Thu, 13 Jun 2013

17:00 - 18:00
L3

Forking in the free group

Chloe Perin
(Strasbourg)
Abstract

Sela showed that the theory of the non abelian free groups is stable. In a joint work with Sklinos, we give some characterization of the forking independence relation between elements of the free group F over a set of parameters A in terms of the Grushko and cyclic JSJ decomposition of F relative to A. The cyclic JSJ decomposition of F relative to A is a geometric group theory tool that encodes all the splittings of F as an amalgamated product (or HNN extension) over cyclic subgroups in which A lies in one of the factors.

Mon, 03 Jun 2013

12:00 - 13:00
L3

Emergent Time and the M5-Brane

Chris Hull
(Imperial College)
Abstract
One of the more dramatic effects that can arise in a theory at strong coupling is the opening up of an extra spatial dimension, as in IIA string theory or 5-dimensional maximally supersymmetric Yang-Mills theory. The aim of this talk is to investigate the possibility of an extra time dimension opening up in a similar way. The main focus will be on supersymmetric Yang-Mills theory in 5 Euclidean dimensions with 16 supersymmetries, which will be argued to have a strong coupling limit that is a theory in 5+1 dimensions, with a new time dimension opening up to give the (2,0) theory. This gives new insight into the elusive (2,0) theory, which also arises as the M5-brane world-volume theory. It is interesting to have a theory formulated with no time dimension but from which time emerges, and may be useful in thinking about cosmological models in which time and/or space are emergent. The discussion raises questions about the role of time in quantum theory, and about the meaning of a compact time dimension.
Mon, 13 May 2013

12:00 - 13:00
L3

Stationary holographic plasma quenches and numerical methods for non-Killing horizons

Pau Figueras
(DAMTP)
Abstract
In this talk I will explain a new method to numerically construct stationary black holes with non-Killing horizons. As an example, I will use AdS/CFT to describe a time-independent CFT plasma flowing through a static spacetime which asymptotes to Minkowski in the flow's past and future, with a varying spatial geometry in-between. When the boundary geometry varies slowly, the holographic stress tensor is well-described by viscous hydrodynamics. For fast variations it is not, and the solutions are stationary analogs of dynamical quenches, with the plasma being suddenly driven out of equilibrium. We find evidence that these flows become unstable for sufficiently strong quenches and speculate that the instability may be turbulent. The gravitational dual of these flows are the first examples of stationary black holes with non-Killing horizons.
Mon, 06 May 2013

12:00 - 13:00
L3

Torsion-free generalized connections and heterotic supergravity

Mario Garcia Fernandez
(EPFL)
Abstract
I will present a new derivation of the equations of motion of Heterotic supergravity using generalized geometry, inspired by the geometric description of 11-dimensional and type II supergravity by Coimbra, Strickland-Constable and Waldram. From a mathematical point of view, this arises from the study of torsion-free generalized connections on a non-exact Courant algebroid. We will find that the freedom provided by the dilaton field in the physical theory can be interpreted as the freedom of choice of Levi-Civita connection in generalized geometry.
Mon, 29 Apr 2013

12:00 - 13:00
L3

Hyperconifold Singularities and Transitions

Rhys Davies
(Oxford)
Abstract
I will discuss a class of isolated singularities, given by finite cyclic quotients of a threefold node (conifold), which arise naturally in compact Calabi-Yau threefolds. These singularities admit projective crepant resolutions, and thereby give rise to topological transitions between compact Calabi-Yau spaces. Among the interesting properties of such 'hyperconifold transitions' is that they can change the fundamental group, and are related by mirror symmetry to familiar conifold transitions. Having established these mathematical properties, I will briefly discuss some applications, as well as the physics of type IIB string theory compactified on a space with a hyperconifold singularity.
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