Strings 2014

Long time since my last post. There is no particular reason, just a mix of circunstances. I write now to advise, if someone is still not aware, about the annual convention about string theory. The slides, and vídeos, are available online at talks online Based on previous experience with similar cases I warm potential watchers about the fact that the videos usually are available online for a limited amount of timpe, typically one or two weeks, so harry up!

A prety good year ending in Arxiv

After a somewhat disappointing year in theorethical physics where the greater topic seems to have been the black holes firewall discussion (seemingly settled in a long - around 90 pages- paper) the end year in a promising way. One one hand we have the second paper of the famous Amplituhedron construction (I told about it in my spanish blog when it appeared) that was announced by Nima Arkani-Hamed, but of which we only had seen a somewhat introductory paper. This is the second one: Into the Amplituhedron. I have not read it, neither I did it with the first. I still am reading, from time to time, a previous long paper about grassmanians and all that. I hope I'll give a reading to the first, and to the one of today, soon, but I guess that Lubos will write a fair better post about it that anything that I could do. I must acknowledge to Lubos the pointing to another paper. Gravitation from Entanglement in Holographic CFTs. I had not paid attention to it because almost like a question of principles I don't pay attention to papers with the fancy names "holographic" or "entanglement" in the topic. But this time it looks pretty important because it looks like is they have made a great advance in a constructive derivation of general relativity from some thermodynamics consideration, improving the previous work by Ted Jacobson. But better read the entry in Lubos Blog Einstein's equations from first law of thermodynamics in AdS. A very different type of article is Stars in M theory (made up of intersecting branes). The title is surprising and the subject certifies that we are in front of a very exotic paper. We study stars in M theory. First, we obtain the analog of Oppenheimer -- Volkoff equations in a suitably general set up. We obtain analytically the asymptotic solutions to these equations when the equations of state are linear. We study perturbations around such solutions in several examples and, following a standard method, use their behaviour to determine whether an instability is present or not. In this way, we obtain a generalisation of the corresponding results of Chavanis. We also find that stars in M theory have instabilities. Therefore, if sufficiently massive, such stars will collapse. We discuss the significance of these (in)stabilities within the context of Mathur's fuzz ball proposal. The last paper I consider is this: Inflationary paradigm after Planck 2013. The first author is the very creator of the idea of Inflation, Alan guth, and at first sight it would seem a review of resoluts. In fact is somewhat of a reply to another paper, but still it is a good way to get an idea of how the Planck results have affected our view on Inflation. One last words for my previous entry. After a discussion of it in an spanish forum I was pointed to an old paper by Tholman - http://authors.library.caltech.edu/2596/1/TOLpr30d.pdf- and, seemengly, it could be considered in the kind of ideas discussed there. My example could be translated to emission of photons between bodies orbitating a central one. The gravitational blue/red shift would play the role of the expanding/contracting universe red/blue shift and that would accommodate into a stationary situation. If the difference of blue/red shift energy is greater than the temperature difference (boltzman factors mediated) then an inverse Clausius behaviour is possible. The key point is that the Clausius Law is not equivalent to the formulation of increase of entropy in a general relativistic setup and this last kind of law still holds, at least in static (probably stationary) gravitational fields, but I am not sure about how the same would still hold in a general spacetime.

Spacetime kills the second law of thermodynamics, real or paradox?

  This summer, among many other things, have read the famous book of Susskind and Lindesay about black hole information paradox. Casually just after finishing ir's reading I have to teach in private tuition (hope I am saying it right ) statistical mechanics (a conventional introduction with the main average topics). In doing that I have revised the foundations of it from the viewpoint of what I had read in the Susskind, and also some other aspects that one must face when triying to apply  statistical mechanic reasoning to non physical problems (for example ecology (I collaborated with a guy working in mathematical ecology for a while).

  There are a few things that I am thinking about, but for the present entry I will concentrate in an easy mental experiment that, at least apparently, violates the second law of thermodynamics. The key of the violation is the lack of a proper definition of energy in general relativity, but for the present case I don't even need to go into mathematical details about it. My idea was clear, take a situation where that problem in the energy definition rises a paradox that violates the second law. I tried a few strategies that possible also work, but in the end I found a really easy one that I think is simple and representative.

 I have found that the easier way of attack is to use the Clausius enunciate: No physical process can transfer heat from a cold body to a warm one. The way to circunvate the law is as follows: Take two  "black bodies", for example the canonical ones consisting of a cavity with photons in equilibrium with the walls that are at different temperatures. Now in the warmer one open the also canonical small hole that allow a photon, or a few ones, to scape. This is made in an expanding universe, the photons that have exit form the body travel in that space time loosing energy. Them they arrive at the coldest body. The photons of the warmer body initially had more energy that the ones in the coldest body, but now, after travelling in the expanding space time, arrive to the second with less energy that the photons in the cold body. That means that the cold body becomes coldest after it gets in equilibrium with that photons. Now we make the reverse procedure, we send some photons from the cold (now coldest) body to the warmest one. But, as this is an imaginary experiment, choose to do so when the universe is contracting (for example we make the experiment in the edge of the time when a FRW goes for the expanding to the contracting phase). In the travel on this contracting universe the photons gain energy and when they arrive to the warmest body they can be, if we wait enough, be more energetic than the ones in equilibrium with the warm body and so they actually  drive it hotter. As far as we can make the experiment as far as we want from anything else we are in a closed system and in that closed system we have effectively transferred heat from a cold body to a hot one, breaking the second law.

 Of course we have not counted the entropy of spacetime, but how could we do so? In the Hawking laws of black holes we learned that classical gravity worked as entropy, the area of a black hole playing the role of entropy. And  it is well known how the Hawking radiation, a semiclassical effect (so taking quantum mechanics into play) gave a further argument. String theory (and ulterior works using  only geometry and CFT, the kerr/CFT correspondence) gave microscopic support to that correspondence among gravity and thermodinamics. And also are were known the, probably wrong, ideas about gravity as entropy, that have gained a rebirth with the paper of Verlinde about "gravity as en entropic force". But in this mind experiment I don't use nothing special in GR, something like an horizon, or quantum effects. Neither is any claim about saying that gravity is entropy. The whole point is that, if there is no mistake, if you don't know how to count the entropy of the spacetime, in this nonstationary case, you can violate the second law of thermodynamics, and that looks very unfunny, isn't it? ;)

 The most similar situation that I know is the famous case of Hawkings telling once (and later felling shame about the idea) that in a contracting universe entropy would go in the opposite direction, and the worries of Sean Carrol and others about the thermodynamic arrow. But as far as I know none made such an explicit case as the one I am presenting here.

 Just to avoid some trivial criticisms I clarify that in GR the energy of a body (or a system of bodies) is the time component of a cuadrivector, so it is not an invariant. As E=Q-W (first law) and \[ \Delta S= \Delta Q /T \] entropy also should be some time component of some cuadrivector and that makes it's precise definition somewhat tricky, but as far as I see this experiment could be suited in a single reference system so we don't need to care about that things.

 Well, this is the idea, and probably I am making some very trivial mistake, or this simply has already been considered and discarded, but as I don't know for sure than that is the case I present the idea here so anyone can blame me if necessary ;).

Conference about the inmeditate future of particle physics afther the Higgs discovering

As you can have read in the Peter Woit blog this week there was a meeting in Madrid, in the institute of theorethical physics, auspiced by the CSIC (consejo superior de investigaciones científicas, close to the UAM , autonomous university of Madrid) with the title " Why mH= 126 GeV? .

 The conferences have been recorded in video and are available online (including the slides). This morning is having place the last set of talks. IT begins at 10 00 so if you live somewhere in the UE maybe you still can get a plane and arrive to the dinner after the last talk, or, if you live in Madrid you can get the next train or bus to Cantoblanco and probably you could arrive to most of the talks.

 I find the topics pretty interesting from a phenomenology viewpoint because some clues on where to be reasonable to watch for new physics. I'll try to post some review of the most interesting ideas exposed if I find time for it.

Proposal for an Ig Milner prize (or trantor prize)

 Ok. Now that we have the Milner  prize I think that it is time to go a little bit further. The nobel prize has it's counterpart, the Ig nobel, so it would be fun to have an  Ig Milner. or Trantor prize`. remember Trantor is the planet that is the centre of the galactic empire in the Asivmovs book's about the foundation.  In that literary universe physics has discovered almost everything and the physicists mainly get prestige by doing formal or ideological revisions of the already established physics which have not consequence in making new discoverings (they even don't care about it).

 Well, the idea would be to give a symbolic prize to works that are superficially correct but are totally useless, or even make no sense at all when one looks at it's closely. I think that they would also be works which are specially ambitious and prepotent. To avoid injuring people who is beginning and could get too injured in their professional life by the bad press I guess that it would be better that the candidates to the prize would be people with a well established position.

 Also it could be given a prize -maybe in an slightly separate category- to papers that are correct but specially fun, or exotic.

 For the first edition I would make a few suggestions.

 One candidate would be the AMPS paper about firewalls. Better than trying to explain the details myself  I suggest reading some posts of Lubos on the subject, this being the last one at the date of writing this:

An apologia for firewalls

 An even better candidate, in my humble opinion, would be Lee Smollin for his last book. I would link the Lubos review but I think that would be to unfair because it is well known that "crackpot" is the more polite word that could characterise Lubo's opinion about Smollin.   That's why I give a link to Sabines blog, who also have a closer knowledge of Lee because she was a former collaborative of him: Book review: “Time Reborn” by Lee Smolin.

 Well, my last candidate would be for the exotic side. It is an article where the author - Benjamin K. Tippett - designs a metric that could fit a literary writing by Francis Wayland Thurston. The whole felling of the paper is something like a scientific description of the worlds of chuthlhu. I give here the link and the abstract:

Possible Bubbles of Spacetime Curvature in the South Pacific

In 1928, the late Francis Wayland Thurston published a scandalous manuscript in purport of warning the world of a global conspiracy of occultists. Among the documents he gathered to support his thesis was the personal account of a sailor by the name of Gustaf Johansen, describing an encounter with an extraordinary island. Johansen`s descriptions of his adventures upon the island are fantastic, and are often considered the most enigmatic (and therefore the highlight) of Thurston`s collection of documents. 
We contend that all of the credible phenomena which Johansen described may be explained as being the observable consequences of a localized bubble of spacetime curvature. Many of his most incomprehensible statements (involving the geometry of the architecture, and variability of the location of the horizon) can therefore be said to have a unified underlying cause. 
We propose a simplified example of such a geometry, and show using numerical computation that Johansen`s descriptions were, for the most part, not simply the ravings of a lunatic. Rather, they are the nontechnical observations of an intelligent man who did not understand how to describe what he was seeing. Conversely, it seems to us improbable that Johansen should have unwittingly given such a precise description of the consequences of spacetime curvature, if the details of this story were merely the dregs of some half remembered fever dream. 
We calculate the type of matter which would be required to generate such exotic spacetime curvature. Unfortunately, we determine that the required matter is quite unphysical, and possess a nature which is entirely alien to all of the experiences of human science. Indeed, any civilization with mastery over such matter would be able to construct warp drives, cloaking devices, and other exotic geometries required to conveniently travel through the cosmos.

 Of course everyone is free to make his own suggestions, and, if they don't like the idea, to explain their reasons.

And the third milnor prizes goes to Polyakov (maybe)

I didn't write a single line about the second milner prize so I'll write a brief post about the third.

 As can be read in "not even wrong" and in the blog of matt strassler it seems (but is not confirmed at the time of writing this entry) that the winner of the this third prize is Alexander Polyakov.

 Everybody who has ever read a book on string theory know the name because of the Polyakov's action and the Polyakov path integral that are the very basics of the worldsheet aspect of string theory. Besides string theory Polyakov has made very valuable contributions to QFT such as instantons and magnetics mopopoles.

 All in one it is clear that the prize is absolutely well deserved. In fact my only concern is why he hasn't also a nobel prize. It is absolutely clear that there are people awarded with a nobel who have by far less merits than Polyakov for the prize. Well, at this point I think that among theorists the milner prize should be more valued than the nobel not only for the money but for the prestige of the previous winners.

 By the way, if someone wonders why I write so few posts in the last times there is an easy answer: The LHC is doing a hard work to hide any possible evidence of physics beyond the standard model. The Higgs is of the most boring type possible and no SUSY, no extra dimensions no nothing. Well, this is not the end of the world for theorethical physcis and it is sure that some surprises could be around the corner (but maybe not where people usually expects them xD) but it would have been fun to have some BTSM physics in colliders at this point.

Mathjax, note 10.1 and more

I have become really tired of the problems to have a proper way to display math in blogspot. I had previously used a latex renderer but I have had problems with it. On one hand the server has changed in a few occasions, and in another the blogspot people have deleted the modifications of the template that allows the to be used.

 Because of that I had become somewhat frustrated. After all some entries with maths in this blog doesn't work any more and I don't like the idea of change the template every time that the server of latex changes in the future.  I have been observing that from a months to now the Lubos blog uses mathajax and it looks like it is an stable solution so I have decided to give it a chance.

 This a text equation using the mathajax plugin:

 $$  ds ^2=H^{-2} dt^2 + H^2 dx^2 $$

Ok, to have an easy way to display formulas is fun, but it is only part of the game. You need to write them. That seams a trivial task, write the latex code. Yeah, it is truth, I know the latex code for math and I know how to write a full Latex document also, of course. But, you know, that is "last year technology. I mean, this year has arrived the note 10.1, the tablet version of the note family of tablets. It was presented in February  at the MWC in Barcelona (Spain) and it arrived the stores (and my home the very first day it was on sell) this September. Among the many goodies of that tablet is the math handwriting recognition that you can see working in this video:

That feature works really ok. it recoginzed well ther formula  (the plain text handwriting recognition works also fine, of course) and send the equation to wolphram alpha (if you want so) but it has an annoying aspect, it doesn't allows you to get the latex code that correspond to the formula. This feature has also been added to the phablets and I have used it in the note 5.3 (the original) form around march, long before getting the note 10.1. That means that writing an equation using latex has become a "too primitive" way to do things.

 Well, I have been learning to do android programs. But as far as I have had a bad experience in the recent past with writing for the symbian platform (I ended an app, send it to the nokia store, passed the quality standards, but never seen it published in the store so I have not made any money of the app) I have been somewhat reluctant to get too involved in that business. Still I have learned enough to know the theory of how to do a virtual keyboard that could allow write latex in a really easy way. I mean, for writing an alpha letter you need to type \alpha, and similar for other greek letters. It would be easiest to have an \[ \alpha \] symbol in the keyboard and that when you type it it would appear in the suggestion bar (candidate view) the option to write the latex code (that would be the default one), the mathajax code or the actual character. And similarly for many other math symbols. Maybe in the future I could make this keyboard, but if somewhat reading this blog entry decides to do it before I wouldn't ask him any money for the idea ;).

 Well, a latex keyboard would be fun. But it would be better still that the math recognition of the note would allow the option to show the latex code. I know that there is a SDK for the S-pen of the note, but I don't know if it allows access to the code of the math recognition program (I seriously doubt it, but I still have not watched too much the SDK). If so it would be fine that somewhat would try to write a variant of the S-note program that would include that latex presentation option. Of course if someone of Samsung reads this entry they are invited to include that functionality in an update. I really like the note tablet and I absolutly think that is the best tablet of the moment, specially for people who need to use equations often. If that LaTeX extra functionality is added it would convert a wonderful product into an even better one :)-
 

The arrival of the Milner prices of fundamental physic

This month came by surprise a new international prize for physicists. It is oriented for works of relevance in theoretic development, even if the theories worked hasn't be tested and they can't probably be for a large amount of time. It is created by a Russian physic doctorate Yuri Milner and it is named Fundamental Physics Prize.

It has been widespread announced in the English blogosphere. I also wrote a brief note about it in my Spanish blog and made publicity of it in some Spanish blogs about science. Since them I have not seen too many news about it. I neither have seen any signal of it in the TVs or newspapers of my country. That is too bad for a prize that in it's monetary recompense gives 3 million € to any winner and it has awarded, for it's first edition to NINE people,. That is, it has given a total amount of 27 million € to the physicist community. I think that only because of it it deserves a lot more of attention ;). Another point about the prize is the relevance of the winners. In the most theoretical size we have to Ed Witten, Juan Maldacena, Nima Arkani-Hamed, Nathan Seiberg and Asoke Sen. All of them are big names of string theory related research. Of course the most famous of them is Ed Witten who already had got in the ninties the field medal (the most famous and prestigious award in mathematics) for it's work in topological field theories. Probably next to him in fame is Maldacena and its broadly known correspondence. Nima is more on the phenomenological size with its work in possibly mesoscopic extra dimensions that have generated a lot of work for a few years (including the variant of them by Lissa Randal) and also had raised the possibility of creation of black holes in the LHC. Now this possibilities have something been gone with the null results of all the experimental searches but still there is a minor possibility of discovery when the LHC gets a bump in energy (or maybe when more data is available). In the last times Nima has been working in twistor techniques for amplitudes in certain kinds of maximal supersymmetric theories that, probably, can in the future be extended to more phenomenologically viable theories. Nathan Seiberg is well known by his works together wit Ed Witten. In particular he is well known for its cohomoloy. A teacher of a seminar in algebraic geometry told us that when his theory was announced during a course all the assistants started to use the new calculational tool to reobtian in a easiest and fastest way the cohomological groups of very well known spaces. Asoke Sen, on the other side, has made work in many areasm, but it is best known by it's work in tachyon condensation. I will not say any more and pont the interested reader to a post on it's work by Lubos motl: Asoke sen and tachyon condensation Well, there are winners in other subjects. For example in cosmology we have two very well known people, Andre Linde and Alan Guth. Guth is the father of inflationary theory and Linde the cofather who got the original idea and mutated it into the "eternal inflation" paradigm. Undoubtedly (at least for me) their work is the main contribution to the field of cosmology in the last decades and only the lack of a firm experimental verification of the idea has prevented them fro winning a nobel. There are also two other winner who works in quantum computing -Alexei Kitaev. and in mathematics (related to physic) Maxim Kontsevich whose work inspired Ashoke sen. I don't know about them so I will not say anything else. Overall the prize and the winners (at least the ones I know) are all really top people. Still the prize didn't get as mediatic as it should and I wonder why. Well, for sure there was a way in which the prize could have deserved a lot more attention from the mass media: awarding to Stephen Hawking who is probably the most famous physic alive. So the question is should Hawking have deserved the prize? Well, in my opinion he would. In fact I think that he was the ideal candidate to win it. It's work in emission of radiation for black holes has inspired lot, lot of work (the last line of research the "firewall" that according to some people is in the inner of old black holes). In fact it's work is considered the most firm candidate as the first quantum effect related to gravity. It the radiation would have been detected in an actual black hole (and not only in condensed matter analogues) hawking would have wined for sure the nobel prize. Hawking now is old (and as everybody knows, it¡s health is weak, because of it's illness), and it is probable that he would die without seeing it's idea tested experimentally. Sooo...YES! he would have wined the Milner prize as a recognition to it's work. Ok, there will be more editions of the prize, but unfortunately (let's hope not) the next year could be too late. Definitively not awarding Hawking looks to me both not of justice and a lost opportunity to make the prized best known. Well, even without Hawking the prize is great, and the awarded people are famous and important so let´s hope it will get the deserved media attention. Ok, I have talk about the winners of this year, who will win the next edition?. The decision is among the winners of this year (that is the dynamic of the prize). Maybe in the next 12 months it is made some really bright works that totally rocks, but among the already well known people, who should be potential winners? I see two major candidates (who, of course, could have already have win this year, but ok, it was necessary to choose someone and not everybody could win). I am talking of Cumrum Vafa and Joseph Polchinsky. I invite the readers to propose some more names. After all this kind of games are a part of the way to make a prize famous. Ok, it is too early, but still it could be interesting, when the date of the next edition arrives it could be made another round to see how the candidatures have evolved ;).

Strings 2012

The LHC rules the physics blogosphere, specially since it's recent announcement about the Higgs. None doubt of the importance of the discovery and, in general, the labour of the LHC. But that doesn't meant that nothing else is going on on physics. On the experimental side we have new experimental results on dark matter detection: on the negative by xenon amd on the possitive (but in a very non fable way because of the early state of the experiment) by COUPP-4 On the theoretical side also there are some news. For example those last days we have assisted to a "black hole firewall" revolution of which I'll make a separate entry soon. But, in the theoretical side, I think that it is quite important to notice that this weak have begun the annual congress in string theory, that is, strings 2012. The place where it is celebrated is the Ludwig-Maximilians-Universität (in collaboration with the Max Planck institute). This is the web page Strings 2012

Some slides are available (and I guess that the links with an x that actually don't really have a pdf although they point to one in the future will have it). Some videos are also available. I'll try to see some of the important conferences and to read the slides. For now my next post is planned on the "firewall revolution" but maybe I'll post also another entries about this conference. Update: You can read a brief analisys of some of the conferences in Lubo's blog Strings 2012, a few words

The LHC blues

Ok, I am being deliberately a little bit negative in the title. But the sad truth is that the runs of the LHC at the 7 TeV centre of mass energy have been somewhat bit disappointing in the respect of bringing us confirmed new physic. It has been a long time since my last post in this blog (I have keep posting in my Spanish blog) and in the while there were some interesting claims, extensively reported in the blogosphera such as the famous OPERA maybe super-luminous neturinos and the corresponding maybe rectifications. Also there were some claims about anomalies in LHC b related to CP symmetry violation, recently confirmed by the CDF analysis. A pity that this can be own to bad calculations in QCD and not to new physic. This negative results have also meant that most of phenomenological models for new physics have been driven into a very bad position. The only real almost discovering has been a Higgs at 125 TeV. Now we are near to a restart of the LHC activity to a somewhat increased energy, 8 TeV. And it is expected a lot more luminosity (around 19 fb^-1). So, maybe some new physics could appear, or maybe not. In the purely theoretical side I have not seen any revolutionary paper, and not even a simple "good although not amazing" one. At most some curious ideas that are beautiful but not particularly useful. Ok, again maybe I have been too negative again, but if you compare the amount of exciting new results in physic with the ones in fields such as consume technology (with a mobile/tablet revolution going on) and the revision that society and economy are undergoing right now high energy physics is going very slow. Anyway, I have keep reading most blogs, some of the new papers and rereading with care some not so new ones so I will come to blog again with some more regularity. Of course if the LHC is merciful enough to give as some new physic to deal with I will be happier with the task ;).

Geometric Models of Matter

The last Friday there was a very interesting paper in arxiv. I am really busy those days (and it will be so until around the 15 of September)so I couldn't still read ot completely. Still I think that I must leave notice of it here.

The paper in question is title like the post entry, Geometric Models of Matter. It has three authors: Michael Atiyah, Nicholas S. Manton, Bernd J. Schroers. Among them the best known one is, of course, sir Michaell Atiyhay, a very well known field medallist in mathemathics.

The abstract of the paper reads:

Inspired by soliton models, we propose a description of static particles in terms of Riemannian 4-manifolds with self-dual Weyl tensor. For electrically charged particles, the 4-manifolds are non-compact and asymptotically fibred by circles over physical 3-space. This is akin to the Kaluza-Klein description of electromagnetism, except that we exchange the roles of magnetic and electric fields, and only assume the bundle structure asymptotically, away from the core of the particle in question. We identify the Chern class of the circle bundle at infinity with minus the electric charge and the signature of the 4-manifold with the baryon number. Electrically neutral particles are described by compact 4-manifolds. We illustrate our approach by studying the Taub-NUT manifold as a model for the electron, the Atiyah-Hitchin manifold as a model for the proton, CP^2 with the Fubini-Study metric as a model for the neutron, and S^4 with its standard metric as a model for the neutrino.

Ok, as I said I still didn't read the full article so I can't say many detaills. But the idea seems simple. They are inspired in the Skyrme modell. There there is a group-valued field from :$$mathbb{R}^3$$

$$U:mathbb{R}^3 \rightarrow G$$.

where the lie group is usually SU(2). In that construction specific characteristics of the proton and neutron (baryon number and so on)are associated to topological constructions, that aare, automatically, conserved quantities.

In the paper they generalize the idea in order to construct another particles, for example the electron. They must choose different kinds of manifolds, and maps. Also they use different topological invariants and so on.

But the idea is that they try to describe matter, and it's associated charges, in basic to purely geometric/topologyc constructions. Of course we are talking about different constructions that the one's involved in gauge theories. The proposal of Atiyah and all somewhat replace the need of an ordinary QFT to begin with. IF I have understood right they only have by now an static construction, that is, they don't have a way to give a dynamics to their theory. That means that it remains a lot of work to be done before they get something remotely similar to the actual world.

But, still, it is a beautiful (at least mathematically) idea. For sure Einstein would have loved it. Let's remember that in GR the space-time has a geometric nature while matter has a non-geometric one. In that sense it is an inelegant theory. If this construction works we would have a fully geometric description of the universe. If that works the immediate answer would be: the resulting theory would be equivalent to ordinary QFT in curved (well, maybe we would first ask for flat space-time9 space time for usual situations? would it give some advantage, other than aesthetic? could it be promoted to a quantum gravity?

Whatever the answer to these questions could be I think that it looks like a theory that deserves some further development. Even if it fails like a viable physical theory it could be a source of new ideas for existing ones.

Great day in arxiv

Today there are in arxiv two articles that look really great.

The firs (in the order that arxiv gives to them) is from Samir D. Mathur: Effective information loss outside the horizon.

It argues that there is no loss of information inside a black hole because he information simply doesn't go inside the black hole. The abstract explains it more carefully:

If a system falls through a black hole horizon, then its information is lost to an observer at infinity. But we argue that the {\it accessible} information is lost {\it before} the horizon is crossed. The temperature of the hole limits information carrying signals from a system that has fallen too close to the horizon. Extremal holes have T=0, but there is a minimum energy required to emit a quantum in the short proper time left before the horizon is crossed. If we attempt to bring the system back to infinity for observation, then acceleration radiation destroys the information. All three considerations give a critical distance from the horizon $d\sim \sqrt{r_H\over \Delta E}$, where $r_H$ is the horizon radius and $\Delta E$ is the energy scale characterizing the system. For systems in string theory where we pack information as densely as possible, this acceleration constraint is found to have a geometric interpretation. These estimates suggest that in theories of gravity we should measure information not as a quantity contained inside a given system, but in terms of how much of that information can be reliably accessed by another observer.

The other article is written by Maldacena: Einstein Gravity from Conformal Gravity.

The abstract is:

We show that that four dimensional conformal gravity plus a simple Neumann boundary condition can be used to get the semiclassical (or tree level) wavefunction of the universe of four dimensional asymptotically de-Sitter or Euclidean anti-de Sitter spacetimes. This simple Neumann boundary condition selects the Einstein solution out of the more numerous solutions of conformal gravity. It thus removes the ghosts of conformal gravity from this computation. In the case of a five dimensional pure gravity theory with a positive cosmological constant we show that the late time superhorizon tree level probability measure, $|\Psi [ g ]|^2$, for its four dimensional spatial slices is given by the action of Euclidean four dimensional conformal gravity.">We show that that four dimensional conformal gravity plus a simple Neumann boundary condition can be used to get the semiclassical (or tree level) wavefunction of the universe of four dimensional asymptotically de-Sitter or Euclidean anti-de Sitter spacetimes. This simple Neumann boundary condition selects the Einstein solution out of the more numerous solutions of conformal gravity. It thus removes the ghosts of conformal gravity from this computation.
In the case of a five dimensional pure gravity theory with a positive cosmological constant we show that the late time superhorizon tree level probability measure, $|\Psi [ g ]|^2$, for its four dimensional spatial slices is given by the action of Euclidean four dimensional conformal gravity.

Unfortunately until the next Friday I am going to be very busy and I badly will have time to read them carefully those days so I can't say too much more about them. I suppose that (at least) Lubos will talk about them so I will read its report before I can read them myself. I write this entry partially to recommend the articles to however could be interested and also to keep a link to them so I could later have a quick access to them from wherever I want.

Update: Well, at last I had no patient and read the first article (after all is a brief one, only 7 pages). I have a mixed filling about it. The author computes a few things related to the fall of a body towards an event horizon. Firstly he does for an Schwarschild one.

There he considers two cases. The first in the free fall. In that case the last light (containing the info about the object) is emitted, because of the red-shift at a frequency bellow the Hawking temperature and so it can't be differentiated from this and he concludes that we actually don't have the information about that object.

The second case is when an observer at infinity holds the infalling object until the last time. In that case it is the unrhu radiation associated to the acceleration of an object at rest respect to a gravitational field which is responsible for a dissipation of the information of the object when it finally is released and cross the horizont.

Later he calculates similar things for a Reissner-Nordstöm like black hole and he finds that somewhat different mechanism operate in order to get similar qualitative and quantitative results.

In the last part he does calculations using string theory and the fuzzball paradigm for black holes (where the notion of event horizon is replaced by an stringy construction). Still he finds equivalent results.

Certainly the fact that many different calculations lead to a similar result is appealing. But still I don't see clear the whole subject. I think that at best he would be saying that the lost of information happens before the horizon (or its fuzzball "equivalent") so the problem of lost of unitarity remains (and even we could say that is getting worst because it happens in a region causally connected with the outsider observer). But the whole thing is that one could think that a priory we could think that if the outside of the black hole is clean of other infalling matter (other that the actual object under study) we could argue that if we know the state of the object at infinity we can apply the laws of quantum mechanics to know t's state when it is falling (eve it we can't actually do a measure to be sure that nothing has perturbed our object). That contrast the case of the object that falls behind the horizon when we have no idea of which it's final state would be because we don't know the laws of quantum gravity near the singularity. Well, I am ware that this last objection is somewhat wrong because the key point of the lost of information is the horizon and not the singularity but I have no more time just now to see what point I am missing. I'll realize it for sure later, but I don't promise to write it here soon. But keep calm, for sure Lubos will write about it sooner or later and will clarify the relevant points ;).

Can we see inside black holes?

The last week there was an article that was commented in the arxiv blog: Planets Could Orbit Singularities Inside Black Holes.

The blog entry discuses this article: Is there life inside black holes?.

The article is a pure classical relativity article. It study the possibility of stable orbits for planets inside a black hole, in particular in a Kerr-Newman black hole, that is, a rotting charged black hole. The classical geometry of a Kerr-Newman black hole is described by it's Penrose diagram:



The essential aspect of the K-N black holes for the work of that people is the presence of the inner horizon (a Cauchy horizon). In a non rotating black hole, described by the Schwarschild metric, once he cross the event horizon the radial coordinate changes it sign acquiring a time sign. That means that one must go in the direction of decreasing radius until one finds the central point-like singularity. A common interpretation of that geometry is to say that inside the black hole the space itself is falling toward the centre at the speed of light and it drags averything with it.

In the K-N case things are somewhat richer. In addition to the outer event horizon there is an inner cauchy horizon. When the black holes spins faster and faster (or when the charge of the black hole increases) both horizons get nearest and nearest until, ultimately, they would converge and it would become an extreme black hole.Beyond that one would have a naked singularity but it is thought that such a possibility should be ruled out.

well, as a I said te key point was the cuchy horizon. AS you can read in the linked wikipedia article a Cauchy Horizon is a boundary for the validity of a well posed Cauchy problem in partial differential equations. It can be shown that light (or whatever wave) crossing the horizon gets an infinite blue-shift. That means that it's energy-momentum tensor diverges. The implication of it would be that the back-reaction would destroy the Cauchy horizon once a particle cross it. Still one could get an stable Cauchy horizon if one throws in it exotic matter violating the AWEC (average weak energy condition) well known for people working in wormholes.

The reason why the cauchy horizon is important is because once it is crossed the radial coordinate becomes once again space like. That opens the possibility of the existence of stable orbits inside the black hole. In previous articles, cited by the author, it was shown the existence of that orbits for Reisner-Nordstöm (charged) and Kerr (rotating) black holes. The present article generalizes the results to the general case. In the article considerations are hold about the tidal forces, sizes, radiation rates and they conclude that in a galaxy centre sized black hole a planet could do an stable orbit around the singularity and hold life.

Well, this is the content of the article. As I have explained it is worked in the ansatz of the validity of classical relativity inside a black hole. Also it depends strongly in the stability of the cauchy horizon that can't be got without exotic matter. Note: the author doesn't mention that point about exotic matter although he is aware of the fact that cauchy horizons are not stable. Without exotic matter the whole paper is of a purely academic interest even if one accepts that classical general relativity is accurate to describe black holes inners.

Of course there are a lot of people who don't like classical GR for doing so. In string theory there are many alternative descriptions. On one side one has the correspondence principle of black holes (due to t' hooft and Suskind) that says that an observer falling into a black hole will not be able to notice when he has crossed the even horizon. That means that the physic he sees is must be equal that the physics seen by an outside observer. The reason of the introduction of that principle is the intent of saving unitarity in the presence of Hawking radiation. The actual reasoning is made not classically but for the Hilbert space of a quantum theory as seen but inner and outer observers.

Another string theory inspired viewpoint is described in a classical article by Maldacena: D-brane Approach to Black Hole Quantum Mechanics . In the last part of that article, after calculating the Beckenstein-Hawking entropy, Maldacena Suggest a view where black holes inners and Hawking radiation is described in terms of D-branes. I am not aware if that suggestion has been further developed. I have made a partial search for "hawking radiation in string theory" but I haven't found too much. In fact, beyond that Maldacena article, I found only an approach written some years before using a very aproximative description.

Another paradigm for black hole inners in the string literature would be the fuzzball approach of Mahupart (or maybe Mithur I am not sure at this point and have not time to do a search just now).

Well, that variety of viewpoints, not very compatible among them, for the black hole inner is disappointing. Even in the simpler case of the general relativity viewpoint is disappointing the possibility of the existence of stable structures (maybe planets of an advanced alien civilization, maybe a much more prosaic rings of dust) existing inside the black hole hidden for us from the event horizon.

But, wait! The title of the post wonders about the possibility of seeing inside the black hole. Of course classically it is impossible because of the very meaning of "event horizon". But when quantum mechanics enter the game thing could change. Of course the key would be Hawking radiation. The semiclassical theory says that the radiation must be purely thermodynamic so we can't get any info from it. But if unitarity is conserved the Hawking radiation can't be purely thermodynamic and it must have some structure that stores all the structure of the matter that formed the black hole and that has fallen inside it after it's formation. Possibly it will also have some information about the inner structure of the black hole. Obviously to get that info is very difficult in practice. The usual analogy is to say that one could reconstruct, in principle, the form of a living object from the ashes that are produced when it is burn.

But if we are a little least ambitious maybe we could actually get some partial information. Maybe we could design some easy mental experiment in which throwing into the black hole some specific kind of matter in some specific way we could analyse the Hawking radiation related to it to get some information of the inside of the horizon. That would actually be very cool because it would give an experimental way to distinguish the competing descriptions of the black hole inner.

Of course I actually don't know the details of how this could be done (only a very vague ideas that probably will not work). But maybe something on this purpose is already made and a kind reader would give me the references ;).

String theory in exotic R^4

Today in arxiv there is a very curious article:Quantum D-branes and exotic smooth R^4 written by Torsten Asselmeyer-Maluga, Jerzy Krol.

The article is actually the second part of a previous one: Exotic smooth R^4 and certain configurations of NS and D branes in string theory.

The abstract of the first (in date order) article reads:

In this paper we show that in some important cases 4-dimensional data can be extracted from superstring theory such that a) the data are 4 Euclidean geometries embedded in standard $\mathbb{R}^{4}$, b) these data depend on NS and D brane charges of some string backgrounds, c) it is of potential relevance to 4-dimensional physics, d) the compactification and stabilization techniques are not in use, but rather are replaced. We analyze certain configurations of NS and D-branes in the context of $SU(2)$ WZW model and find the correlations with different exotic smoothings of $\mathbb{R}^{4}$. First, the dynamics of D-branes in $SU(2)$ WZW model at finite $k$, i.e. the charges of the branes, refers to the exoticness of ambient $\mathbb{R}^{4}$. Next, the correspondence between exotic smoothness on 4-space, transversal to the world volume of NS5 branes in IIA type, and the number of these NS5 branes follows. Finally, the translation of 10 dimensional string backgrounds to 4 Euclidean spaces embedded as open subsets in the standard $\mathbb{R}^{4}$ is achieved.

I still haven't had time to read the full article, but it looks quite interesting, and beautifully, specially from the perspective of someone whose favourite area of maths is topology. The idea is to see if someone can make string theory in a background of R^4 with a different differential structure than the usual one. One of the most amazing discoveries of differential topology was that there were different differential structures for R^4 than the usual one. That means that although like a topological manifold R^4 is unique there are different differentiable manifolds that are compatible with it's topological structure. An explicit characterization of that exotic structures in terms of coordinate is difficult and their existence is proved by means of topological techniques. In the paper it is made use of h-cobordism.

Later he begins the string theoretical construction, using SU(2) WZW (wess-zumino-witten= CFT's, NS 5 branes, D branes, etc. AS I still haven't read the article carefully, nor the continuation of it (the today's arxiv article ) wouldn't give more details. Only to say that it looks like a very intriguing area of research.

Also today in arxiv there is a very interesting, but much more conventional article of Miche Dine: Supersymmetry from the Top Down whose abstract is:

If supersymmetry turns out to be a symmetry of nature at low energies, the first order of business to measure the soft breaking parameters. But one will also want to understand the symmetry, and its breaking, more microscopically. Two aspects of this problem constitute the focus of these lectures. First, what sorts of dynamics might account for supersymmetry breaking, and its manifestation at low energies. Second, how might these features fit into string theory (or whatever might be the underlying theory in the ultraviolet). The last few years have seen a much improved understanding of the first set of questions, and at least a possible pathway to address the second.">If supersymmetry turns out to be a symmetry of nature at low energies, the first order of business to measure the soft breaking parameters. But one will also want to understand the symmetry, and its breaking, more microscopically. Two aspects of this problem constitute the focus of these lectures. First, what sorts of dynamics might account for supersymmetry breaking, and its manifestation at low energies. Second, how might these features fit into string theory (or whatever might be the underlying theory in the ultraviolet). The last few years have seen a much improved understanding of the first set of questions, and at least a possible pathway to address the second.

The article is quite pedagogical, and even begins with an ultra fast introduction to supersymmetry. Certainly recommendable.

On the subject of supersymmetry in the LHC era I thing that everybody would must read the last entry of Jester's blog: What LHC tells about SUSY that discussed the paper of the ATLAS collaboration: Search for supersymmetry using final states with one lepton, jets, and missing transverse momentum with the ATLAS detector in sqrt{s} = 7 TeV pp.

Well, certainly the expectations of Lubos of an early discovery of SUSY in the LHC are gone, but still there are good reasons to be patient, as explained by Lubos himself or by the Dine's paper.

By the way, while writing this entry I have seen that Lubos himself has written an entry about the Dines paper, you can read it here. At the moment of writing my entry he hasn't given many details about the article, but possibly he will edit his post and discuss the paper in more detail.

Update: Lubos has read this entry and has written a very intersting essay about the general relevance (or irrelevance) of the pathological mathematical structures in physics .

About the actual series of papers in exotic R^4 he doesn't say too much because he claims that he doesn't understand the paper. I have been studying the subject, including some references, and I am still going on. Much of the mathematics (differential topology: h-cobordism, topological surgery, tubular neighbourhoods) are familiar to me, but some more recent concepts are new to me. Still I think that I can follow the general argumentative line of the mat part. I get somewhat more loose in other points of the WZW model in an SU(2) background, but still I think that I get the general argumentation. As soon as I end reading a few more references I'll try to expose the key ideas.

Anyway, there is a difference here with the case commented but Lubos. R^4 is the only R^n that admits different smooth structures for the same topological structure. In that aspect it is not a case of searching for a pathology but a case where the pathology appears by itself. In fact most people who have heard about that particularity of R^4 have always though that maybe that could be the ultimate reason that we live in a four dimensional manifold. Of course what they lacked is a way to relate that peculiarity of R^4 to any actual physic. This people seem to have advanced somewhat in that direction, but as far as I understood they are far of the objective (if that is their objective, that probably it is not the case).

String theory and nanotechnology meet today in arxiv

String theory deal mainly with physic at the planck scale, although it's goal is to connect to with the electroweak scale.

On the other hand, nonotehcnology, deals with physics at sizes similar to the Bohr radius. There are, consequently, many orders of magnitude of difference among that two branches of physics.

Because of that it is absolutely amazing to see in the title of the paper a reference to a relation among them. But today in arxiv we have such a paper: Fermionic condensate and Casimir densities in the presence of compact dimensions with applications to nanotubes.

The abstract reads like this:

We investigate the fermionic condensate and the vacuum expectation value of the energy-momentum tensor for a massive fermionic field in the geometry of two parallel plate on the background of Minkowski spacetime with an arbitrary number of toroidally compactified spatial dimensions, in the presence of a constant gauge field. Bag boundary conditions are imposed on the plates and periodicity conditions with arbitrary phases are considered along the compact dimensions. The boundary induced parts in the fermionic condensate and the vacuum energy density are negative, with independence of the phases in the periodicity conditions and of the value of the gauge potential. Interaction forces between the plates are thus always attractive. However, in physical situations where the quantum field is confined to the region between the plates, the pure topological part contributes as well, and then the resulting force can be either attractive or repulsive, depending on the specific phases encoded in the periodicity conditions along the compact dimensions, and on the gauge potential, too. Applications of the general formulas to cylindrical carbon nanotubes are considered, within the framework of a Dirac-like theory for the electronic states in graphene. In the absence of a magnetic flux, the energy density for semiconducting nanotubes is always negative. For metallic nanotubes the energy density is positive for long tubes and negative for short ones. The resulting Casimir forces acting on the edges of the nanotube are attractive for short tubes with independence of the tube chirality. The sign of the force for long nanotubes can be controlled by tuning the magnetic flux. This opens the way to the design of efficient actuators driven by the Casimir force at the nanoscale.

I haven't read in deep the paper, but in a superficial reading I have got a confirmation that they are actually claiming that actual aspects of the compactified extra dimensions of string theory could, through the Casimir effect observable consequences in the characteristics of nanotechnologic materials, in particular nanotubes. My guess is that there must be some error, or some trick, somewhere. If not this paper would be driving string theory from the realms of cute edge speculative high energy physics to actual applications in one of the most economicaly profitable industries. Too good to be truth probably, but, who knows? Well, I'll read the article carefully sooner and I'll comment more details. But I doubt that I would be the only one to say something about it ;)-

Update: Ok, the article actually doesn't relate string theory extra dimensions and carbone nanotubes. IT only applies the formalism of compactificactions to nanotubes, based on the premise that a nanotube is a cylinder, i.e. a compactified plane. The introductions, and many other parts of the article are somewhat misleading and seem to suggest what I had explained. Also it is misleading the fact that it appears inhep-th. The reason for that possibly is that they make some development of the formalism of compactifications in a general, multidimensional, framework. Possibly that general development could be usefull for people working in string theory, that possibly justifies the inclussion of the paper in hep-th although the primal subjecto of the paper is condensed matter physic.

Two cosmology papes and the question of extra dimensions

First an advert, the two topics are unrelated, well, or may be not ;).

The first paper I would want to mention is this form Luis Alvarez Gaumé. Gaumé is one of the best known spanish string theorists. He is famous for the 80's paper with Witten about the (absence of) anomalies in string theory.

Now he has a paper in arxiv. A Minimal Inflation Scenario. This is the abstract:

We elaborate on a minimal inflation scenario based entirely on the general properties of supersymmetry breaking in supergravity models. We identify the inflaton as the scalar component of the Goldstino superfield. We write plausible candidates for the effective action describing this chiral superfield. In particular the theory depends (apart from parameters of O(1)) on a single free parameter: the scale of supersymmetry breaking. This can be fixed using the amplitude of CMB cosmological perturbations and we therefore obtain the scale of supersymmetry breaking to be 10^{12-14} GeV. The model also incorporates explicit R-symmetry breaking in order to satisfy the slow roll conditions. In our model the eta-problem is solved without extra fine-tuning. We try to obtain as much information as possible in a model independent way using general symmetry properties of the theory's effective action, this leads to a new proposal on how to exit the inflationary phase and reheat the Universe.

I am far from expert in the inflation technicalities but it looks like a quite natural proposal (well, maybe except for explicit R-parity breaking, which, otherwise, is good for some very specific models of dark matter tht were popular the last year trying to accommodate some now not too much discussed anomalies from PAMELA, ARTIC and others).

The other cosmology paper I wanted to mention is this: Holographic unification of dark matter and dark energy. The abstract is as follows: Using a new version of the holographic principle, a constant term was introduced, which conduces to the description of the standard cosmological LCDM model, and unifies under the same concept the dark matter and dark energy phenomena. The obtained model improves the results of previously considered holographic models based on local quantities. The inclusion of constant term is interpreted as a natural first approximation for the infrared cutoff which is associated with the vacuum energy, and the additional terms guarantee an appropriate evolutionary scenario that fits the astrophysical observations. The model allows to reproduce the standard LCDM model without explicitly introducing matter content, and using only geometrical quantities. It is also obtained that the model may describe the dark energy beyond the standard LCDM.

The idea of unifying dark matter and dark energy as one and the same phenomena looks interesting aesthetically. Against it it is the question of many independent observations leading to the existence of dark matter. Specially he galactic ones seem a little bit out of the sight of this paper. But I remember a paper a few time ago that could fit the rotation of galaxies by mean of an stringy phenomena without need of dark matter. Still there is a good reason for expecting the existence of dark matter: Supersymmetry. Specially if R-parity is conserved the LSP is stable and would be a component of dark matter. And one expect that supersymmetry would break near the electroweak scale, isn't it?

Well, maybe. But one of the main supports of low energy SSB is the resolution of the hierarchy problem. Fortunately there is another option for solving the hierarchy problem, the Randall-Sumdrum scenaries. Recently there were two results from the tevatron announcing two anomalies at more than 3 sigmas related to top quark physics. One is described in the article: Evidence for a Mass Dependent Forward-Backward Asymmetry in Top Quark Pair Production., Annalized in entries in the Lubos and Jester blog, among others.

The other, more recent one, was first announced in the Jester's blog: Another Intriguing Result from Tevatron's CDF that also was analysed in Lubos blog (I don' put the link because everybody reads Lubos blog, isn't it? ;) ).

Reading the entries, and the comments one sees that there is a main natural candidate for both phenomena, a KK (kaluza-klein) partner of the gluon. For that KK gluon being in the right track one must be in a Randall-Sumdrum scenario. That means that if the LHC, that restart it's operation in fabruary, confirms the anomalies we would have a (very likely) confirmation of the existence of extra dimensions which would be possibly the most amazing all times experimental discovery in physics.

But don't shout Eureka still. Recently the LHC published an article advertising that they had not seen evidence of micro black holes, which is a bad thing for the R-S sceneries. Well, one wouldn't care too much about it. Microscopic B-h are not the former prediction of R-S scenaries and the paper had some drawbacks also.

Yesterday there was a paper about this R-S braneworlds proposal and it's measure in the LHC: LHC bounds on large extra dimensions based on the analysis of 3.1 inverse picobarns of LHC data that fixed new limits in the possible extent of the extra dimensions. AS far as I understand that bounds don't forbid the KK gluon as the author of the top quarks anomalies. Of course there are another possible answers to that anomalies. For example Marco Frasca claims (The Tevatron affair and the “fat” gluon )that his model of "fat gluon" could do the job (although he acknowledges that the KK gluon fits better).

Well, the thing is that if braneworlds are proved valid the motivation for weak scale SSB would be lacking (because R-S would solve the hierarchy problem), and with it the best candidates for dark matter. Still one could think that another KK particle are candidates for dark matter. But maybe the theories that unifies dark matter and dark energy would gain some points, or maybe be not, cosmology you know ;).

The ADSC/CFT bible

Today in arxiv has appeared something that I had never seen before, a whole collection of review articles devoted to a single topic, the ADS/CFT. There is an "overview" article that serves as reference of the others: Review of AdS/CFT Integrability: An Overview.

If I haven't counted bad there are 23 articles. And this is a review!. Clearly that is an indication of the fact that AdS/CfT is a very active line of research with many branches.

Behind the super event horizont

This is a very brief note to tell that I have not forgotten about the blog.

There are a couple of combined reason for my long absence. Possible one of the most important ones is that my favourite (and expensive) wireless keyboard is out of work because it's USB connector seems to be slightly broken. Now I must return to writing with a wired keyboard, which is uncomfortable and it results in laziness for writing.

I also have had a few math exams, which take time out, I have been busy with activities out of physics (doing an small software develpment, have returned to get piano lesson because of a very good economic offer, deciding which martial art I want learn now that there is no instructors here in the one I did any more, etc).

In the physics side I have been refreshing my knowledge in some topics (for example renormalization) and studying new ones. Specially I am very involved in black hole physic. Both in classical semiclassical gravity (dynamic horizons, vadya metrics, etc) with the idea of see if it is possible to realize some idea I have about "entropic force" effects in black hole physics. Also I am haveing a close attention to a somewhat technical points of black holes in supergravity and string theory, far beyond the basic (although very important) results of the nineties that appear in most books. I hope that these two areas could result in something useful, but who know?

Anyway, I am recovering from the trauma that has supposed the dead of my favourite keyboard and hopefully I'll return to write more often here.

Where to hide a vacuum?

This last mounth happened many interesting things and there were a few good papers. Possibly I´l mention some of them in the future, but today just want to write a brief entry about a pre-idea.

The energy of vacuum is an ugly problem for QFT and string theory. Both in the theoretical side, the vacuum energy would be a lot greater than 0, and in the experimental side, the universe seems to be accelerating according to the famous last nineties observations. The observational side is less worrying: It is due to an imbalance among redshifth and brightness of supernovae. Well, one could try to imaginate an missing light theory (MLT) where some photons are lost in the travel from the distant supernovae to us. This would result in a lost of intensity that could explain the observations. The way to "loose photons" would be by means that some (very rare, of course) interaction with K gravitons in a warped scenario, or in F-theory like scenaries where unification of gauge bossons is in upper dimensional branes one could try to search ways to loose some photon in that branes. Ok, this an idea of myself, and I have not purchased it too much, there are good reasons to think it is not an excellent/important idea.

Or it could be that the accelerated expansios is real but not due to a cosmological constant. One way would be the entropic force scenarie of Gerge Smooth where the acceleration is due to a entropic force caused by the entropy of the cosmological horizon. Or, maybe some "dark energy" theorie.

All of those phenomemenological problems and solutions don't address the fundamental problem that there is a vacuum energy in QFT that should be great. The most extended way is a"landscape + antropic" solution based on the Weimberg ideas and developed by Polchinsky, Bousso, etc and has derived in the multiverse scenario.

Reading the articles in the CC problem (for example, this of WItten: The Cosmological Constant From The Viewpoint Of String Theory or this other of Polchinsky: The Cosmological Constant and the String Landscape ) one can learn that the main objective is to get an small vacuum energy. But I thin that there is another alternative. To put the vacuum energy out of the reach of gravity. That sound like a hard problem because gravity couples to everything, that seems to be one of the most imprtant insights of general relativity. Well, of course I have not a mechanism to hide the vacuum energy for gravity or I would be trying to publish the paper where I develop it to some important review (and it would be in the arxiv just now), butt he key point is that if there is a way to do so it would give an elegant solution to a long standing problem and it could be, possibly, consistent with all the cosmological constraints, including accelerated expansion, inflation, CMB, etc. Well, maybe one could try to relate the mechanism to the observation that some people has made among the CC (presumably) observed value and the neutrino masses. Or, better, to try to relate it to the (presumably) recent observation of different masses among a neutrino and an antineutrino.Or one could forget neutrinos at all, of course.

Well, it is a pre-idea, as I said. For the most conservative readers my recommendation is to look at the two papers I linked (if they haven't already made it) and forget my comments ;-).

Verlinde videos on emergent gravity

Reading the kea's new blog I advertised that there were two videos of Verlinde talking about emergent gravity:

1)KIPT video

2) Perimeter video

At the beginning of this summer Verlinde had announced it's presence in a string theory conference in Madrid but, unfortunately, at last he didn't assist. Because of that I was curious to see in video what I missed in direct and looked at the first of the videos. The realization is somewhat poor in various moments and you can't see Verlinde typing equations in the blackboard or you can't see complete the slides. Still you can follow the conference so it is ok.

In the conference he argues some thing that dn't appear in his paper On the Origin of Gravity and the Laws of Newton. Searching on verlinde's papers n arxiv one can find that some of the things he says on the conference about Berry phase or Born-Oppenheimer/adiabatic could be somewhat rooted on this other paper: Black Hole Berry Phase

While watching the video I have got the impression that it could be interesting to see it one could get an alternative construction of general relativity using entropy instead of geometry as the way to go from special relativity, that is, constant speed reference systems, to accelerated systems. In particular I thought about it when Verlinde does sme considerations about Rindler space.

If that would be possible one would have a different way to consider the questions of general relativity. Also if that entropic viewpoint of general relativity makes sense one could use as a framework to formulate actual theories, including string theory, and, maybe, it could seed some light on it. In that sense it would be more an entropic formulation of general relativity that anything else.

Of course he last two paragraphs are just my considerations, and wouldn't be confused with what Verlinde says.

To end this entry just to mention that a few days ago Peter G.O. Freund posted the paper Emergent Gauge Fields where he argues that the entropic force scenario for gravity could mean, because of ADS/CFT, that all other forces are emergent. The paper is discussed in Peter WOits blog Everything is Emergent. Verlinde mentions similar things at the end of the talk.

Well, as I said in my previous post about entropic gravity I agree with the common view that this idea is very vague. Still I liked the talk over the paper. Even if the idea of gravity through entropy fails the talk is entertaining and worth to watch.