Ioannis Papadimitriou constructs new domainwalllike solutions that are asymptotic to the nearhorizon geometry of M2branes, namely "AdS4 x S7" (supergravity in four noncompact dimensions). He uses the method of "fake supergravity".
HanXin He claims that he or she has calculated the full, exact threepoint function of two fermionic fields and one gauge field in QED, just by algebraic manipulations with the WardTakahishi identities. An anonymous reader has pointed out that the resulting formula depends on the full nonperturbative fermionic propagator, so it is not quite a "free lunch", but it still seems nontrivial. The result has been checked up to oneloop level but it seems that it is being argued that it is valid up to all orders. This sounded like an extraordinary statement, especially because the formulae didn't seem to have the complexity and transcendentality of the higherloop contributions, but I am not able to say what's wrong with it. If it's correct, it means that all the complexities of the fermion propagator and the threepoint function are isomorphic. Comments will be certainly welcome.
Shinji Tsukikawa studies how the higherderivative corrections to general relativity arising from string theory may regulate the Big Bang singularity and generate various bouncing, ekpyrotic, and other solutions. Topics of this kind were very popular five years ago. Do you remember papers such as Seiberg's paper on the Big Bounce?
Shibaji Roy and Harvendra Singh propose a technique to generate accelerating cosmological solutions from "Sbranes" of another kind that has not yet been used. I have not understood how can one decide whether something is an Sbrane or not. Sbranes seem to be a label for rather general timedependent solutions, unlike Dbranes that are the lowest energy states in a given charge sector. This makes it difficult for me to understand why a solution should become important just because it is connected with the term "Sbrane". The only meaningful purely spacelike counterpart of Dbranes is a Dinstanton or Dbraneinstanton which is just a continuation of the usual Dbranes to a Euclidean spacetime. Sbranes are supposed to be something else but I don't think there is any room for something else.
Giuseppe Bimonte, Enrico Calloni, Giampiero Esposito, and Luigi Rosa include the firstorder corrections from a gravitational field to a conventional calculation of the Casimir effect. They must be computing the first subleading correction, I think. Up to the leading order, the Casimir force and the gravitational force are simply added together. They argue that the Casimir force becomes a little bit stronger if you add acceleration (or gravity).
T.Khachidze, A. Khelashvili, T.Nadareishvili only offer a PDF file, and they argue that the Dirac equation does not reduce to the usual nonrelativistic equation in the appropriate limit. They say that it has extra solutions  maybe hydrinos? :)  and some new qualitative features. I am convinced that their paper is based on elementary errors. For example, when they square the Dirac Hamiltonian, they only keep the "V^2" terms but omit the main term proportional to "V" that actually survives in the Schrödinger limit. The "V^2" term is only a relativistic correction, analogous to the "p^4" corrections to the kinetic energy. The authors are recommended to study the derivation of the nonrelativistic limit of the Dirac equation from one of the standard textbooks.
Daniel Litim promotes the idea that the UV fixed point of general relativity exists. An ultraviolet fixed point is the hypothetical scaleinvariant theory valid at the subPlanckian distances whose relevant deformation turns flows to general relativity at long distances. Its existence could make quantized general relativity predictive but it is believed that such a fixed point does not exist. Litim admits that classically, such a scaleinvariant theory only exists in the critical dimension "d=2". We know purely fieldtheoretical gravitational theories in this spacetime dimension; the most useful twodimensional fieldtheoretical gravity in "d=2" is called the string worldsheet. However, Litim claims that a fixed point of the renormalization group exists in every dimension. To support this claim, he considers the cosmological constant (Lambda) and the EinsteinHilbert term (G) only, and studies the evolution of "Lambda,G" under the RG flow. With an appropriately chosen anomalous dimensions of the operators, he can find a fixed point in every dimension. I find this argument unconvincing because in the subPlanckian distance regime, gravity is strongly coupled and it is not justified to neglect all other terms except for "Lambda,G". Equivalently, we can see that the "Lambda,G" approximation is bad because the anomalous dimensions of the fields are greater than one. It is unsurprising that whenever we include a few couplings only, we will formally find some fixed points, but those that appear in strongly coupled regions of the parameter space can't be quite trusted, unless a more complete argument is found.
Itzhak Bars continues in his provoking research of physics with two time coordinates. Today, he considers the Standard Model derived from a 4+2dimensional starting point. In the past, Bars has argued that one can obtain very different physical systems  such as the Hydrogen atom and the harmonic oscillator  by different gaugefixing choices applied to the same doubletemporal starting point with an Sp(2,R) gauge symmetry. I don't understand how can one ever get different physics by choosing a gaugefixing condition. Gauge symmetry and gaugefixing choices should not affect observable physics, by definition. This is why I can't quite understand either of Bars' twotime papers. Nevertheless, if someone can do better than me, this paper constructs the Standard Model from a 4+2dimensional starting point. Not only that, it explains the strong CPproblem because the "F /\ F" term cannot arise in this framework.
Martin Land discusses aspects of generalized Ddimensional Dirac magnetic monopole. I feel 85% certain that if you know Maxwell's equations in D dimensions and the simple power law solutions for electric and magnetic pointlike sources, you won't find anything new in this paper.
Gottfried Curio and Vera Spillner jump into a seemingly random place of the KKLT landscape. They start with the elliptic CalabiYau manifold called P_{11169}[18]  known from "building a better racetrack"  and write down all relevant terms in the superpotential that is usually used in the context of flux compactifications, coming from fluxes and gaugino condensation. They modify the usual wisdoms by an unexpected dependence on the dilaton "tau". They seem to argue that the superpotential "W(tau)" itself is not holomorphic which I misunderstand because superpotentials should be holomophic. But because they seem to be really powerful technologically, I am inclined to believe that the paper makes a perfect sense.
Kazumi Okuyama studies the topological partition sum on the conifold  both in the Amodel (the math of melting crystals) as well as Bmodel (ChernSimons matrix model). He or she finds a very interesting integral transform that transform one to the other. The integral transform involves an integral over matrices with a Gaussian measure.
U. Gran, J. Gutowski, G. Papadopoulos, D. Roest argue that there are no configurations with exactly 31 supercharges in type IIB supergravity. More precisely, if you give them one with 31 supercharges, they are able to find the 32nd supercharge, proving that the background is maximally supersymmetric. It is analogous to "N=7" theories in "d=4" that are automatically "N=8", too. Their finding, if correct, invalidates the type IIB version of the "preon" speculation. These "preons", in the SUGRA context, were proposed to preserve 31 supercharges. Their central charge was hypothesized to be a "32 x 32" matrix whose rank equals one (instead of more usual values such as 16) which could turn them into basic building blocks of other systems that preserve a smaller number of supercharges. Such a bizarre rankone central charge would clearly be a combination of central charges associated with branes of all possible dimensions, and this paper shows that this can't work at the level of type IIB supergravity.
Wednesday, June 07, 2006 ... //
Hepth papers on Wednesday
Vystavil
Luboš Motl
v
8:09 PM



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