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First Term: Tuesdays 1300 in 54/8033
Second Term: Tuesdays 1300 in 58/1039
**** Note: due to time constraints on the reservation of room 54/8033, seminars will begin promptly at 1300 and move to the Ketley Room at 1355 if continued discussion is desired. ****
Schedule:
09/10/18 - Aaron Poole
16/10/18 - Federico Capone
23/10/18 - George Katsianis
30/10/18 - Sami Rawash
06/11/18 - Pulastya Parekh (IISER - Pune)
13/11/18 - Ben Withers
20/11/18 - Tristan Madeleine
27/11/18 - Joan Garcia-Tormo
04/12/18 - Eliot Bridges
11/12/18 - Paul Rodgers
[Winter break: 17 December 2018 through 4 January 2019]
29/01/19 - Linus Too
05/02/19 - Brandon Robinson
12/02/19 - Marika Taylor
19/02/19 - Kostas Skenderis
26/02/19 - Ronnie Rodgers
05/03/19 - Carlos Mafra
12/03/19 - Eliot Bridges
19/03/19 - Andy O'Bannon
26/03/19 - Adam Chalabi
[Spring break: 1 April 2019 through 26 April 2019]
30/04/19 - Aaron Poole
07/05/19 -
14/05/19 - C. Robin Graham (University of Washington)
21/05/19 - C. Robin Graham (University of Washington)
Details:
09/10/18: Aaron Poole
Title: Bondi-Sachs approach to AdS/CFT [Slides]
Abstract:
In the 1960s; Bondi, Metzner, van der Burg, and Sachs (BMS) introduced the Bondi gauge, a particularly useful tool to study asymptotically flat spacetimes. In this talk I will discuss how one extends this gauge to describe asymptotically locally anti de-Sitter (AdS) spacetimes, with particular emphasis on the holographic interpretation of such spacetimes via the AdS/CFT correspondence.
I will begin with a review of asymptotically flat spacetimes and introduce the Bondi gauge. I will also give an introduction to AdS spaces and their asymptotics, as well as the holographic importance of the Fefferman-Graham gauge. In the second half of the talk I will discuss some recent results including a comparison of the Einstein field equations for the AdS and flat cases as well as giving a holographic description of the Bondi-Sachs metric functions. Time permitting, I may mention some applications of this work to gravitational radiation in AdS and de-Sitter (dS) space-times.
This talk will be based on soon to be published work in collaboration with Kostas Skenderis and Marika Taylor.
16/10/18: Federico Capone
Title: Cosmic Membranes and Higher Dimensional BMS Superrotations
Abstract:
In this talk I am going to give you the motivations behind my work with Marika on "cosmic membranes and higher dimensional BMS superrotations". To this end I will review well known facts about four dimensional asymptotically flat spacetimes, the BMS group and its role in the infrared gravitational physics, in particular the soft graviton theorems by Weinberg and Cachazo-Strominger. I will introduce locally asymptotically flat spacetimes through the example of the cosmic string and I will point out how superrotations are related to the dynamical process of snapping or creation of such defects. I will argue that such a process is the footprint of a superrotation and hence the analysis of cosmic membranes described in Bondi gauge can be the key to the definition of higher dimensional superrotations.
23/10/18: George Katsianis
Title: Anomalies in Supersymmetric Quantum Field Theories [Slides]
Abstract:
Recent holographic computations show the existence of new anomalous terms in the conservation law of the supersymmetric current of N=1 superconformal theories in 4 dimensions. In this talk I will discuss how we tried to confirm these results using quantum field theory techniques in the simplest possible superconformal model. The massless Wess-Zumino action.
I will begin with a review of the U(1) axial anomaly and describe one of the regularization procedures that were used to find it, the momentum routing regularization. In the second part of the talk I will briefly discuss earlier results on supersymmetric anomalies in Ward identities for three-point functions. Then I will focus on our work and describe the Wess-Zumino model that we used and the correlation functions that we computed. Using momentum routing regularization we confirm the holographic computations and find the existence of a new supersymmetric anomaly in the Ward identity for four-point functions. I will argue why this result may have implications for the applicability of localization techniques.
30/10/18: Sami Rawash
Title: A Holographic Description of Black Hole Microstates [Slides]
Abstract:
In this talk I will start reviewing the basic features of black holes in General Relativity: their thermodynamic behaviour and the paradoxes that emerge when one takes this behaviour seriously.
This motivates the study of black hole physics in string theory, which enables for a correct count of the microstates responsible for the entropy of the black hole, and suggests a resolution of black hole issues: the fuzzball proposal. To establish the validity of this proposal, it would be important to provide a precise map between the geometries and the microstates. This map could be constructed using the tools from the AdS/CFT correspondence. In particular one can compare the vacuum expectation values of some operators in a CFT state, with those extracted from the geometry via holographic methods. In the last part of the talk I will discuss these methods for operators of dimension 1, and its generalization to operators of dimension 2.
06/11/18: Pulastya Parekh (Pune)
Title: Tensionless strings : A perspective from the worldsheet [Slides]
Abstract:
I will be speaking about the construction of the tensionless limit of closed bosonic string theory in the covariant formulation in the light of Galilean conformal symmetry that rises as the residual gauge symmetry on the tensionless worldsheet. I will show how the analysis of the fundamental tensionless theory is related to the tensionless limit that is viewed as a contraction of worldsheet coordinates. The connection to massless higher spin states can be seen naively. This analysis can be extended to the closed superstring to obtain the Super Gallilean Conformal Algebra (SGCA), that can be realised in two distinct ways : the Homogenous and the Inhomogenous SGCA. I will also comment on the hermiticity properties of fermions in case of the Inhomogeneous tensionless superstring. We will see that the analysis of the quantum regime uncovers interesting physics. The degrees of freedom that appear in the tensionless string are fundamentally different from the usual string states. Through a Bogoliubov transformation on the worldsheet, one can link the tensionless vacuum to the usual tensile vacuum. As applications, I will discuss how tensionless strings can be connected to Hagedorn Physics and Ambitwistors.
13/11/18: Ben Withers (Geneva)
Title: The bulk description of conformal families
Abstract:
At the heart of the AdS/CFT correspondence lies the matching of symmetries. Operators in a CFT are naturally arranged into conformal families. I will construct the AdS description of a conformal family in terms of tensor fields transforming under the action of AdS isometries. The outcome is the Fierz-Pauli system of equations in AdS, with a mass set by the dimension and spin of the associated primary operator. For a spin-2 conserved current operator we obtain linearised Einstein gravity in AdS equipped with covariant gauge fixing conditions. Analogous results are given for dS and flat space.
20/11/18: Tristan Madeleine
Title: Non supersymmetric fuzzball
Abstract:
In General relativity a black hole is fully determined by its globally conserved charges: it is the "no hair theorem". Within this description, the information regarding what is inside the event horizon is hidden from an observer outside the black hole but not lost. However a semi-classical description gives rise to a thermodynamic interpretation of black holes (Bekenstein entropy) and to a black body like radiation resulting in the evaporation of the black hole in a way that seems to violate the unitarity of the quantum evolution of a system: it is the "Information paradox". A consistent theory of quantum gravity should be able to solve this paradox and give an interpretation to the origin of the Bekenstein entropy. In string theory, a promising attempt is the fuzzball proposal which postulates that a black hole is in a quantum superposition of microstates, whose number gives rise to the entropy of the black hole and have the peculiarity to modify the Hawking radiation allowing information to escape. So far this approach has been successful to describe black holes that saturates the BPS bound, but it is not the case for Non BPS black holes. During this presentation, I will briefly review the information paradox and how it is solved by the fuzzball proposal, then I will show how to try to build fuzzball microstates corresponding to non BPS black holes using a known family of solutions to supergravity.
27/11/18: Joan Garcia-Tormo
Title: Black hole microstates in the D1D5 CFT
Abstract:
Following Sami's talk on the fuzzball proposal, in this talk I will focus on the field theory side of the microstates. Namely, I will review the D1D5 system, which is the best understood system for black hole microstates, and then I will present in detail the CFT side and argue what can be seen within supergravity. If time allows I will present results on 1-point function calculations in this system, also on higher point functions for twist operators and preliminary results on countings/classification in short and long strings of the microstates that have been constructed recently.
04/12/18: Eliot Bridges
Title: Tree level amplitudes in the pure spinor formalism
Abstract:
In this talk I'll start by giving a brief overview of some of the basics of the pure spinor formalism, which allows for covariant quantization of the superstring. I'll then move on to how you compute tree level amplitudes within this formalism, and run over some of the work that has been done to simplify this process considerably.
04/12/18: Paul Rodgers
Title: Dirac perturbations on Reissner Nordstrom (RN) AdS (Global).
Abstract:
I will motivate the talk by giving a review of classical super radiance, moving on to talk about the more recent discovery of the so called near horizon instability which is present for scalar fields. Here we can have a solution which is stable in the UV (satisfies the BF bound), but the scalar field charge can drive the extremal black hole unstable in the near horizon region. The holographic superconductor programme motivated the study of this for both scalars and fermions in planar RN AdS. The scalar field case has been studied extensively in the global AdS case too, therefore we want to study the problem for fermions in the global AdS case. I will set up the variational problem, discuss boundary conditions and finally present some results.
29/01/19: Linus Too
Title: Holographic entanglement entropy and its application
Abstract:
Holographic correspondence provides a window to look at entanglement geometrically. Given an entangling region of the CFT, the entanglement entropy is equal to the area of the minimal surface that extend from the boundary of the entangling region into the AdS bulk. Holographic entanglement entropy has been applied to gravity, black holes information and quantum codes. Holographic renormalization is necessary for calculating dual quantities. Holographic entanglement entropy can be properly renormalized. Many application of holographic entanglement entropy needed renormalization.
05/02/19: Brandon Robinson
Title: Weyl Anomaly and Entropies in 2D defect CFTs
Abstract:
In this talk, I will explain some recent results regarding the appearance of Weyl anomaly coefficients in physical observables in static 2D boundary and defect conformal field theories. The primary focus will be elucidating the relationship between boundary/defect central charges, entanglement entropy, and thermal entropy and the distinction from known results in ordinary conformal field theories. A new positivity bound for a particular defect Weyl anomaly coefficient will be proposed.
12/02/19: Marika Taylor
Title: Black hole microstates in SYK
Abstract:
The main goal of the black hole microstate programme is to find a "geometric" description of individual black hole
microstates. The SYK model is an interesting arena to study black hole microstates, as the model is sufficiently solvable that one can calculate correlation functions in generic pure states. Maldacena and collaborators have proposed that SYK pure states can be captured by certain gravity configurations in two-dimensional Einstein dilation theories; this proposal (1707.02325) has been explored and extended in many subsequent works.
This talk will review 1707.02325 (touching on later papers as well, particularly work in progress by Almheiri), and discuss how this picture of black hole microstates fits with our understanding of microstates in well studied string systems such as the D1/D5 black holes.
19/02/19: Kostas Skenderis
Title: Anomalous Supersymmetry
Abstract:
I will show that supersymmetry in N=1 SQFT in d=4 is anomalous if it has an anomalous R-symmetry.
This anomaly was originally found in holographic SCFTs at strong coupling. We show that this anomaly is present in general and demonstrate it for the massless superconformal Wess-Zumino model via an one loop computation. The anomaly appears first in the 4-point functions of two supercurrents and either two R-currents or an R-current and an energy momentum tensor. In fact, the Wess-Zumino consistency condition together with the standard R-symmetry anomaly implies the existence of the anomaly. I will discuss the implications of this anomaly.
26/02/19: Ronnie Rodgers
Title: Self-dual string central charges from holography
Abstract:
M-theory is a candidate for a theory of quantum gravity. Its fundamental objects are called M2-branes and M5-branes. The low-energy theory describing coincident M5-branes is poorly understood in many respects, with holography providing one of the most useful tools to further that understanding. It is known that the theory should possess solitonic solutions called "self-dual strings". I will use holography to compute quantities which may measure the massless degrees of freedom on a self-dual string, and study the behavior of these quantities along RG flows.
05/03/19: Carlos Mafra
Title: The superstring n-point 1-loop amplitude
Abstract:
In 2011, the superstring n-point amplitudes at tree level have been explicitly computed using the pure spinor formalism.
In this talk I will discuss the recent work on extending these calculations to one loop.
Several very interesting patterns emerge in this quest. One of them is a general Lie-polynomial structure of superstring correlators (also conjectured to hold at higher genus). There is also a double-copystructure between kinematic factors in pure spinor superspace and generalized elliptic integrands. Their construction in terms of objects satisfying shuffle-symmetries
may suggest an underlying combinatoric explanation.
12/03/19: Eliot Bridges
Title: Multiparticle superfields to arbitrary order
Abstract:
String and field theory amplitudes have previously been constructed in terms of multiparticle superfields assumed to satisfy generalised Jacobi identities. In this talk I will run over what multiparticle superfields are, what generalised Jacobi identities are, and some work that has been done to find a general method for constructing such multiparticle superfields to arbitrary order.
19/03/19: Andy O'Bannon
Title: The Renormalization Group and Boundary Conditions in 2D
Abstract:
One of the central questions of quantum field theory is: if we perturb a conformal field theory (CFT) by a relevant scalar operator, triggering a renormalization group (RG) flow, what results at the longest length scales? Do we find a gapped/massive vacuum, another CFT, or something else? In general this question is difficult to answer because along RG flows couplings typically grow large enough to render perturbation theory unreliable. I will discuss an alternative approach to this question, namely integrating the relevant deformation over half of space, so that the RG flow occurs only over half of space. If the result is a gapped/massive vacuum, then we find the original CFT with a conformal boundary condition. If the result is another CFT, then we find a conformal interface between two CFTs. Classifying vacua thus maps to classifying conformal boundary conditions or interfaces, which should be easier since the power of conformal symmetry can be brought to bear. I will provide an overview of the current state of the art of this approach, including a summary of proposals by Gaiotto (1201.0767), Konechny (1610.07489) and Cardy (1706.01568) for 2d minimal models.
26/03/19: Adam Chalabi
Title: Defect Central Charges from the AGT Correspondence
Abstract:
In 2009, Alday, Gaiotto and Tachikawa (AGT) discovered a remarkable correspondence between observables in a certain class of 4d N=2 gauge theories and 2d Liouville CFT. In this talk, I will introduce the field theories on both sides of the correspondence and then motivate it by studying particular examples. I will then briefly sketch how surface operators in the 4d theory enter into the correspondence, and how one may use AGT to extract central charges of DCFTs.
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