## Monday, July 10, 2006

### Hep-th papers on Monday

There are seven papers on hep-th on Monday.

Jarah Evslin and Hisham Sati focus on some subtle differences between K-theory and homology in defining charges of D-branes. Recall that homology used to be viewed as the right classification of the possible lattices of charges of D-branes, in agreement with the simple picture in which a D-brane wraps a submanifold. However, work based on tachyonic considerations of Ashoke Sen has led Edward Witten to argue that K-theory, a structure analogous to homology but based on gauge bundles, is a better more accurate description of the charges because it treats some torsion classes more carefully. The present authors study homology cycles of nice smooth manifolds that cannot be represented by a nonsingular submanifold. I suppose they mean a connected submanifold. For cycles that can be represented by submanifolds, the Freed-Witten anomaly must cancel for the cycle to be usable as a living room for a D-brane. K-theory, a huge hobby of Jarah Evslin, contains most cycles that homology does, but as the authors argue, some of them must be non-representable by submanifolds. Examples are provided.

Tamiaki Yoneya reviews the Penrose (BMN) limit of the AdS/CFT correspondence. It's really one talk so don't expect that he gets to things as complex as integrability or magnons.

Kazunobu Maruyoshi studies gauged N=2 supergravities with various U(1) and U(N) vector multiplets that he breaks spontaneously to N=1 supergravity. Although the resulting theory probably can't have chiral couplings and can't be semi-realistic (which makes it different from the usual mechanisms of supersymmetry breaking in phenomenology), and although the implementation into the big picture of string theory is not given, I must say that it is a very impressive master thesis.

Larus Thorlacius looks at the black hole information loss. Most of the time, we would study the causal subtleties of the propagation of information with the example of a Schwarzschild black hole that has a spacelike singularity. However, the Penrose diagram of a black hole that is charged just a little bit looks different. The Reissner-Nordström diagram can be maximally extended to an infinitely long vertical strip with infinitely many timelike singularities. This picture should be, in some sense, more generic than the exact Schwarzschild picture. Thorlacius offers an argument that assumes black hole complementarity - the conjecture that the degrees of freedom inside a black hole are not independent from those outside - and concludes that the evolution can't respect unitarity or that the information must be lost because of the Cauchy horizons. My interpretation, of a person who thinks that the preservation of information at infinity is an established result, is that Thorlacius did not take the non-locality properly into account, and if he did, then it means that the interior of charged black holes must look very differently than the classical causal diagram indicates, even though the difference could be less dramatic than what Samir Mathur proposes.

Hristu Culetu likes to view the horizons in general relativity as one-way membranes. While everyone agrees that the entropy "S=A/4G" should be assigned to all macroscopic horizons including the Rindler horizons - recall that Rindless space is a flat Minkowski space interpreted by a uniformly accelerating observer - the author notes a discrepancy about the energy that we normally assign to the Rindler horizon. He modifies the formalism in such a way that the horizons always carry the energy of "E=r/2", including the Rinder horizons that were previously said to carry no entropy. I am not sure whether these additive shifts have a well-defined meaning anyway. Overall additive background-dependent shifts to the energy only matter if you can compare two different backgrounds by a physical experiment. The exact result depends on the details how you do the Wick rotation and how you treat the boundary terms in the action or energy.

Baumann, Dymarsky, Klebanov, Maldacena, McAllister, and Murugan look at type IIB (or F-theory) flux compactifications that involve warped geometry which makes them perfectly suited for a stringy realization of the Randall-Sundrum ideas (besides being the canonical stringy playground for the anthropic speculations). Take this warped geometry with the throats and throw a D3-brane into it. Study its motion. You will be able to describe the motion by a potential. Recall that there is more structure in these backgrounds. The sizes of four-cycles - Kähler moduli - are stabilized by certain objects wrapping the four-cycles. The authors confirm a qualitative form proposed by Ori Ganor and compute the superpotential relevant for the D3-brane to be a function of the warped four-cycle volume. Because their SUGRA / closed string loop calculations remain nicely holomorphic, they can also tell you how the holomorphic gauge couplings can be expressed as functions of the geometric parameters of the compactification.

David Kastor and Jennie Traschen study how the thermodynamics of black holes in Kaluza-Klein theories changes under infinitesimal changes of the moduli parametrizing the compactification. From thermodynamics, you know the "dE" contains terms of the form "p dV", and they also tell you something about the similar terms in which "dV" is replaced by various variations of the compactification moduli. Well, if you know the energy "E=Mc^2" as a function of the moduli, you really don't need more than basic knowledge thermodynamics. The new quantities analogous to "p" and "dV" will carry two extra indices for toroidal compactification. And I am not quite sure whether the experts learn something new from the nicely-written text.