2019-20 Postgraduate Seminars
Time/Place Semester 1: Tuesday 12:15pm, Building 13, Room 3021, unless otherwise stated.
Time/Place Semester 2: Tuesday 13:00pm, Building 7, Room 3027, unless otherwise stated.
Semester 1
08/10/19 - Federico Capone
15/10/19 - Perseas Christodoulidis (Groningen) - String seminar
22/10/19 - Elliot Bridges
29/10/19 - Aaron Poole
05/11/19 - Davide Bufalini
19/11/19 - Bidisha Chakrabarty (ICTS-TIFR Bangalore) - String seminar - time 12:00/12:15pm
26/11/19 - Ivan Muñoz
03/12/19 - Fabrizio del Monte (SISSA Trieste) - String seminar - time 12:00/12:15pm
10/12/19 - Josh Kirklin (Cambridge) - String seminar - time 12:00/12:15pm
[Winter break: 14 December 2019 - 5 January 2020]
Semester 2
28/01/20 - Paul Rodgers
04/02/20 - Marika Taylor, part 1
11/02/20 - Marika Taylor, part 2
18/02/20 - Marika Taylor, part 3 - 1:00-1:45 [note Honda lecture 2:00-3:00]
25/02/20 - Andrew Spiers
03/03/20 - Adam Chalabi
10/03/20 - Adam Chalabi, part 2
17/03/20 - Sami Rawash
[Spring break: 21 March - 19 April 2020]
21/04/20 - Marika Taylor, part 1 (part 2 on Wednesday 22/04/20, at 1:00 PM)
28/04/20 - Alex Davey
05/05/20 - Daniel Hasenbichler
12/05/20 - Linus Too
19/05/20 - Enrico Parisini
26/05/20 - Jacopo Sisti
02/06/20 - Aaron Poole
16/06/20 - Davide Bufalini
23/06/20 - Elliot Bridges
07/07/20 - Federico Capone
08/07/20 - Federico Capone (Part 2) **note unusual time Wed 1pm**
14/07/20 - Davide Bufalini
15/07/20 - Adam Chalabi **note unusual time Wed 1pm**
21/07/20 - José Manuel Penín
Topics: (reverse chronological order)
José Manuel Penín: T-duality and non-Abelian T-duality.
In this talk I will give a short introduction to non-Abelian T-duality. After reviewing the concept of T-duality and Buscher's rules, we will focus on its non-Abelian generalization, showing the transformation rules and its properties, and the profound differences between both symmetries. We will also discuss its applications in holography, with some examples.
Adam Chalabi: Aspects of 3d gauge theories from type IIB string theory (Hanany-Witten part 2).
In this talk I will summarise the paper of Hanany and Witten (hep-th/9611230). Picking up where we left off yesterday in Davide’s talk, I will review the type IIB brane set-ups that Hanany-Witten consider, and describe the resulting 3d N=4 gauge theories in the IR. Many properties of the field theory are naturally encoded in the string theory, and I will explain how the brane picture incorporates highly non-trivial field theoretic phenomena such as 3d mirror symmetry. I will also explain the emergence of a brane creation/annihilation mechanism that has become known as the Hanany-Witten transition. These ideas will be illustrated with many examples.
Davide Bufalini: From D-Branes to Field Theories.
I will briefly explain the importance of k-Symmetry for D0-Brane, Superstring and Dp-Branes. After that, the notion of Killing spinor and SUSY-preserving conditions for Dp-branes are recalled. I will also show, in a very simple case, how to obtain a field theory on the worldvolume of a Dp-brane, at low energies. Thanks to the above notions, I will be able to introduce the Hanany-Witten brane setup (see https://arxiv.org/abs/hep-th/9611230) which will be used and further discussed tomorrow by Adam.
Federico Capone: On soft theorems and asymptotic symmetries.
In the first part I will provide a general field theoretic proof of gravitational soft theorems due to A. Sen (& collaborators) and in the second part I will address the conjecture that such generic IR properties can be derived from asymptotic symmetries of an asymptotically flat gravitational system. I will point out the assumptions needed for the conjecture to work in d=4, and comment on the d>4 generalization and the associated difficulties. This conjecture is taken as an important input of a putative model of holography in asymptotically flat spacetimes. We will discuss to which degree the optimism in this direction is justified.
The first part is mostly based on https://doi.org/10.1007/JHEP10(2017)065 . The second part builds up on several papers, the most recent of which is arXiv:2005.03439 + unpublished notes.
Elliot Bridges: One loop BCJ numerators from string amplitudes.
Recent work has led to the development of string amplitude formulae at one loop up to seven points using pure spinors. However, in field theory one loop amplitudes satisfying BCJ identities have only been constructed with pure spinors up to five points. In this talk I will briefly review how the string amplitudes were found, and then discuss some recent work I have been a part of to take these results from string theory and produce field theory amplitudes which satisfy BCJ identities.
Davide Bufalini: WZW Model, Part II. From Little String Theory to Black Hole Microstates.
In this second episode of the Wess-Zumino-Witten saga, I will give a brief introduction to the so-called “Little String Theory” from different perspectives and highlight its relation with gauged WZW models and Black Hole Microstates. Finally, I will present my current work and possible future directions.
Aaron Poole: The Λ-BMS_4 charge algebra
In this talk I will discuss the recent paper https://arxiv.org/abs/2004.10769 by Compère, Fiorucci and Ruzziconi. I will begin with an introduction to the main technology they use, namely the covariant phase space formalism (as developed by Wald et. al in the 90s) and how one uses this to compute charges in diffeomorphism covariant physical theories (such as general relativity). I will briefly review some of the past literature regarding how these techniques have been applied to both asymptotically flat and asymptotically (locally) AdS spacetimes, with particular regard to deriving black hole entropy and describing gravitational radiation. Finally, I will summarise the main results of the paper and comment upon how they extend the current literature on charges in asymptotically locally (A)dS_4 spacetimes.
Jacopo Sisti: Introduction to Topological Insulators and Symmetry Protected Topological Phases of Matter
SPT phases are gapped phases which cannot be deformed to a trivial phase without undergoing a phase transition if the symmetry is preserved. Even though they have short-range entanglement and a unique ground state, they show either gapless excitations or topological order degrees of freedom localized at their boundary. In this talk, I will present an introduction to SPT phases by first considering some specific examples such as the integer quantum Hall effect and the time-reversal band topological insulators. Then, I will briefly describe their classification and how it is affected by the introduction of interactions.
Enrico Parisini: An introduction to the ambient space
I will present the main features of the ambient metric construction by Fefferman and Graham. To this end, I will start by reviewing the embedding space from a geometrical point of view and show how the ambient space can be seen as one possible generalization of it. Finally, I will describe how the theory of ambient spaces relates to the usual Fefferman-Graham near-boundary expansion in holography.
Linus Too: Quick review on entanglement wedge reconstruction and the information paradox
There have been many interesting approaches trying to resolve the black hole information paradox. In the paper ‘Entanglement Wedge Reconstruction and the Information Paradox’ (1905.08255), it claims by using entanglement wedge reconstruction in the setting of AdS/CFT the information inside the evaporating black hole eventually gets out of the horizon. More precisely, one of the results of the paper shows if we extract the Hawking radiation of the evaporating black hole from asymptotic boundary of AdS into an auxiliary space then the plot of the entanglement entropy between this auxiliary space and the boundary CFT as a function of time reproduces the Page curve. In the presentation, I will be giving a quick review on the background and results of this paper.
Daniel Hasenbichler: Strong Cosmic Censorship in 4D-Reissner-Nordstrom-de Sitter (RNdS)
In 2018 it was shown [1] that the Strong Cosmic Censorship (SCC) conjecture is violated in 4D-RNdS for a massless scalar field matter model in a near-extremal region of the RNdS parameter space. This was done by computing the quasinormal mode (QNM) spectrum of the scalar field in that background. A substantial part of my MSc dissertation was devoted to the reproduction of the numerical results in that paper. My calculations were done using two different methods. One is a generalisation of the pseudospectral approach developed in [2] from the Schwarzschild-de Sitter case to the more difficult case of the RNdS black hole. This method is also the one used in [1], although the brief treatment in the Phys. Rev. Lett. paper does not explain explicitly how the quasinormal modes have been computed. The second approach is a shooting method and involves fitting Frobenius expansions to the numerical solution of the ODE describing the QNMs. I will discuss the SCC conjecture in this context and go through the calculations that I did for my MSc dissertation.
References:
[1] https://arxiv.org/abs/1711.10502
[2] https://arxiv.org/abs/1709.09178
Alex Davey: Strong Cosmic Censorship in Higher Dimensions
The Strong Cosmic Censorship (SCC) conjecture asserts that Cauchy Horizons are generically singular. I will give an introduction to SCC, the state of this conjecture in 4d, and then discuss work that I am currently doing with Óscar Dias and Paul Rodgers on SCC in higher-dimensional spacetimes.
Marika Taylor: The holographic dictionary
Sami Rawash: 4-point functions in AdS_3/CFT2
I will discuss a version of the information loss problem in AdS, which comes from observing that 2-point functions exhibit exponential decay at late times. In order to understand how information is stored in the black hole, it is interesting to study 4-point functions with two heavy (H) and two light (L) operators. I will summarize the techniques used in 1606.01119, 1705.09250 and 1710.06820, where a simple class of these correlators has been studied both in the D1-D5 CFT and in the dual AdS_3xS^3 description. For large central charge c, the authors showed that unitarity is recovered already at leading order in large c. I will then summarize how to extract the LLLL correlator from the HHLL result using some bootstrap techniques (1812.06479).
Adam Chalabi: A lightning introduction to higher-form symmetries
Determining the vacuum structure of 4d Yang-Mills theory at non-vanishing theta-angle is a notoriously hard problem. Even though to this day it is still a poorly understood question, considerable progress has been made in recent years by studying a generalisation of ordinary symmetries – so-called higher-form symmetries – and their interplay with oridnary discrete symmetries through anomalies (1703.00501). In my talk, I will focus on these higher-form symmetries which were first discussed in (1412.5148). I will give a pedagogical introduction, and use numerous examples to illustrate these new ideas.
Andrew Spiers: Perturbatively Constraining the BMS Freedoms for Second-Order Self-Force Calculations
Second-order Self-Force calculations will be required for highly accurate parameter extraction from LISA measurements of extreme mass ratio inspirals (EMRI). Such data will allow for precise tests of General Relativity in the strong field regime. A second-order Teukolsky equation seems the most viable avenue for second-order Self-Force calculations in Kerr; we wish to design the equation such to constrain all the non-physical degrees of freedom (in order to obtain gauge-invariant results). The BMS symmetry group’s surprisingly infinite dimensional nature has intrigued physicists for nearly 60 years, in this case the abundance of freedoms become troublesome. In this talk I will discuss practical methods of constraining the BMS frame in perturbation theory, the limitations of constraining the BMS freedoms within a Kerr background spacetime, and give potentially practical measures to overcome these difficulties for EMRI models. After, to address the BMS frame of first-order perturbations, I will present our progress in constraining the infinitesimal BMS degrees of freedom.
Marika Taylor: Replica wormholes
I will discuss recent work on replica wormholes, including e.g. 1911.12333.
Paul Rodgers: Evading no hair theorems: Hairy black holes in a Minkowski box.
The "black hole bomb" system, whereby a Reissner-Nordstrom black hole confined inside a reflecting "box" is perturbed by a scalar field, can become unstable. I will talk about recent work to numerically extend the perturbative solutions of https://arxiv.org/abs/1802.01603 in order to find the full non-linear "hairy" solutions to the system. First I will motivate the above discussion and then present some preliminary results.
Ivan Muñoz: A Witten diagram primer
In AdS/CFT computing correlation function in the CFT side corresponds to compute Witten diagram in the bulk. The ingredients for Witten diagrams are the set of bulk vertices, and the bulk-to-boundary propagator and the bulk-to-bulk propagators. We will work in the embedding space formalism to simplify the computation of Witten diagram and using the harmonic decomposition of the bulk-to-bulk propagator to split it into two bulk to boundary propagator.
Davide Bufalini: Unoriented 4D Quiver Gauge Theories from Toric Calabi-Yau Singularities
I will present, in the most pedagogical way possible, the content of my Master's degree thesis. After recalling some basic definitions in complex geometry and explaining the importance of Calabi-Yau manifolds in String Theory compactifications, I will discuss how (in the context of AdS/CFT) we can obtain 4D N=1 or N=2 quiver gauge theories living on the worldvolume of (fractional) D3-branes probing (toric) Calabi-Yau singularities. I will discuss how fractional branes on (abelian) orbifold singularities can naturally lead to quiver gauge theories, whose superpotential can be easily derived by looking at the associated dimer (brane tiling). I finally discuss how the presence of O3/O7 Orientifold planes affects the local physics of these models and the anomaly/RR Tadpole cancellation condition.
If time permits, I will briefly discuss the meaning and the importance of (crepant) resolutions of orbifold singularities. The resolution of the well-known conifold theory will be an example.
A very well-written reference for complex and toric geometry that I strongly suggest is: https://arxiv.org/abs/hep-th/0702063
A standard reference for the physical motivations is: https://arxiv.org/abs/hep-th/9810201
For unoriented quiver gauge theories see: https://arxiv.org/abs/1307.0466
For applications of dimers see: https://arxiv.org/abs/0803.4474
Aaron Poole: Cosmic footballs from superrotations
In this talk I will be discussing the recent paper https://arxiv.org/abs/1910.05435. In order to do this, I will first give a brief introduction to 4-dimensional asymptotically flat gravity in the Bondi gauge and the BMS group, with particular emphasis on the role that the `superrotations’ play. Once I have set this up (and reviewed some of the recent literature), I will discuss the main conclusions of the paper.
Elliot Bridges: BCJ relations and the double copy
In 2008 Bern, Carrasco, and Johansson proposed a new set of relations between Yang-Mills amplitudes. With this came an associated means of construction which allows for the expression of gravity amplitudes as the double copy of Yang-Mills amplitudes. In this talk I intend to provide an overview of the original paper on this, https://arxiv.org/pdf/0805.3993.pdf , a follow up paper where the link with gravity amplitudes was further developed, https://arxiv.org/pdf/1004.0476.pdf , and then how these relations can be reproduced using techniques from the pure spinor formalism of sting theory https://arxiv.org/pdf/1510.08846.pdf
Federico Capone: Quantum chaos and holography
I will discuss the main ideas and technical points concerning some of the original papers on quantum chaos and holography:
1. https://doi.org/10.1007/JHEP08(2016)106