2016-2017 Postgraduate Seminars (String Lunch)

Second term meetings take place

on Tuesdays at 1pm, Room B02/5033 -- unless otherwise specified --

Schedule:

04/10/16

11/10/16

18/10/16

25/10/16 - Dimitrakopoulos Fotios

01/11/16

08/11/16 - Nick Poovuttikul

15/11/16 - Panagiotis Betzios

22/11/16

29/11/16

06/12/16

13/12/16

10/01/17 - Andrea Marzolla

[ winter break ]

31/01/17

07/02/17

...

Talks Details:

Fotios Dimitrakopoulos ``Non--linear dynamics and the (in)stability of AdS"

AdS is one of the three vacuum solutions to General Relativity and, as such, the natural question to ask is whether it is stable or not. Although the other two vacuum solutions, namely Minkowski and de Sitter spaces, were proven long ago to be non-linearly stable, a similar study had not taken place for AdS until a few years ago. The first model developed to study this problem was that of a perturbation of AdS in the form of a spherically symmetric, massless scalar field. One then traces the evolution of the scalar field to see whether it collapses to form a Black Hole, or the perturbation remains small for ever. Although initially it was conjectured that AdS is generically unstable, later developments on the subject revealed a large class of perturbations that do not collapse, raising in this way questions about the initial conjecture. In this talk, I will review the model of spherical symmetric perturbations in AdS explaining at the same time the underlying dynamics, I will refer to both numerical and analytic results and I will try to explain the phase space of solutions in the limit where the amplitude of the perturbation vanishes.

Nick Poovuttikul ``On universality of transport phenomena in holography"

In this talk, I will discuss transport phenomena in two classes of theories with holographic dual. In the first part, I will discuss systems where U(1) current is non-conserved due to anomaly and illustrate how one can show that the anomalous conductivities are non-renormalised in a large class of holographic RG flow. The holographic RG flow we considered is generated by arbitrary dilaton potentials and arbitrary higher derivative terms that do not break global symmetries, incorporating coupling constant corrections to the boundary theory in an expansions around infinite coupling.In the second part, I will focus on systems where translational symmetry is broken by slowly varying scalar fields and show that the bound shear viscosity/entropy density is violate. I will also discuss how to understand the violation in the language of forced fluid dynamics.

Panagiotis Betzios ``Matrix Quantum Mechanics on the S^1/Z_2 orbifold"

We revisit c=1 non-critical string theory and its formulation via Matrix Quantum Mechanics (MQM). In particular we study the theory on an S^1/Z_2 orbifold of Euclidean time and try to compute its partition function in the grand canonical ensemble that allows one to study the double scaling limit of the matrix model and connect the result to string theory (Liouville theory). En route, we take advantage of some beautiful mathematics related to Pfaffians and Elliptic functions. We also compare the partition function with the cases of the circle and the Euclidean 2d black hole. Finally, we will make some comments regarding the possibility of using this model as a toy model of a two dimensional big-bang big-crunch universe.

Andrea Marzolla ``Holographic Ward identities for symmetry breaking"

In any QFT, explicit and spontaneous breakings of a global symmetry correspond to specific Ward identities among two-point functions, which can be derived in a very general fashion. We retrieve those identities in a minimal AdS/CFT setup, thanks to holographic renormalization. The same story holds, even if with some interesting subtleties, also in two boundary dimensions, showing the breakdown of Coleman theorem in the strict large N limit. We provide as well some explicit analytic solutions, for 3 and 2 dimensions, which exhibit the gapless Goldstone pole in the scalar correlator.