In person, Tuesdays, 1:00-2:45, unless otherwise stated.
Room location is variable, please check each week.

Note: dates are subject to change, to give flexibility for topics to possibly run over into a second session.

Semester 1 

03/10/23 - Chandramouli Chowdhury & organizational group meeting - 54/7033 (7C)

10/10/23 - David Vegh (QMUL) (String Seminar - 1:00-1:20 student session, 1:20-2:20 main seminar) - 06/1081 (L/R B)

17/10/23 - arXiv club - 06/1083 (L/R C)
                          All attendees are encouraged to talk for ~5 minutes about a recent paper they find interesting. Please add these papers to this list.

24/10/23 - Kuo-Wei Huang - 54/7033 (7C)

31/10/23 - Deepali Singh - 54/7033 (7C)

07/11/23 - arXiv club - 54/7033 (7C)
                          All attendees are encouraged to talk for 5 minutes about a recent paper they find interesting. Please add these papers to this list.

14/11/23 - Zezhuang Hao - 54/4001 (Ketley room)

21/11/23 - Arvind Shekar, TBC - 02/1083 (L/T B)

28/11/23 - arXiv club, TBC - 54/7033 (7C)
                          All attendees are encouraged to talk for 5 minutes about a recent paper they find interesting. Please add these papers to this list.

05/12/23- Junggi Yoon (APCTP) (String Seminar - 1:00-1:20 student session, 1:20-2:20 main seminar) - 06/1081 (L/R B)

[Winter break: 11 Dec 2022 - 12 January 2023]
[Exam period: 15-26 Jan 2023]

Semester 2 

30/01/24 - arXiv club - 54/10031 (10C)
                          All attendees are encouraged to talk for 5 minutes about a recent paper they find interesting. Please add these papers to this list.

06/02/24 - Javier Carballo - 54/10031 (10C)
Followed by a discussion on University library book retention 

13/02/24 - David Sola Gil        - 54/10031 (10C)

20/02/24 - Group meeting: discussion with the RSG (https://rsgsoton.net) followed by an arXiv club - 54/10031 (10C)
  An opportunity to discuss our group's scientific computing requirements.

27/02/24 -  Elisa Marieni - 06/1081 (L/R B)

05/03/24 -  Alex Ratcliffe - 54/10031 (10C)

12/03/24 -  arXiv club - 06/1081 (L/R B)

19/03/24 - Radu Moga - 32/3077 *note unusual location*

[Spring break: 25 March - 19 April 2024]

17/04/24- Gong show! - 54/4011 (L/T A) *note: Wednesday*
All string theory students and postdocs are expected to present a 3 minute (TBC), one slide presentation of their project. Staff and members of other groups are also welcome to participate!

23/04/24 - arXiv club - 58/1007 (L/R C)
All attendees are encouraged to talk for 5 minutes about a recent paper they find interesting. Please add these papers to this list.

30/04/24 - Altay Etkin - 58/1007 (L/R C)

07/05/24 - Adrián Sanchez Garrido - 58/1007 (L/R C)

14/05/24 -  Ernesto Bianchi  - 54/10031 (10C)

Topics: (reverse chronological order)

Link to journal club topics

 Ernesto Bianchi: AdS/CFT @ Loop Order

Even though AdS/CFT is conjectured to be true non-perturbatively, most work have been done at the leading order of the correspondence, where the AdS side is approximated by classical gravity and the CFT side by the most leading terms in the 1/N expansion. In this talk we will go one step further and analyse the effect of subleading corrections to both sides of the duality. As a testing ground we will study the simplest yet non-trivial case, an interacting scalar theory in the bulk. At loop order, new divergences appear. We will discuss their renormalization and physical implications.

Adrián Sanchez Garrido : Krylov complexity, chaos and black holes

Krylov complexity is a recently proposed notion of quantum complexity, applicable to both quantum states and to observables, which is unambiguously well-defined given an initial state, a time evolution generator, and an inner product in the Hilbert space of interest. It has been suggested as a probe of quantum chaos and, relatedly, as a plausible candidate for being the complexity item in the holographic dictionary. In this talk I will present recent results on the Krylov complexity profile of operators up to exponentially late time scales for the SYK model as well as for the integrable XXZ spin chain, together with a spin chain that interpolates between integrable and chaotic regimes. In the final part of the talk I will present a recent result where we showed analytically that Krylov complexity of the infinite-temperature thermofield double state in the low-energy regime of the double-scaled SYK model maps to bulk length in JT gravity, providing the first explicit matching between a notion of quantum complexity and bulk length in a concrete instance of two holographically dual systems.

Altay Etkin: Unimodular time and the topological evolution of invariants.

The absence of a physical time in gravitational physics is an important issue that has been studied in various theories, notably unimodular gravity. This talk is based on my master's project back at Imperial. We relate physical time with the topology of magnetic field vortices. We base ourselves on a formulation of unimodular gravity where the cosmological constant appears as the canonical dual to a variable which on-shell becomes four-volume time. If the theory is restricted to a topological axionic form (viz. a parity-odd product of an electric and a magnetic field),  such a time variable becomes the spatial integral of the Chern-Simons density. The latter equates to helicity, so that unimodular time is transmuted into the linking number of the vortices of the topological magnetic field, times their flux. With the added postulate  that this flux is a universal constant, the flow of time can thus be interpreted as the progressive weaving of further links between magnetic field vortices, each link providing a quantum of time with value related to the fixed flux. Non-abelian extensions, and lower dimensional gravitational settings are also briefly examined.

Reference: arXiv:2311.11160

Radu Moga: A Review of Holographic Renormalization 

The AdS/CFT correspondence has been extensively studied at tree-level for the past 25 years. More recently, there has been a better understanding of subleading large-N corrections on the CFT side. Through the duality, these subleading corrections correspond to loops in the bulk theory. This motivates the development of a solid understanding of how to do loop computations in AdS. In this talk, I will give a review of the correspondence from the bottom-up point of view, introducing the relevant ingredients for computing correlation functions of gauge invariant boundary operators. I will illustrate the formalism through an example of a simple scalar theory. Since scalar loops have recently been computed, the next step is to consider spin-one fields, for which I will present the main issues as well as some ongoing work. 

 Alex Ratcliffe: The Hydrodynamic Attractor via Resurgent Transseries

In many problems tackled by perturbation theory we encounter diverging series. Moreover, there are non-perturbative contributions which are missed by this approach.

In this talk I will cover how we can obtain analytic results from diverging series via resummation, how the perturbation theory "knows" about non-perturbative effects via resurgence and how transseries provide the framework to capture these relations. As an explicit example I will discuss how solutions to relativistic viscous hydrodynamics in the MIS model are given by a resurgent transseries. This then explicitly reveals how the system settles to an attractor regardless of the initial condition given and provides results more in line with numerical calculations.

Elisa Marieni: 2d Yang-Mills: a lattice approach 

We do not have a mathematically rigorous definition of quantum field theory, but we have a definition of topological quantum field theory. It is believed that this definition could be used as a guiding principle to define QFTs as well. The main reason for this is that QFTs seem to be organised by the same categorical structure that organises TQFTs.

In this talk I will present the setting for my work with Itamar, which is pure 2d Yang Mills theory. This theory is well known and it has been extensively studied. I aim to show some key features of 2dYM, predominantly its exact solvability, to motivate why this should be a good testing ground for the purpose of connecting the definition of TQFTs to QFTs.

David Sola Gil: Strong Cosmic Censorship in Kerr-Newman de-Sitter

Predictability is an important characteristic of any physical theory. Within the context of the initial value formulation of GR, Penrose asserted predictability via the Strong Cosmic Censorship conjecture. Such conjecture claims that, given well-posed initial data for Einstein’s and matter field equations, classical physics should be uniquely determined from such initial data. An updated version of Penrose’s Strong Cosmic Censorship: Christodoulou’s Strong Cosmic Censorship (CSCC), has been classically shown to be respected by Kerr de-Sitter black holes and violated by near-extremal Reissner-Nordström de-Sitter black holes. To understand the transition from violating to respecting CSCC, we have studied the Kerr-Newman de-Sitter spacetime, which contains both Kerr de-Sitter and Reissner-Nordström de-Sitter as limiting cases. In this presentation, I will motivate and review CSCC, explain how QNMs arising from massless, uncharged linear scalar field perturbations in Kerr-Newman de-Sitter allow us to probe CSCC and present our main results.

Javier Carballo: Spectral stability of de Sitter QNMs 

There has been recent interest in studying the stability properties of the quasinormal mode (QNM) spectra of black holes under additional “environmental perturbations” to the system for astrophysically motivated reasons. Using pseudospectra techniques and arbitrary perturbations to their potential, asymptotically flat and (anti-)de Sitter black holes’ QNM spectra have been deemed highly unstable under very small perturbations. This is particularly interesting in the case of AdS black holes, since the real-time holographic dictionary identifies their QNM frequencies as the poles of the retarded 2-point function (in momentum space) in the dual CFT. In this talk, I will present ongoing work in my PhD with Ben in which we explore the stability of QNM spectra in various contexts. 

Junggi Yoon (APCTP, Pohang): Gravitational edge mode for 2D gravity.

In this seminar, I will discuss the gravitational edge mode of the two-dimensional gravity. First, I will review the gravitational edge mode from the bosonic JT gravity. I will discuss the issues related to dilaton and PSL(2,R) gauging in the derivation of the Schwarzian action. I will revisit the derivation of Schwarzian action for the edge mode by using the inversion formula of Schwarzian derivative. I will explain how to obtain N=1 super-Schwarzian action. I will also discuss the relation between osp(2,1) gauging and isometry.

Arvind Shekar: Entanglement Entropy via replica trick and theories with islands in d dimensional static backgrounds 

Entanglement entropy quantifies the degree of entanglement between two quantum systems or between two subregions in a QFT and hence is an important tool to understand the quantum system. Certain tricks (Replica) and holographic duals (Ryu-Takayanagi Area) have been used to calculate this measure. However, its study in dimensions > 2 has been mostly limited to flat backgrounds and CFT vacuum states in specific subregions due to technical as well as conceptual difficulties. I will present our work trying to bridge this gap, by calculating the EE for a CFT state in a subregion in higher dimensional static black hole backgrounds using the replica trick. In the presence of gravity, the quantum corrections to the black hole entropy is crucial in resolving the information paradox. Recently, its study in 2 dimensions led to the island proposal as a potential correction required to preserve unitarity in black hole evaporation. We will use our result for EE in higher dimensional black hole background, to discuss if islands exist in higher dimensions (d > 2) and obtain constraints on the QFT for islands to exist.  

Zezhuang Hao: Flat holography dictionary

Abstract: Ads/CFT correspondence has been brought up over 25 years and, from the bottom-up point of view, the duality is clearly classified by the match of the data between the effective theory in the AdS bulk and the conformal theory on the boundary. Here in this talk, based on the AdS/CFT dictionary, we introduce the construction of flat/CFT dictionary which gives us the duality relation between the effective theory on Minkowski and the proposed QFT theory on the celestial sphere.

Deepali Singh: Bosonic and Fermionic fluctuations in type IIB string theory

Abstract: I will talk about Bosonic and Fermionic fluctuations over a classical string solution in a general curved background. The goal is to obtain compact expressions for these fluctuations in semi classically quantized strings and interpret the results in terms of the intrinsic and extrinsic geometric invariants of the background. This work extends the results of the 2015 paper by Forini et al (arxiv: 1507.01883v3). I will consider a few examples and see how these results can be used to calculate corrections to gauge invariant quantities like the vev of the Wilson loop operator, free energy, etc, and match with existing results. For this analysis, we have considered a general target space background along with the antisymmetric 2-tensor, the B field. 

Kuo-Wei Huang (Southampton): From d=2 Virasoro blocks to something analogous in d=4 

Abstract: Motivated by generalising the Virasoro vacuum block in d=2 CFTs to higher dimensions, I will discuss recent progress in understanding the multi-stress tensor sector of the heavy-light four-point scalar correlator in holographic CFTs in d=4. By focusing on the correlator in the near-lightcone regime, I will point out that, at certain values of the conformal dimension of the light probe operators, the d=4 correlator obeys a system of linear differential equations that share striking similarity with the d=2 BPZ equations at large central charge. 

Chandramouli Chowdhury (Southampton): Witten Diagrams in Momentum-Space at Loop Order

Abstract: I will review the computation of the Wave function of the universe in momentum space for scalars. I will then describe how to compute these more efficiently and describe some new results for loop-level integrals. If time permits, I will discuss some work in progress related to how these are useful in computing in-in correlators in cosmology, some hidden simplifications for integrands, and also some thoughts on renormalizing them.