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 

01/10/24 - Organizational group meeting, 54 / 8031 (8C)

08/10/24 - ArXiv Club - 54 / 4001 (Ketley)
Group members are encouraged to sign up to talk for ~10-15 minutes about a recent (ish) paper they find interesting. Please add these papers to this list.  

15/10/24 - Javier Carballo - 54 / 7035 (7B)

22/10/24 - Paolo Arnaudo - 54 / 7035 (7B)

29/10/24 -  ArXiv Club - 54 / 5027 (5A)
Group members are encouraged to sign up to talk for ~10-15 minutes about a recent (ish) paper they find interesting. Please add these papers to this list.  

05/11/24 - Chandramouli Chowdhury - 54 / 5027 (5A)

12/11/24 - Radu Moga - 54 / 4001 (Ketley)

19/11/24 - ArXiv Club - 54 / 7035 (7B)
Group members are encouraged to sign up to talk for ~10-15 minutes about a recent (ish) paper they find interesting. Please add these papers to this list.  

26/11/24 - Jan Becker - 54 / 7035 (7B)

03/12/24 - Chen Huang - 54 / 7035 (7B)

10/12/24 - ArXiv Club - 54 / 7035 (7B)
Group members are encouraged to sign up to talk for ~10-15 minutes about a recent (ish) paper they find interesting. Please add these papers to this list.  

[Winter break: 16 Dec 2024 - 10 Jan 2025]

[Exam period: 13-24 Jan 2025]

Semester 2 

28/01/25 - Elisa Marieni - 54 / 7033 (7C)

04/02/25 - Ben Suzzoni - 54 / 7033 (7C)

11/02/25 -String+Modelling seminar. Pizza in Ketley room at 12:30 (sign up for pizza) - Johannes Hofmann - 54 / 7035 (7B)

18/02/25 - James Ratcliffe - 54 / 10031 (10C)

25/02/25 - Ritam Sinha (KCL) Student session until 13:20 -  54 / 10031 (10C)

04/03/25 - David Sola Gil - 54 / 7033 (7C)

11/03/25 - Ernesto Bianchi -  54 / 7033 (7C)

18/03/25 - Orestis Papadpoulos - 54 / 7033 (7C)

25/03/25 - Altay Etkin - 54 / 7033 (7C)

[Spring break: 31 March 2025 through 25 April 2025]

29/04/25 - Alex Ratcliffe - 54 / 7033 (7C)

06/05/25 - NO JOURNAL CLUB - STAG COLLOQUIUM  (54 / 7035 (7B) )

13/05/25 -  Arvind Shekar - 54 / 7033 (7C)

[Exam period 19 May 2025 - 6 June 2025]

Titles and Abstracts (reverse chronological order):

Topics: (reverse chronological order)

Link to journal club topics

Arvind Shekar: Virasoro identity block and the reparametrization mode formalism.

The Virasoro identity block in a 2-dimensional CFT is universal, and gives the dominant contribution to pure gravity processes in AdS$_3$ in the context of the AdS$_3$/CFT$_2$ holography. The identity block has been explored in the HHLL limit where two operators are heavy (conformal dim $\sim$ central charge $c$) and the other two are light (conformal dim $\sim O(c^0)$), where only certain quantum ($O(1/c)$) corrections have been explored. Extending this analysis to incorporate all quantum corrections in a tractable form remains a challenge.

In a generic CFT$_2$ the identity block corresponding to the four-point function $\langle AABB \rangle$ can be expressed, as a correlator involving two Bilocal operators built from “reparametrization modes”, accompanied by an effective theory governing their correlators/ dynamics. This can be organised as a perturbative expansion in the order of the reparametrization modes and has a systematic set of Feynmann diagrams. It enables efficient computation of the identity block as simple contractions of the reparametrization modes or as simple sums involving Virasoro mode correlators, with no heavy computations such as certain integrals that are typically required in other methods. The reparametrization mode also admits a natural interpretation as graviton exchanges in AdS$_3$. I will discuss this formalism and how we can use this to compute quantum ($O(1/c)$) corrections to the HHLL correlator and certain challenges in its computation. I will also discuss some applications of this formalism and how it can be generalised to other computations.  

Alex Ratcliffe: The exact WKB method for quasinormal modes

Black holes have a characteristic ringing known as quasinormal modes. Studying these modes and their corresponding frequencies is vital for understanding problems in gravity and hydrodynamics via holography. One way to study these modes is by the WKB method either with a natural small parameter or by introducing an artificial one. This finds the modes perturbatively as a series in this small parameter but can miss non-perturbative contributions which are essential for some problems. Fortunately, such issues are well understood in quantum mechanics and a generalisation known as the exact WKB method allows one to find these non-perturbative contributions. In this talk, I will discuss how the exact WKB method can be adapted to study quasinormal modes by exploring a toy problem and the issue of strong cosmic censorship for charged de Sitter black holes.

 Altay Etkin: Unimodular Jackiw-Teitelboim gravity and de Sitter quantum cosmology

In this talk, we show that a gauge-theoretic description of Jackiw-Teitelboim (JT) gravity naturally yields a Henneaux-Teitelboim (HT) unimodular gravity via a central extension of its isometry group, valid for both flat and curved two-dimensional spacetimes. HT gravity introduces a unimodular time canonically conjugate to the cosmological constant, serving as a physical time in quantum cosmology. By studying the mini-superspace reduction of HT2 gravity, the Wheeler-DeWitt equation becomes a Schrodinger-like equation, giving a consistent and unitary quantum theory. Analysis of the wavefunction's probability density reveals a quantum distribution for the scale factor, offering a quantum perspective on the expansion and contraction of the universe. In this perspective, the possibility of reaching the singular point signals that topology change could occur.

arXiv number: 2501.17213

Orestis Papadpoulos: Improved Nuclear Matter in Holographic QCD

The AdS/CFT correspondence can give us new insight into the strong coupling regime of QCD. With the aim of studying dense nuclear matter, we employ the top-down Witten-Sakai-Sugimoto model, which in the large t’Hooft coupling limit enables us to translate calculations of classical gravity to calculations in our field theory. After introducing the model, along with how to include isospin (a)symmetric baryons, we use the so-called homogenous ansatz to find all possible solutions containing one and two baryon layers. This not only helps us in checking which phase is energetically preferred, and where, but also to improve previous results for basic low density nuclear matter properties in holography. This is a first step in constructing more realistic holographic neutron stars.

Ernesto Bianchi: AdS amplitudes as CFT correlators

One of the fundamental checks of the AdS/CFT conjecture has been the obtention of CFT correlators starting from an AdS gravitational path integral. Early work explored this relation to leading order in the correspondence where the bulk side gets approximated by classical supergravity, with more recent work extending this beyond tree-level. In this talk, I will show that the boundary correlators obtained from the AdS/CFT correspondence do indeed correspond to CFT n-point functions to all order in the bulk loop expansion, by explicitly showing they satisfy the conformal Ward identities. Based on: 2412.09503.

David Sola Gil:  Scalar instabilities in BTZ, boson stars and hairy black holes.

In this talk, I will present ongoing work on the study of scalar perturbations in the BTZ background. In asymptotically AdS backgrounds, for scalar masses between the unitarity and the Breitenlohner-Freedman bound, both asymptotic decays of the scalar field yield normalizable modes. This allows us to impose the so-called mixed or double-trace boundary condition, which interpolates between Dirichlet and Neumann boundary conditions. The BTZ and the global AdS backgrounds can be unstable, to superradiance and/or scalar condensation, under such perturbations with mixed boundary conditions (although not with Dirichlet or Neumann). There are regions where BTZ is unstable, but AdS is stable. This BTZ instability signals the existence of solitonic solutions and hairy BTZ black holes, which we aim to construct numerically and perturbatively. I will summarize our setup, current findings and expectations. 

Ritam Sinha (KCL): On the Fusion of Conformal Defects

We consider the fusion of a pair of p-dimensional conformal defects, embedded in a d-dimensional conformal bulk manifold, that are approximately parallel to each other. We argue that the appropriate description of the resulting geometry in the fusion limit is in terms of an RG flow, whose IR limit consists of an effective field theory (EFT) supported on a new emergent conformal defect. We provide an explicit construction of terms in the EFT in a derivative expansion in the distance between the defects and using diffeomorphisms and Weyl invariance. Our construction holds for a set of defects with any co-dimension, shape and topology, as long as they are non-intersecting. As an application of our framework, we compute the high energy asymptotics of the density of bulk one-point functions (with and without spin) in the presence of a defect.

James Ratcliffe: BPS bounds and meromorphic mass in AdS/CFT with flavour

In 10 and 11 dimensional flat space, branes and certain intersections of branes are known to obey BPS bounds with associated spacetime and worldvolume central charges, respectively. In particular curved spacetimes, the stability of such branes and their intersections is understood in the context of calibrated geometries. However, less attention has been paid to the explicit form of worldvolume central charges in curved backgrounds. I will present a D7-D7-brane intersection in AdS_5 x S^5 and introduce a BPS bound involving a generalised worldvolume central charge. I will show that meromorphic embedding functions saturate this bound, and so simultaneously satisfy the appropriate equation of motion exactly. I will also discuss the holographic interpretation of such solutions, in the context of matter multiplets in the dual field theory with a meromorphic mass term.

Johannes Hofmann  (University of Gothemburg, Sweden):  Exploring pairing effects and scale symmetries in mesoscopic Fermi quantum gases

I will introduce recent advances in the preparation of mesoscopic Fermi gases, which are gases that contain only a small number of atoms stored at ultracold temperatures in optical traps, and discuss how fundamental quantum-mechanical properties, such as the energy levels and the wave function, can be directly measured in experiments. Since both the atom number and the interatomic interactions are precisely controlled these systems provide a unique experimental platform to explore how collective behaviour of many-body systems emerges from interactions between individual particles. In particular, I will highlight how the emergence of superfluid pairing can be probed and how the crossover from few to many particles is captured theoretically. Finally, I will present a proposal to test the implication of fundamental symmetries — scale and conformal invariance — in these confined few-body gases.

Ben Suzzoni:  An interpolating index for 4d N=2 SCFTs 

By coupling a flat-space supersymmetric theory to a rigid supergravity background, we may define its curved space analogue, while preserving some amount of supersymmetry. The partition function of such a theory placed on S3xS1 determines an index with fugacities related to the various background supergravity fields. In the first part of the talk, I will briefly review how this construction works for 4d N=2 SCFTs. With a suitable choice of supergravity background fields, I will show how to recover the superconformal index and the twisted index. In the second part of the talk, I will construct a so-called "interpolating" supergravity background which takes us from the superconformal to the twisted configuration, while preserving one supercharge. I will further show that this interpolation is in fact exact and the resulting partition function is independent of the interpolating parameter. This allows us to identify the superconformal index with the twisted one. I will conclude by commenting on that identification of the indices. 

Elisa Marieni: Lattice defect networks in 2d Yang-Mills

We construct defect networks in pure Yang-Mills theory in two dimensions using a refinement of the lattice approach. The refinement preserves the locality properties of individual defects, and is compatible with solvability of the theory via subdivision invariance. We explicitly demonstrate closure of the building blocks under fusion. 

Chen Huang: Higher-point gauge-theory couplings of massive spin-2 states in four-dimensional string theories

We explicitly compute the Neveu-Schwarz sector conventional type-I superstring tree-level amplitudes at five points after compactifying to 4D, express the QFT building block in the helicity basis, and give several attempts toward arbitrary points. More specifically, we consider the interaction of one first excited level and otherwise massless states of conventional type-I superstrings, where the four-dimensional states can, for instance, be realized via D3 branes. We construct the amplitude by using the Berends-Giele currents. From the recursion of Berends-Giele currents, we can generate the higher point amplitude. We also apply the BCFW recursion with massive external legs shifted and get the amplitude for arbitrary points.

Jan Becker: Two Charge Black Holes and the Fuzzball Proposal 

One of the proposed frameworks in string theory which aims to resolve the entropy problem for black holes, and the black hole information paradox is the fuzzball proposal. The fuzzball proposal conjectures that the quantum gravitational effects in a black hole are not confined to a Planck-scale region near the curvature singularity but are significant on the scale of the horizon. The proposal claims that the classical description of black holes is an approximate effective description, valid for certain physical processes, and that the underlying microscopic description is given by horizon-scale and horizonless quantum stringy bound states, known as fuzzballs. In this talk, I will first discuss the results of the fuzzball program for the most-studied example of the two-charge D1-D5 black hole. I will describe the construction of the microstates for the D1-D5 black hole from three different perspectives: in terms of bound states of branes, as supergravity solutions, and as heavy pure states in the holographically dual D1-D5 CFT. I will conclude by discussing the work that I have done so far during my PhD and possible future research directions. This work involves the extension of the precision holographic dictionary for the D1-D5 system, in order to study three-charge D1-D5-P microstates. 

Radu Moga: Recursions, Singularities, and Soft Limits for Fermions in AdS

Study of correlation functions in AdS/CFT and in-in correlators in de Sitter space often requires the computation of Witten diagrams. Due to the complexity of evaluating radial integrals for these correlators, several indirect approaches have been developed to simplify computations. However, in momentum space, these methods have been limited to fields with integer spin. In this talk I will present tools for evaluating Witten diagrams with massless spin$-\frac12$ fields in momentum space and discuss where they differ from the corresponding integer-spin analysis. I will discuss their singularities and soft limit which leads to a Weinberg-like soft theorem for gauge fields coupled to matter in AdS and show that the universal terms in the leading soft factor are sensitive to the spin of the matter field. Time permitting, I will show how appropriate Weight shifting operators with respect to the external kinematics can be used to obtain the generalization to fermions with integer mass.

Paolo Arnaudo: Analytic perturbative methods for black hole perturbations 

In this talk, we will discuss analytic methods that provide the quantization condition for the quasinormal mode frequencies of linear perturbation fields around different black hole geometries. More precisely, we will consider the asymptotically de Sitter, anti-de Sitter, and flat Schwarzschild black holes in four dimensions.

This talk is based on the research I conducted throughout my PhD. 

Javier Carballo:  Arbitrarily long-lived linear black hole perturbations

Quasinormal modes (QNMs) of black holes are governed by a non-normal Hamiltonian with respect to a natural choice of inner product, and thus do not form a complete and orthogonal basis. I will show that non-orthogonality leads to the existence of arbitrarily long-lived linear perturbations constructed from finite sums of M QNMs, whose lifetimes scale as log(M). Such perturbations are realised as localised energy packets travelling along the future horizon (and future null infinity). I will discuss simple analytical examples for a massive scalar field in the static patch of de Sitter, as well as numerical examples for gravitational perturbations in the Schwarzschild black hole, using hyperboloidal foliations.