John Voight : On Moduli of Nondegenerate Curves
- String Theory ( 244 Views )We study the conditions under which an algebraic curve can be modelled by a Laurent polynomial that is nondegenerate with respect to its Newton polytope. Such nondegenerate polynomials have become popular objects in explicit algebraic geometry, owing to their connection with toric geometry; however, despite their ubiquity, the intrinsic property of nondegeneracy has not seen much detailed study. We prove that every curve of genus $g \geq 4$ over an algebraically closed field is nondegenerate in the above sense. More generally, let $\mathcal{M}_g^{\textup{nd}}$ be the locus of nondegenerate curves inside the moduli space of curves of genus $g \geq 2$. Then we show that $\dim \mathcal{M}_g^{\textup{nd}} = \min(2g+1,3g-3)$, except for $g=7$ where $\dim \mathcal{M}_7^{\textup{nd}} = 16$; thus, a generic curve of genus $g$ is nondegenerate if and only if $g \geq 4$
Lenya Ryzhik : How an incompressible flow helps diffusion to mix things
- String Theory ( 237 Views )I will describe some recent results that concern various aspects of the mixing properties of a strong incompressible flow acting together with a diffusion. In particular, we will discuss the short-time decay of solutions of the corresponding initial value problem, asymptotics of the principle Dirichlet eigenvalue and the behavior of the explosion threshold in the Zeldovich problem when the incompressible flow is strong. When the flow is prescribed, the "enhancement" of these characteristics comes from the geometric properties of the flow. We will also show that flows arising from the Stokes-Bousisnesq problems possess these "enhancement" features.
Josh Garretson : T-duality and Generalized Geometry
- String Theory ( 230 Views )T-duality has long been well understood locally via the Buscher rules. Global T-duality in the presence of an arbitrary background is much more involved. It relates backgrounds of different topology and can be seen to map 'regular' or commutative geometries to noncommutative ones. I will give a brief overview of these attempts at studying T-duality and show how T-duality acts very naturally in the context of Hitchin's generalized geometry. I will show that T-duality is an automorphism of the Courant bracket in the most general sense and give an example. If time permits, I will discuss applications to Poisson-Lie T-duality.
Mainak Patel : Temporal Binding Emerges as a Rapid and Accurate Encoding Tool Within a Network Model of the Locust Antennal Lobe
- String Theory ( 224 Views )The locust olfactory system interfaces with the external world through antennal receptor neurons (ORNs), which represent odors in a distributed, combinatorial manner. ORN axons bundle together to form the antennal nerve, which relays sensory information centrally to the antennal lobe (AL). Within the AL, an odor produces a stimulus-specific temporal progression of neuronal spiking, inspiring the hypothesis that the AL encodes odors through dynamically evolving ensembles of active cells. Such a coding strategy, however, requires higher olfactory centers to integrate a prolonged dynamic profile of AL signals prior to stimulus assessment, a process that is likely to be slow and inconsistent with the generation of quick behavioral responses. Our modeling work has led us to propose an alternate hypothesis: the dynamical interplay of fast and slow inhibition within the locust AL induces transient correlations in the spiking activity of an odor-dependent neural subset, giving rise to a temporal binding code and allowing rapid stimulus detection by downstream elements.
Mark Jackson : Superstring Cosmology: New Physics in the Sky
- String Theory ( 222 Views )Striking advances in observational cosmology over the past two decades have ushered in a golden era in cosmology, where our focus has turned from what the universe is made of to why it has the form we observe. The leading theory capable of answering such a question, Superstring Theory, does not appear capable of being tested using conventional accelerator-based experiments, forcing us to be more creative in our goal to verify or dismiss it. Focusing on brane inflation as a string theory-inspired model of inflationary cosmology, I will review how the cosmic microwave background (CMB) will provide a deluge of high- precision data into otherwise inaccessible energy scales. These data include possible "Transplanckian" signatures in the power spectrum, indications of variable sound speed or extra dimensions in non- Gaussianity, or constraining the inflation model parameter space using polarization. I then describe how the production of cosmic (super)strings in brane inflation would provide an additional means to verify superstring theory, and which would yield much detailed information about the underlying theory parameters.
Bobby Acharya : M Theory, G_2 Holonomy spaces and N=1 Theories in Four Dimensions
- String Theory ( 42 Views )After reviewing some reasons why one might be interested in G_2 holonomy compactifications of M theory, I shall review some of the recent results about such vacua, which are mainly due to Atiyah, Maldacena, Vafa, Witten, and myself in various collaborations. Topics covered will mainly center around how various aspects of super Yang-Mills theory can be related to properties of M theory in such spacetime backgrounds. These will include instantons, domain walls and confinement.
David Morrison : Introduction to F-Theory, IV
- String Theory ( 34 Views )This is the fourth of a series of lectures on F-theory. The lectures will present the theory of elliptically fibered Calabi-Yau manifolds in considerable detail, explain how these manifolds are used to produce string vacua by means of the ``F-theory'' construction, and how various properties of these string vacua are determined by the corresponding Calabi-Yau manifolds.
David Morrison : Introduction to F-Theory
- String Theory ( 33 Views )This is the first of a series of lectures on F-theory. The lectures will present the theory of elliptically fibered Calabi-Yau manifolds in considerable detail, explain how these manifolds are used to produce string vacua by means of the ``F-theory'' construction, and how various properties of these string vacua are determined by the corresponding Calabi-Yau manifolds.
Albion Lawrence : Worldsheet RG and target space time evolution
- String Theory ( 33 Views )A useful tool to study tachyon condensation in string theory is the renormalization group flow of the associated relevant perturbation of a conformal field theory. One assumes that the worldsheet RG flow reflects the actual time evolution of the target space. The relationship is not completely direct: for example, the RG equations are first order differential equations, while the target space equations of motion are at least second order in time. To put the correspondence on firmer footing we discuss the coupling of non-conformal 2d field theories with $n\geq 25$ scalar fields to 2d gravity, following previous discussions of Polyakov and of Schmidhuber and Tseytlin. The conformal mode of the metric becomes a timelike target space direction with a Liouville action, so that the Liouville dressing of an operator is related to its scaling properties. We will give a conjecture for the equations of motion for the worldsheet couplings, which reduces to the standard 2d RG equations in the limit that the Liouville mode becomes semiclassical, and describe our current attempts to prove this conjecture.
Peter Miller : The Zero-Brane as a Soliton
- String Theory ( 33 Views )We present an exact construction of the D0-brane as a soliton on the worldvolume of a BPS D2-brane in the presence of a large B field. We match the mass and spectrum of fluctuations of this soliton to the D0-D2 conformal field theory. This spectrum includes a tachyon: the endpoint of tachyon condensation represents the ``disappearance'' of the D0-brane, and all modes associated with it, as it dissolves in the D2-brane. This construction thus sheds some light on issues related to tachyon condensation.
David Morrison : Non-Spherical Horizons
- String Theory ( 33 Views )The ``horizon'' of N coincident branes in Minkowski space is the unit sphere in the transverse directions; in a certain scaling limit, string or M-theory in the presence of branes is dual to a compactification on the product of an anti-de Sitter space with the horizon. We study the analogous limit when N branes are placed at a singular point, so that the horizon becomes the so-called ``link'' of the singularity, and is no longer a sphere. A similar scaling argument leads to a natural extension of Maldacena's celebrated ``AdS/CFT correspondence conjecture'' to this situation. The conformal field theories in question have less supersymmetry than the cases studied by Maldacena, with the amount being determined by the Killing spinors on the horizon manifold. An important mathematical tool is the relationship -- derived some years ago by C. Bär -- between Killing spinors on a manifold and covariantly constant spinors on the cone over that manifold.
Brian Wecht : Towards a c-theorem in Four Dimensions
- String Theory ( 33 Views )Intuitively, renormalization group flow is irreversible, since one would expect the number of massless degrees of freedom of a theory to decrease as one goes to progressively larger distance scales. This has been proven rigorously in two dimensions (the "c-theorem"), but is otherwise an open problem. I will discuss recent efforts to prove an analogous theorem in four dimensions for supersymmetric theories, for which we can make exact statements not usually accessible in their non-supersymmetric counterparts.
David Morrison : Introduction to F-Theory, II
- String Theory ( 32 Views )This is the second of a series of lectures on F-theory. The lectures will present the theory of elliptically fibered Calabi-Yau manifolds in considerable detail, explain how these manifolds are used to produce string vacua by means of the ``F-theory'' construction, and how various properties of these string vacua are determined by the corresponding Calabi-Yau manifolds.
Washington Taylor : Counting flux vacua with special properties
- String Theory ( 32 Views )I will discuss the statistical analysis of flux vacua with special properties such as (no-scale) supersymmetry and discrete symmetries. Unlike generic flux vacua, enumeration of such special vacua can require number-theoretic methods.
Lilia Anguelova : R^4 Corrections and Flux Superpotential in Heterotic M-theory
- String Theory ( 31 Views )I will describe two kinds of corrections to the previously known effective action of heterotic M-theory. The first is due to taking into account the 11d R^4 term. This results in a new contribution to the Kahler potential of the four-dimensional effective theory. The second comes from the flux-generated superpotential. To derive systematically the form of this superpotential, one has to use the approach of group structures. It is essential to consider SU(3), not G_2, structure on the internal 7d manifold. Finally, I will address the issue of how these corrections affect the existence of dS vacua in heterotic M-theory, which were originally found by balancing open membrane instanton effects and gaugino condensation.