible if the NEC-violating elements are inhomogeneously distributed in thecompact dimensions and vary with time in precise synchrony with the matter and energy density in the non-compact dimensions. Although our proofs are derived assuming general relativity applies in both four and higher dimensions and certain forms of metrics, we argue that similar constraints must apply for more general compactifications."

http://arxiv.org/abs/1003.2815
Exploring extra dimensions through observational tests of dark energy and varying Newton's constant
Paul J. Steinhardt, Daniel Wesley
27 pages, 11 figures.
(Submitted on 14 Mar 2010)
"We recently presented a series of dark energy theorems that place constraints on the equation of state of dark energy w_DE, the time-variation of Newton's constant G', and the violation of energy conditions in theories with extra dimensions. In this paper, we explore how current and future measurements of w_DE and G' can be used to place tight limits on large classes of these theories (including some of the most well-motivated examples) independent of the size of the extra dimensions. As an example, we show that models with conformally Ricci-flat metrics obeying the null energy condition (a common ansatz for Kaluza-Klein and string constructions) are highly constrained by current data and may be ruled out entirely by future dark energy and pulsar observations."

We had a thread about the 2008 paper soon after it came out. No need to get bogged in technical detail. Steinhardt's papers have a good introduction at the beginning that gives the general gist, and then summary and overview at the end. Plus feel free to ask us for intuitive interpretation and paraphrase, as needed. The papers are fairly comprehensible as they are, but we can try to explicate if anyone wants.

As I recall the essential is that if good old Gen Rel that we know and love governs the rolled-ups as well as the visible space dimensions, then during both episodes of accelerated expansion (early U inflation and today's dark energy acceleration) the rolled-ups will not be stable, will either tend to come undone, or at least behave in ways having observable effects on the acceleration rate.

Steinhardt is a world-class physicist who make significant contribution to String back when it was a hot subject (e.g. 1990s) and used to like extra dimensions (even used them in cosmological modeling back in the day!) but now seems to be taking a different view. Interests appeared to have shifted in the 2000s.

Two earlier papers by Wesley about this:
http://arxiv.org/abs/0802.2106
http://arxiv.org/abs/0802.3214

 

[Lievo]

I can't comment on the particular argument against extra dimensions which you present here

My title is misleading. In fact this not exactly against the existence of compactified dimension per see, but rather against their radius to be smaller than the smalest BH.

You might be interested in looking at these papers because they have the same research goal, ruling out XD's.

This is interesting, thx. However, it seems common sense to argue that many physicists still works on strings, thus are not convinced that these papers provide killing arguments. From an outsider perspective, it seems that this kind of no go proofs is usually very fragile because it relies on many hidden hypothesis. So I doubt extra dimensions have been ruled out properly -more likely is that some models relying on some false assumptions have been excluded. Maybe the smart move is to wonder what hidden assumption is false?

Of particular interest (at least given the evidence I referred to) is the assumption that compactified dimension is an accurate description when one goes to the bottom. It supposed too (as far as I can tell this what one should expect if strings are to provide the accurate TOE), but maybe this is the problem in the first place. I find suggestive that some of the main sucesses of strings have been (as far as I can tell) reproducing general relativity at low energy limit and more recently to provide some promising tools for condensed matter physics, neither of which being at the bottom/highest energy.

So is it possible strings describe something intermediate between the TOE at one extreme and general relativity at another? Just my two cents guess ;-)

 

[marcus]

...it seems common sense to argue that many physicists still works on strings, thus are not convinced that these papers provide killing arguments. From an outsider perspective, it seems that this kind of no go proofs is usually very fragile because it relies on many hidden hypothesis.

Steinhardt Wesley do not claim to "provide killing arguments". They exclude some cases. They constrain (narrow down the possible parameterspace for) other cases.

This is only one of the directions from which stringy physics is getting pressured. It is certainly not the most important---there are others that are harder to ignore.

Steinhardt Wesley make their assumptions explicit. The conclusions follow from the assumptions. You don't have to look for "hidden" hypotheses, you can simply read the papers and they will tell you about cases which their arguments do not cover. They will spell out assumptions which you can then choose to reject---thereby rejecting their conclusions.

===================

...many physicists still works on strings,...

That seems largely irrelevant, Lievo. What people work on is determined in large part by their past education, career considerations, professional contacts, and institutional inertia.

It might be more relevant to ask about rates of change. Do we see a decline in string conference attendance? Do we see top people getting out of string research and shifting over to other fields? Do we see a drop in citations to recent stringy papers, compared with a few years back. Do today's research papers get cited as references as much as earlier ones, in other words?

Those are better indicators. People don't all exit from a field en masse. Indeed many of the middle-level people may have no alternative but to continue doing what they know how to do. I've noticed that many of the top people seem to be more able to change what they work on---seem to have greater mobility in other words.

All that does not imply anything about Steinhardt Wesley's papers however. I think they are interesting papers, but just a small part of a larger picture.

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.

 

[Lievo]

Steinhardt Wesley do not claim to "provide killing arguments".

Sure. I was reacting to your 'or even rule out', not to Wesley's claims that are more cautious.

That seems largely irrelevant, Lievo. What people work on is determined in large part by their past education, career considerations, professional contacts, and institutional inertia.

Well, you'll have to trust my word on this, but this is very overstated. Academic position gives you a large freedom to choose what you're working on. I'm not saying that academics are not at all influenced by the priorities of funding agencies, or that there is no inertia (mostly because you won't leave a topic that you putted a student on until (s)he finished his/her thesis) but ultimly if people work on something that's because they want to.

It might be more relevant to ask about rates of change. (...) Those are better indicators.

Loosely speaking, I do agree (loosely speaking because of course the smaller the easier to have large growth rate). I just tried to have a look using scholar.google. Results are very interesting! Below is the number of papers, sorted by date, that includes either "loop quantum gravity" or "string theory" plus cosmology, divided by the number of papers retrieved with cosmology alone.

cosmology "loop quantum gravity"
80-85
85-90
90-95 0.1%
95-00 0.3%
00-05 1.4%
05-10 5.0%

cosmology "string theory"

80-85 0.7%
85-90 8.5%
90-95 8.6%
95-00 8.9%
00-05 15.3%
05-10 30.1%

I have to say I'm impressed and surprised by the recent rate of growth of loop quantum gravity. It's about half as big as the so call first revolution in string theory! I was certainly not aware of such progression -thx for having attracted my attention on it.

However, to get back to the point, obviously the growth of string theory is even more impressive even in the most recent years. Clearly it's the dominant theory now, and no, there's no chance that this kind of statistics can be attributed to institutional inertia ;-)

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]

Sure. I was reacting to your 'or even rule out', not to Wesley's claims that are more cautious.
...

Perhaps I wasn't clear, I didn't mean they were looking at ways to rule out all cases of extra dimensions, but how some common cases might be ruled out. I was paraphrasing what Steinhardt and Wesley actually said in their abstract to the second paper I cited:

" we show that models with conformally Ricci-flat metrics obeying the null energy condition (a common ansatz for Kaluza-Klein and string constructions) are highly constrained by current data and may be ruled out entirely by future dark energy and pulsar observations."

I did not say Steinhardt had already ruled out these cases but that he was "looking at ways" to constrain or even rule out. Sorry if I wasn't clear enough.

But that said, I protest that I was speaking no less cautiously than Steinhardt and Wesley :

...one of the top people at Princeton, Paul Steinhardt, has written about ways to constrain or even rule out extra dimensions based on cosmology observations---parameters of cosmological models determined by data-fitting.

 

[marcus]

Lievo, I was intrigued by your statistics. I had not used scholar.google this way before. Instead I have been using the Spires database (Stanford-SLAC) which is based on DESY keywords classification.

So I looked into scholar.google. I chose "advanced scholar search" and it seems to be based not on a librarian's keyword search but on occurrence of the words in either the title or the main body of text. I see how you can restrict by date, e.g. 2001-2005.

 

[Lievo]

Argh I lost my post

I protest that I was speaking no less cautiously than Steinhardt and Wesley :

Pardon my misunderstanding that you were making a stronger claim

" we show that models with conformally Ricci-flat metrics obeying the null energy condition (a common ansatz for Kaluza-Klein and string constructions) are highly constrained by current data and may be ruled out entirely by future dark energy and pulsar observations."

Perhaps I should to clarify what I was saying about hidden assumptions. Let's suppose they found a way to clearly and definitly rule out models with conformally Ricci-flat metrics obeying the null energy condition.

As an external observer (a cute way to say I'm not qualify in anyway to judge this ), I'd not interpret this as extra dimension idea is dead. I would interpret this as: relying on models with conformally Ricci-flat metrics obeying the null energy condition is equivalent to make a series of assumptions. At least one of these assumptions is false, but maybe just one. Maybe the assumption of continuity?

 

[Lievo]

Lievo, I'm intrigued by your statistics. (...) it seems to be based not on a librarian's keyword search but on occurrence of the words in either the title or the main body of text.

Yes this is not based on keyword (one of the clear advantages in my mind!). Here are the raw numbers I got plus the links to some of my searchs

http://scholar.google.ca/scholar?as...s_publication=&as_ylo=2000&as_yhi=2005&hl=fr"
80-85 4
85-90 5
90-95 24
95-00 169
00-05 819
05-10 2070

http://scholar.google.ca/scholar?hl...hercher&lr=&as_ylo=2000&as_yhi=2005&as_vis=0"

80-85 97
85-90 1380
90-95 2530
95-00 4420
00-05 8980
05-10 12400

http://scholar.google.ca/scholar?hl...hercher&lr=&as_ylo=2000&as_yhi=2005&as_vis=0"

80-85 13600
85-90 16100
90-95 29400
95-00 49800
00-05 58700
05-10 41200

I have used the Spires database (Stanford-SLAC) which is based on keywords.

I looked into it!

Good! It'd be interesting to have a point of comparison with the Spires database. The raw numbers will change but it'd be interesting to check if the pattern is the same for the rate of growth.

 

[Lievo]

80-85 13600
85-90 16100
90-95 29400
95-00 49800
00-05 58700
05-10 41200

Notice that the decreased number of papers for 05-10 may well be an artefact with some further document to be indexed latter. That's why I think it's more carefull to look at proportion rather than raw numbers.

 

[marcus]

Yes this is not based on keyword (one of the clear advantages in my mind!). Here are the raw numbers I got plus the links to some of my searchs

...
Good! It'd be interesting to have a point of comparison with the Spires database. The raw numbers will change but it'd be interesting to check if the pattern is the same for the rate of growth.

I'm glad you are interested. You have given a reason why there could be some correlation with the perceived scientific validity or the fertility of a subject---to the extent that (at least secure academics with tenure) are free to change fields.

As you suggest the raw numbers will be much lower. And I have not checked Spires for string+cosmology. I only have loop+cosmology numbers. I think we are talking numbers like 40-60 per year, at best. We'll see.

If you know the Spires database, we will be typing things like this into the box:
dk quantum cosmology and date=2009

dk quantum cosmology and dk loop space and date > 2006 and date < 2009

Just for illustration.

Lets use the German mirror, Stanford Spires has been very slow lately:
http://www-library.desy.de/spires/hep/

Here is the result for "dk quantum cosmology and dk loop space and date = 2010"
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=dk+quantum+cosmology+and+dk+loop+space+and+date+%3D+2010&FORMAT=WWW&SEQUENCE=citecount%28d%29

It produces only 58 papers. I have them ranked by cite count so that one can see the papers at the top which the researchers seem to consider the more interesting.
So that is 58 in 2010.

Here is the same thing for 2005:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DK+LOOP+SPACE+AND+DATE+%3D+2005&FORMAT=www&SEQUENCE=citecount%28d%29

It gives 18 papers for that year. You can see how to change the date if you are curious. The search window is simple. The code "dk" stands for DESY keyword. The DESY librarians do the classification and tagging. You have pointed out that this could introduce error. However I have followed the LQC (loop quantum cosmology) literature rather closely and I am not used to seeing misclassification or failure to tag in this case. I can only state this for LQC and for the period since, say, 2005.

In case you might be interested I decided to save you the trouble. Here are the numbers for 2005,...,2010:
18, 28, 42, 51, 47, 58.

 

[Lievo]

You can see how to change the date if you are curious.

I am, but what keywords would you advice to have the moral equivalent of string theory, loop quantum gravity and cosmology as a whole? Also, is it not a big limitation to look at HEP only?

 

[marcus]

I am, but what keywords would you advice to have the moral equivalent of string theory, loop quantum gravity and cosmology as a whole? Also, is it not a big limitation to look at HEP only?

the HEP is not a limitation, I find. Spires began many years ago as a narrower database and it was called HEP. But it has broadened out and includes astrophysics and cosmology and much more. It is not just narrowly HEP.

I actually don't know the keywords to get string papers but you can get a list of DESY keywords from SEARCH HELP. (right under the box where you type in search terms)

Here is the list of keywords:
http://www-library.desy.de/schlagw2.html

And also you can do an author search for your favorite string author and get a list of papers beside each paper it will say "keywords". You click on that and it will tell you what keywords that paper has been tagged with. So this way you learn what are the keywords that match your interests.

for example to get Alain Connes papers you type
"find a Connes, Alain" where a stands for "author" and you can omit "find" so simply
type "a Connes" and you will get his papers. I think. I didn't try it.

If there is a danger of getting too many papers, then restrict the date to narrow it down.

You asked about getting Loop Gravity papers. I would include spinfoam and group-field-theory. But for starters type in:

"dk quantum gravity, loop space and date > 2005"

or type in:

"(dk quantum gravity, loop space OR dk spin, foam) AND date > 2006"

It is a good idea to have it sort papers by cite-count so you get the more important papers first. At least for my purposes.

 

[smoit]

Steinhardt is a world-class physicist who make significant contribution to String back when it was a hot subject (e.g. 1990s) and used to like extra dimensions (even used them in cosmological modeling back in the day!) but now seems to be taking a different view. Interests appeared to have shifted in the 2000s.

Dear Marcus,
Contrary to your claim regarding the extra dimensions, for the past 10 years Paul and his collaborators have been working on a relatively new class of models directly inspired by the Heterotic M-theory (see the abstract below), which present an alternative to inflation - the so-called "Ekpyrotic scenario". He has been a very outspoken critic of inflation both in the context of string theory and in general while promoting his own string-inspired model.

http://arxiv.org/abs/hep-th/0103239
The Ekpyrotic Universe: Colliding Branes and the Origin of the Hot Big Bang
Justin Khoury, Burt A. Ovrut, Paul J. Steinhardt, Neil Turok

"We propose a cosmological scenario in which the hot big bang universe is produced by the collision of a brane in the bulk space with a bounding orbifold plane, beginning from an otherwise cold, vacuous, static universe. The model addresses the cosmological horizon, flatness and monopole problems and generates a nearly scale-invariant spectrum of density perturbations without invoking superluminal expansion (inflation). The scenario relies, instead, on physical phenomena that arise naturally in theories based on extra dimensions and branes. As an example, we present our scenario predominantly within the context of heterotic M-theory. A prediction that distinguishes this scenario from standard inflationary cosmology is a strongly blue gravitational wave spectrum, which has consequences for microwave background polarization experiments and gravitational wave detectors."

 

[marcus]

I'm well aware of Steinhardt and Turok's ekpyrotic cosmo, clashing brane cyclic.
You cite a 2001 paper.

Steinhardt's interest seems to have dwindled. Citations to cyclic braneclash has dwindled.

That is basically my point.

Every now and then one sees a paper on that stuff.

In the 1990s Steinhardt contributed to string IIRC. And I think he saw an inconsistency between inflation and string. That motivated him (since he liked string) to find some alt cosmology which did not need inflation.

So around 2000 he and Turok invented braneclash. It has some intriguing aspects and there was a flurry of interest. They also wrote a popular book.

But since then inflation has taken a stronger hold, because of WMAP and BAO cosmology data I think. The CMB spectrum. I think that is the main reason many people have lost interest in braneclash cosmo.

Now to our surprise we find Steinhardt writing papers that EMBRACE inflation as an assumption (instead of trying to dispense with it). And following through on the old idea he had at the start. That inflation was kind of difficult to mix with string.

What is interesting is the CHANGE in a top person. And of course the 2001 paper you mentioned is a good illustration of that.

I cited a 2008 and 2010 paper of Steinhardt. Most of the braneclash stuff is like 2001-2006 as I recall.

Oh, why not keep a copy of your post, Mr. Smoit

Dear Marcus,
Contrary to your claim regarding the extra dimensions, for the past 10 years Paul and his collaborators have been working on a relatively new class of models directly inspired by the Heterotic M-theory (see the abstract below), which present an alternative to inflation - the so-called "Ekpyrotic scenario". He has been a very outspoken critic of inflation both in the context of string theory and in general while promoting his own string-inspired model.

http://arxiv.org/abs/hep-th/0103239
The Ekpyrotic Universe: Colliding Branes and the Origin of the Hot Big Bang
Justin Khoury, Burt A. Ovrut, Paul J. Steinhardt, Neil Turok...

 

[smoit]

Dear Marcus,
Your claim that Steinhardt's interest in the Ekpyrotic scenario has dwindled is simply not true. He's been actively pursuing this direction since the original paper came out and never stopped working on it. FIY, here is the latest paper of Steinhardt:

http://arxiv.org/abs/1101.3548"
Generating Scale-Invariant Perturbations from Rapidly-Evolving Equation of State

Justin Khoury, Paul J. Steinhardt
(Submitted on 18 Jan 2011)
"Recently, we introduced an ekpyrotic model based on a single, canonical scalar field that generates nearly scale invariant curvature fluctuations through a purely "adiabatic mechanism" in which the background evolution is a dynamical attractor. Despite the starkly different physical mechanism for generating fluctuations, the two-point function is identical to inflation. In this paper, we further explore this concept, focusing in particular on issues of non-gaussianity and quantum corrections. We find that the degeneracy with inflation is broken at three-point level: for the simplest case of an exponential potential, the three-point amplitude is strongly scale dependent, resulting in a breakdown of perturbation theory on small scales. However, we show that the perturbative breakdown can be circumvented -- and all issues raised in Linde et al. (arXiv:0912.0944) can be addressed -- by altering the potential such that power is suppressed on small scales. The resulting range of nearly scale invariant, gaussian modes can be as much as twelve e-folds, enough to span the scales probed by microwave background and large scale structure observations. On smaller scales, the spectrum is not scale invariant but is observationally acceptable."

Here is a couple of papers from last year:

http://arxiv.org/abs/1008.4567"

Dynamical Selection of the Primordial Density Fluctuation Amplitude

Jean-Luc Lehners, Paul J. Steinhardt
(Submitted on 26 Aug 2010 (v1), last revised 18 Feb 2011 (this version, v2))
"In inflationary models, the predicted amplitude of primordial density perturbations Q is much larger than the observed value (~10^{-5}) for natural choices of parameters. To explain the requisite exponential fine-tuning, anthropic selection is often invoked, especially in cases where microphysics is expected to produce a complex energy landscape. By contrast, we find examples of ekpyrotic models based on heterotic M-theory for which dynamical selection naturally favors the observed value of Q."

http://arxiv.org/abs/1007.2875"

Unstable growth of curvature perturbation in non-singular bouncing cosmologies

BingKan Xue, Paul J. Steinhardt
(Submitted on 16 Jul 2010)
"We consider non-singular bouncing cosmologies, such as the new ekpyrotic model, in which the universe undergoes a slow contraction phase with equation of state $w \gg 1$, followed by a bounce that occurs at a finite scale factor when quantum gravity corrections are still negligible. Such a non-singular bounce requires a violation of the null energy condition in which $w$ falls below -1 at some time before the bounce. In this paper, we show that a component of the adiabatic curvature perturbations, though decaying and negligible during the ekpyrotic phase, is exponentially amplified just before $w$ approaches -1, enough to spoil the scale-invariant perturbation spectrum. We discuss how the problem may be avoided, for example, in singular bounces."

 

[marcus]

Dear Marcus,
Your claim that Steinhardt's interest in the Ekpyrotic scenario has dwindled is simply not true...
http://arxiv.org/abs/1007.2875"

Unstable growth of curvature perturbation in non-singular bouncing cosmologies

BingKan Xue, Paul J. Steinhardt
(Submitted on 16 Jul 2010)
"We consider non-singular bouncing cosmologies, such as the new ekpyrotic model, in which the universe undergoes a slow contraction phase with equation of state $w \gg 1$, followed by a bounce that occurs at a finite scale factor when quantum gravity corrections are still negligible. Such a non-singular bounce requires a violation of the null energy condition in which $w$ falls below -1 at some time before the bounce. In this paper, we show that a component of the adiabatic curvature perturbations, though decaying and negligible during the ekpyrotic phase, is exponentially amplified just before $w$ approaches -1, enough to spoil the scale-invariant perturbation spectrum. We discuss how the problem may be avoided, for example, in singular bounces."

There is the "old" ekpyrotic scenario involving braneclash. It depended heavily on extra dimensions (XD). We live in a brane in some higher dim space. Our brane clashes against another etc etc.

There is a "new" ekpyrotic scenario that runs in plain 4D without XD. By introducing special fields and potentials you get a prior contracting phase of the U, just like in Loop Cosmology, and a "bounce" just like in Loop Cosmology.

The nice thing is you don't need Quantum Gravity. The whole analysis can be done with classic GR! (with the special machinery added).

Now the thing to do is look at how this affects the "primordial density fluctuations" that we can observe in the CMB (the microwave background).

I am happy that Steinhardt's interest has turned to purely 4D modeling without needing XD.

I am happy to see that he is still looking for alternatives to inflation. (acceptance of inflation rests on the absence of alternatives to explain certain features of the early U, so to put inflation on trial as good scientists should, they ought to attack it by developing alternatives.)

Thanks for calling the Xue Steinhardt paper to my attention.

 

[marcus]

Earlier I mentioned that interest in bumping brane cosmology has dwindled. I think that is right. Some time back I checked the citations for papers about braneclash cosmology.
Other people do not seem to have taken up the idea. There are fewer papers now and they are less cited. Anyway that was my impression.

But the word "ekpyrotic" has broadened its meaning to include scenarios not unlike what happens in Loop Cosmology, where you have the universe in a contracting phase, and then undergoing a bounce, which initiates the expansion we observe.

Acording to Steinhardt-Xue the bounce can be singular or non-singular. I think by nonsingular they mean that it can be modeled throughout by ordinary classic GR. The contraction turns around long before extreme densities are reached.

In this paper Steinhardt-Xue critically examine one of the "new ekpyrotic" ideas. It appears not to required extra dimensions, but they find it wanting in at least one respect:

==quote Steinhardt-Xue page 1==
The second possibility is a “non-singular bounce”, such as in the “new ekpyrotic model” [5], where the universe stops contraction and reverses to expansion at a finite value of a(t) where classical general relativity is still valid. A significant advantage of this scenario is that the entire cosmological history can be described by 4d effective field theory and classical general relativity, without invoking extra dimensions or quantum gravity effects. However, for the bounce to happen within classical general relativity, w must fall below −1 for a sustained period, i.e. a violation of the null energy condition (NEC). The NEC violating energy component is commonly chosen to be a scalar field that undergoes ghost condensation [6, 7].
In this paper, we show that the non-singular bounce creates a serious problem for cosmological perturbations. Observations of the cosmic microwave background (CMB) and large-scale structure have provided evidence for a scale-invariant perturbation spectrum. However, in the ekpyrotic model when the scale-invariant curvature perturbation is generated during or just after the ekpyrotic phase, a potentially dangerous component of...
==endquote==

 

[marcus]

There seems to have been a broad shift of interest away from cosmological models involving extra dimensions (XD). There are a lot of different bits of evidence of this and an interesting bit that just surfaced is the change in research interests of one person, Paul Steinhardt, and the redefinition of the word "ekpyrotic".

In 2001 ekpyrosis involved XD, we lived on a brane that had clashed against another brane.
You can see that simply from the title of this 2001 paper:
http://arxiv.org/abs/hep-th/0103239
The Ekpyrotic Universe: Colliding Branes and the Origin of the Hot Big Bang
Justin Khoury, Burt A. Ovrut, Paul J. Steinhardt, Neil Turok...

In 2009 there was a "new" ekpyrosis that works in 4D and doesn't involve extra dimensions. It resembles Loop Cosmology where there is a prior contracting phase of the universe which turns around---the reversal of contraction into expansion is called a "bounce".

In this new 4D ekpyrosis Steinhardt's mechanism is a "dark energy" field with a changing equation of state (EOS) w. This w is the ratio of pressure to energy density. Basically if you can vary w, the dark energy EOS, you can get a lot of stuff you want to happen---expand contract accelerate decelerate...

This wasn't the first paper along these lines but it serves to represent the shift in interest:
http://arxiv.org/abs/0910.2230
Adiabatic Ekpyrosis: Scale-Invariant Curvature Perturbations from a Single Scalar Field in a Contracting Universe
Justin Khoury, Paul J. Steinhardt
5 pages
(Submitted on 12 Oct 2009)
"The universe can be made flat and smooth by undergoing a phase of ultra-slow (ekpyrotic) contraction, a condition achievable with a single, canonical scalar field and conventional general relativity. It has been argued, though, that generating scale-invariant density perturbations, requires at least two scalar fields and a two-step process that first produces entropy fluctuations and then converts them to curvature perturbations. In this paper, we identify a loophole in the argument and introduce an ekpyrotic model based on a single, canonical scalar field that generates nearly scale-invariant curvature fluctuations through a purely 'adiabatic mechanism' in which the background evolution is a dynamical attractor. The resulting spectrum can be slightly red with distinctive non-gaussian fluctuations."

 

[Dmitry67]

There is the "old" ekpyrotic scenario involving braneclash. It depended heavily on extra dimensions (XD). We live in a brane in some higher dim space. Our brane clashes against another etc etc.

I was always wondering what is a 'collision' of the brains. Collision is something that happens in TIME. But time is confined within the brane, isn't it?

And it is even not garanteed that both branes are +++-
And even if they are, why their times point in the same direction?

 

[smoit]

Dear Marcus,
FYI, here is the reference to the "new" ekpyrotic scenario, note that Steinhardt is not one of the authors:
http://http://arxiv.org/abs/hep-th/0702154"

New Ekpyrotic Cosmology
Authors: Evgeny I. Buchbinder, Justin Khoury, Burt A. Ovrut
(Submitted on 20 Feb 2007 (v1), last revised 22 Nov 2007 (this version, v4))

Abstract: "In this paper, we present a new scenario of the early Universe that contains a pre big bang Ekpyrotic phase. By combining this with a ghost condensate, the theory explicitly violates the null energy condition without developing any ghost-like instabilities. Thus the contracting universe goes through a non-singular bounce and evolves smoothly into the expanding post big bang phase. The curvature perturbation acquires a scale-invariant spectrum well before the bounce in this scenario. It is sourced by the scale-invariant entropy perturbation engendered by two ekpyrotic scalar fields, a mechanism recently proposed by Lehners et al. Since the background geometry is non-singular at all times, the curvature perturbation remains nearly constant on super horizon scales. It emerges from the bounce unscathed and imprints a scale-invariant spectrum of density fluctuations in the matter-radiation fluid at the onset of the hot big bang phase. The ekpyrotic potential can be chosen so that the spectrum has a ``red'' tilt, in accordance with the recent data from WMAP. As in the original Ekpyrotic scenario, the model predicts a negligible gravity wave signal on all observable scales. As such ``New Ekpyrotic Cosmology" provides a consistent and distinguishable alternative to inflation to account for the origin of the seeds of large scale structure."



Let me give you a quote from that paper:
"While the approach presented here is closely related to and borrows key elements of the original
ekpyrotic scenario [1], our new scenario resolves all of its short-comings, such as generating
a bounce. It also resolves the important issues of singularity-avoidance and the fate
of the perturbations that occurs in [2, 3, 4]. For these reasons, we call it “New Ekpyrotic
Cosmology”.
As it stands the new ekpyrotic scenario is precisely that, a scenario. In principle, it might
be implemented in various ways in different fundamental theories of particle physics. That
being said, this scenario maintains the motivations of the original ekpyrotic model [1], namely,
as the cosmology associated with the singularity free collision of a bulk five-brane [37, 38]
in heterotic M-theory with the observable boundary wall. Indeed, we have computed the
potential in such a theory and find that it can satisfy many of the constraints required
in the new ekpyrotic model. This potential will be presented elsewhere [39]. A further
motivation is that a realistic matter spectrum can appear on the observable wall of such
theories, see, e.g., [40]. Hence, our scenario can potentially occur in a realistic context."

 

[marcus]

Dima, this shift in interest is really interesting to watch. I agree that braneclash seems a bit far-fetched, hard to swallow (just my personal subjective take) but instead of discussing my own reaction to it I want to track its citation history. (He does keep going back to it, it just gets progressively de-emphasized. You can invoke braneclash to explain variation in the dark energy EOS, or not.) Let's pick the most cited Steinhardt braneclash paper for each year starting in 2001.
I will show two papers from 2001 to illustrate the very considerable interest in colliding branes at that time.

2001: The Ekpyrotic universe: Colliding branes and the origin of the hot big bang. hep-th/0103239
643 cites!

2001:Cosmic evolution in a cyclic universe. hep-th/0111098
368 cites

2002: A cyclic model of the universe. Science 296:1436-1439,2002.
173 cites

2003: Cosmological Perturbations in a Big Crunch/Big Bang Space-time. hep-th/0306109
99 cites

2004: M theory model of a big crunch / big bang transition. hep-th/0408083
54 cites

2005: Solution of a braneworld big crunch / big bang cosmology. hep-th/0512123
27 cites

2006: Why the cosmological constant is small and positive. astro-ph/0605173
39 cites

2007:Generating ekpyrotic curvature perturbations before the big bang. hep-th/070215
66 cites

2008: Intuitive understanding of non-gaussianity in ekpyrotic and cyclic models. 0804.1293
38 cites

2009: (essay for general audience) The Return of the Phoenix Universe. 0910.0834
12 cites

2010: (paper observationally constraining XD models) Exploring extra dimensions through observational tests of dark energy and varying Newton's constant. 1003.2815
2 cites

I guess to sum up, braneclash was originally a motif/inspiration but now you can have a Steinhardt paper about new ekpyrotic that does not even mention extra dimensions, much less colliding branes. For instance this one: http://arXiv.org/abs/1007.2875
Braneclash has become something of an optional motivation which may or may not be invoked.

It is clear Steinhardt is less interested in it now---not even bothering to mention the old brane scenario in several recent ekpyrotic papers.

But more important, I think, is the loss of interest by the research community as a whole, reflected in the above numbers.

 

[Lievo]

I actually don't know the keywords to get string papers

Well.. the problem with the searchs that are based on key words is that you already need some expertise to know which one to choose. If even you is not sure what keywords to use, there is no point me doing it, plus it becomes doubtfull that this engine may help to figure out this kind of question.

There seems to have been a broad shift of interest away from cosmological models involving extra dimensions (XD).

Sorry marcus, but I don't get how could you be confident in such claim if the engine you rely on is not suitable for a proper look at it.

Here's a result that would support the claim that there was a shift of interest away from cosmological models involving brane collision

cosmology "brane collision" / cosmology
99-00 0.01%
01-02 0.15%
03-04 0.20%
05-06 0.19%
07-08 0.15%
09-10 0.11%

Here's a result that does not support the claim that there was a shift of interest away from cosmological models involving extra dimension

cosmology "extra dimension" / cosmology
99-00 1.8%
01-02 2.5%
03-04 3.1%
05-06 3.4%
07-08 4.1%
09-10 5.3%

As you can see, one should conclude that there is a continuous and increasing interest in cosmological models involving extras dimensions, which goes contrary to your intuition.

instead of discussing my own reaction to it I want to track its citation history.

Again sorry, but this (lack of) methodology is clearly inappropriate. This is exactly how one can fool himself -believe a human scientist on this topic ;-)

 

[marcus]

...
Here's a result that does not support the claim that there was a shift of interest away from cosmological models involving extra dimension

cosmology "extra dimension" / cosmology
99-00 1.8%
01-02 2.5%
03-04 3.1%
05-06 3.4%
07-08 4.1%
09-10 5.3%

As you can see, one should conclude that there is a continuous and increasing interest in cosmological models involving extras dimensions, which goes contrary to your intuition.


Again sorry, but this (lack of) methodology is clearly inappropriate. This is exactly how one can fool himself -believe a human scientist on this topic ;-)

Those are interesting numbers, Lievo. They do in fact go against my intiuition which is about trends post 2005.

I gather you are counting papers which mention the word "cosmology" in the main text, and then among those you count those which mention "extra dimensions" in the main text.

My experience has been primarily with actual cosmology papers. I watch gr-qc (general relativity and quantum cosmology) on the arxiv. And also astro-ph (where you get cosmology papers which are not QC).

There may be a vast body of papers that are not actual cosmology research, by that standard, but which mention the word "cosmology". It is a hot topic because of new instruments (especially during the past 10-15 years) and new results (dark matter, dark energy, gammaray bursts, neutrino astronomy etc.).

Your googlesearch numbers may be influenced by increasing "buzz" about cosmology in papers which are not strictly speaking in that field. However I have no idea of how to get a quantitative grip on that effect. Maybe you will think of a way.

 

[marcus]

Lievo, you raised the issue of methodology: how can one easily gauge the interest of the research community, and get signals about possible changes of interest? You have shown me the google method of counting scholarly works where certain words appear in the text. However I still value assessment based on citecounts. So I will make a comparison.

We have two important figures with two different "bounce" cosmologies: Steinhardt and Ashtekar. Both of their models accomplish much the same thing---explaining features of the early universe now being observed. Both replace the former singularity with a bounce. Steinhardt no longer stresses the connection to branes and extra dimensions very much when writing about his ekpyrotic bounce--he bases the scenario on a "dark energy" scalar field with varying equation of state.
Ashtekar's bounce is based on quantum gravity essentially a quantized general relativity in which quantum corrections at very high densities cause gravity to be repellent.

Let's pick the most cited bounce cosmology paper by each author, in each year. I already did this for Steinhardt.

2006: Why the cosmological constant is small and positive. astro-ph/0605173
39 cites

2007:Generating ekpyrotic curvature perturbations before the big bang. hep-th/070215
66 cites

2008: Intuitive understanding of non-gaussianity in ekpyrotic and cyclic models. 0804.1293
38 cites

2009: (essay for general audience) The Return of the Phoenix Universe. 0910.0834
12 cites

2010: (paper observationally constraining XD models) Exploring extra dimensions through observational tests of dark energy and varying Newton's constant. 1003.2815
2 cites

I guess to sum up, braneclash was originally a motif/inspiration but now you can have a Steinhardt paper about new ekpyrotic that does not even mention extra dimensions, much less colliding branes. For instance this one: http://arXiv.org/abs/1007.2875 As I recall it also got 2 cites.
Braneclash has become something of an optional motivation which may or may not be invoked...

The corresponding citecounts for Ashtekar are 231, 81, 54, 32, 24.

Citecount of most cited paper in given year        2006   2007  2008  2009  2010
Steinhardt with a brane-inspired bounce              39    66   38    12    2
Ashtekar with a quantum geometry bounce             231    81   54    32    24

At this point I won't try to draw conclusions. Of course you expect some downward slope because the older papers have had a longer time to accumulate cites

 

[fzero]

Let's pick the most cited bounce cosmology paper by each author, in each year. I already did this for Steinhardt.


The corresponding citecounts for Ashtekar are 231, 81, 54, 32, 24.

Citecount of most cited paper in given year        2006   2007  2008  2009  2010
Steinhardt with a brane-inspired bounce              39    66   38    12    2
Ashtekar with a quantum geometry bounce             231    81   54    32    24

At this point I won't try to draw conclusions. Of course you expect some downward slope because the older papers have had a longer time to accumulate cites

This is quite misleading. The original ekpyrotic universe paper (Khoury et al) has had somewhere around 236 citations since the original Ashtekar PRL appeared (http://inspirebeta.net/search?ln=en&as=1&p1=refersto:recid:554783&jrec=226). In the last 12 months, the Khoury et al paper has had 39 citations vs 25 citations for the Ashtekar et al paper.

I wouldn't attempt to draw conclusions from so little information, but the point is that making sense of citation counts is a bit more involved than looking at total citations.

 

[marcus]

This is quite misleading. The original ekpyrotic universe paper (Khoury et al) has had somewhere around 236 citations since the original Ashtekar PRL appeared (http://inspirebeta.net/search?ln=en&as=1&p1=refersto:recid:554783&jrec=226). In the last 12 months, the Khoury et al paper has had 39 citations vs 25 citations for the Ashtekar et al paper.

I wouldn't attempt to draw conclusions from so little information, but the point is that making sense of citation counts is a bit more involved than looking at total citations.

Thanks for the pointer to the InSpire beta version. It will be quite useful, I can see.

Certainly the 2001 Khoury et al paper you mention is still accumulating cites at a great rate. It has 643 total so far.
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+EPRINT+HEP-TH/0103239
We have already posted the link and abstract to that paper several times here I think. It was the first to present the braneclash idea and of course one cites it for historical reasons when one has occasion to mention braneclash or related scenarios.

In it's more modest way the 2006 Ashtekar paper is analogous. It revolutionized Loop Cosmology by formulating the dynamics differently, and restarted it on a new footing.
Since 2006 it has only accumulated some 200 cites, but then of course the Loop community is much smaller, fewer researchers by at least an order of magnitude, easily.

As you can well imagine, when I look at citations as a way helping out my own judgment of what's going on in a field, what interests me is not citations to OLD papers but citations to more current ones. And other trends recent research publication.

 

[Lievo]

So I will make a comparison.

Sorry but again this does not make sense. Suppose (or agree that) there is a difference in the number of 'important figures' in the two fields you're comparing (whatever which one includes more important figures), then the distribution of the citation will differ, and you will never be able to sort out whether the difference you see is due to this bias or a true result.

Thanks for the pointer to the InSpire beta version. It will be quite useful, I can see.

You bet!

cosmology

10 9361
09 10183
08 9259
07 9175
06 7073

extra dimension + cosmology

10 2193 23%
09 2357 23%
08 2329 25%
07 2285 25%
06 1737 24%

brane collision + cosmology

10 240 2.5%
09 271 2.7%
08 254 2.7%
07 244 2.6%
06 179 2.5%

loop quantum gravity + cosmology

10 1168 12%
09 1247 12%
08 1207 13%
07 1102 12%
06 802 11%

string theory + cosmology

10 2426 26%
09 2712 27%
08 2704 29%
07 2570 28%
06 2050 29%

So Scholar and Inspires agree for string theory, QLG is two times more mentioned using inspires, extra dimension and brane collision 10 times more. No idea where the difference comes from nor if the dynamic differs too (we can't look at older dates using inSpires).

However, it seems clear there is no major change in the 5 last years, isn't it?

 

[smoit]

I guess to sum up, braneclash was originally a motif/inspiration but now you can have a Steinhardt paper about new ekpyrotic that does not even mention extra dimensions, much less colliding branes. For instance this one: http://arXiv.org/abs/1007.2875
Braneclash has become something of an optional motivation which may or may not be invoked.

It is clear Steinhardt is less interested in it now---not even bothering to mention the old brane scenario in several recent ekpyrotic papers.

Dear Marcus,
Here is a very recent paper of Steinhardt, published in Phys.Rev.Lett.106:081301,2011 , where the heterotic M-theory colliding branes solution was explicitly used and not just mentioned as an optional motivation! I had listed this paper in my earlier post but you conveniently ignored it while trying to convince everyone that Steinhardt's interest in string-inspired models has dwindled, simply because he does not mention heterotic M-theory in every single paper . You need to come up with a stronger argument than that!

http://arxiv.org/abs/1007.2875"

Dynamical Selection of the Primordial Density Fluctuation Amplitude

Jean-Luc Lehners, Paul J. Steinhardt
(Submitted on 26 Aug 2010 (v1), last revised 18 Feb 2011 (this version, v2))
In inflationary models, the predicted amplitude of primordial density perturbations Q is much larger than the observed value (~10^{-5}) for natural choices of parameters. To explain the requisite exponential fine-tuning, anthropic selection is often invoked, especially in cases where microphysics is expected to produce a complex energy landscape. By contrast, we find examples of ekpyrotic models based on heterotic M-theory for which dynamical selection naturally favors the observed value of Q.

 

[marcus]

InSpire is fun! and likely to be quite useful. I just did a simple search of papers that appeared since 2009 in quantum cosmology. I asked it to order by citecount. It gave me a list of 320 papers. Here are the first 10 (the ten most cited QC papers that appeared 2009 or later):

==quote Inspire quantum cosmo top ten 2009-present==
1.
(198)
Cosmology of the Lifgarbagez universe.
Gianluca Calcagni (Penn State U.). IGC-09-4-2. Apr 2009. 21 pp.
Published in JHEP 0909 (2009) 112
e-Print: arXiv:0904.0829 [hep-th]
References | BibTeX | LaTeX(US) | LaTeX(EU) | Harvmac | EndNote
Abstract and Postscript and PDF from arXiv.org
Journal Server
Detailed record - Similar records - Cited by 198 records

2.
(54)
Pathological behaviour of the scalar graviton in Horava-Lifgarbagez gravity.
Kazuya Koyama (Portsmouth U., ICG), Frederico Arroja (Kyoto U., Yukawa Inst., Kyoto). Oct 2009. 7 pp.
Published in JHEP 1003 (2010) 061
e-Print: arXiv:0910.1998 [hep-th]
References | BibTeX | LaTeX(US) | LaTeX(EU) | Harvmac | EndNote
Abstract and Postscript and PDF from arXiv.org
Journal Server
Detailed record - Similar records - Cited by 54 records

3.
(32)
Loop quantum cosmology of Bianchi I models.
Abhay Ashtekar, Edward Wilson-Ewing (Penn State U.). Mar 2009. 33 pp.
Published in Phys.Rev. D79 (2009) 083535
e-Print: arXiv:0903.3397 [gr-qc]
References | BibTeX | LaTeX(US) | LaTeX(EU) | Harvmac | EndNote
Abstract and Postscript and PDF from arXiv.org
Journal Server
Detailed record - Similar records - Cited by 32 records

4.
(28)
On Inflation with Non-minimal Coupling.
Mark P. Hertzberg (MIT & KIPAC, Menlo Park & Stanford U., ITP). Feb 2010. 13 pp.
Published in JHEP 1011 (2010) 023
e-Print: arXiv:1002.2995 [hep-ph]
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Abstract and Postscript and PDF from arXiv.org
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Detailed record - Similar records - Cited by 28 records

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

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

7.
(24)
Loop Quantum Cosmology and Spin Foams.
Abhay Ashtekar, Miguel Campiglia, Adam Henderson (Penn State U.). IGC-09-9-1. Sep 2009. 11 pp.
Published in Phys.Lett. B681 (2009) 347-352
e-Print: arXiv:0909.4221 [gr-qc]
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8.
(23)
Entropic cosmology: a unified model of inflation and late-time acceleration.
Yi-Fu Cai, Jie Liu, Hong Li (Beijing, Inst. High Energy Phys. & TPCSF, Beijing). Mar 2010. 9 pp.
Published in Phys.Lett. B690 (2010) 213-219
e-Print: arXiv:1003.4526 [astro-ph.CO]
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9.
(22)
Towards Spinfoam Cosmology.
Eugenio Bianchi, Carlo Rovelli (Marseille, CPT), Francesca Vidotto (Marseille, CPT & INFN, Rome & Pavia U. & INFN, Pavia). Mar 2010. 8 pp.
Published in Phys.Rev. D82 (2010) 084035
e-Print: arXiv:1003.3483 [gr-qc]
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Detailed record - Similar records - Cited by 22 records

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