Format of the Journal club

1) PGR seminar, usually about own work (30 min + questions)
2) OPTIONAL: Deep dive into a recent paper (30 min incl. questions and discussion)
3) arxiv Discussion. Please add these papers to this list.  

Schedule

Semester 1 

24/09/25 - 54 / 5025 (5B) (TBC) - Organizational group meeting

01/10/25 - 54 / 7035 (7B) - Rowan Wright (External speaker)

08/10/25 - 54 / 7035 (7B) - No Journal club (Maths Forum)

15/10/25 - 54 / 7035 (7B) - Radu Moga + arxiv discussion

22/10/25 - STAG Lecture - No Journal club

29/10/25 - 54 / 7035 (7B) - Orestis Papadopoulos + Deep dive (Javier Carballo) + short arxiv discussion

05/11/25 - 54 / 7035 (7B) - Dongsheng Ge (External speaker)

12/11/25 - 02 / 3043 - (Student session from 13:00-13:20) - Ayngaran Thavanesan (External speaker)

19/11/25 - 02 / 3043 - Andrea Conti (External speaker)

26/11/25 - 02 / 3043 - Javier Carballo (1 hr seminar) + short arxiv discussion

03/12/25 - 02 / 3043 - Arxiv club. 

10/12/25 - NO PGR SEMINAR THIS WEEK DUE TO STAG COLLOQUIUM

17/12/25 - 54 / 7035 (7B) - Arvind Shekar

[Winter break: 14 Dec 2025 - 4 Jan 2026]

[Exam period: 12-23 Jan 2026]

Semester 2 

28/01/26 - Altay Etkin

04/02/26 -  David Sola Gil

11/02/26 - Daniel Baig

18/02/26 - Giorgio Belleri

25/02/26 - James Ratcliffe

04/03/26 - Chen Huang

11/03/26 - Jan Becker

18/03/26 - Elisa Marieni

[Spring break: 22 March 2026 through 19 April 2026]

22/04/26 - Alex Ratcliffe

29/04/26 - 

06/05/26 - 

13/05/26 - 

[Exam period 18 May 2026 - 6 June 2026]

Titles and Abstracts (reverse chronological order):

Javier Carballo: Complete mode decomposition of black hole perturbations

Abstract: It is well known that quasinormal modes (QNMs) do not form a complete basis for linear black hole perturbations. In particular, retarded Green's functions of black holes can only be decomposed as a convergent sum of QNMs at sufficiently late times. In this talk, I will show that Green's functions can be expanded as a convergent mode sum everywhere in spacetime. When QNMs fail at early times, a sum of Matusbara modes (MMs) associated to the horizon temperature converges instead. I will present the case of the Schwarzschild black hole, where the branch cut is resolved into a set of de Sitter modes by adding a small positive cosmological constant. Based on 2510.18956.

Andrea Conti: Defect entanglement entropy for Super-(conformal) monodromy defects

Abstract: Recently, there has been an increasing interest in the study of defects in quantum field theories, with holography providing a powerful framework to explore various aspects of these super-(conformal) gauge theories.

In this talk, I will discuss supergravity solutions that are dual to codimension-2 superconformal monodromy defects. These solutions are obtained using gauged supergravities in D=4,5,6 and 7 dimensions. I will present a prescription to compute the defect entanglement entropy, outlining the renormalization procedure needed to regularise its divergencies, which I will discuss in detail. We are also able to express this quantity in terms of the free energy/Weyl anomaly of the defect and its conformal weight. In addition, we examine whether the defect entanglement entropy obeys a monotonicity theorem under RG flows.

If time allows, I will also discuss some new results for non-conformal monodromy defects.

Ayngaran Thavanesan (External speaker): Kosmic Field Theories: Towards Holographic Duals for Cosmology

Abstract: In this talk, I will present a non-perturbative formulation of bulk unitarity in FLRW cosmologies (including de Sitter space), addressing a long-standing question of how bulk unitarity is realised in the non-reflection-positive boundary theories that appear in holographic cosmology.

I will show how this framework applies to the cosmological wavefunction and how the resulting constraints can also be expressed perturbatively at each loop order. In particular, these constraints determine the exact phase of the wavefunction to all orders, placing powerful restrictions on cosmological correlators and carrying significant implications for quantum gravity. Remarkably, our non-perturbative unitarity condition also enables the identification of a new class of large-$N$ gauge theories—which we call Kosmic Field Theories—that may serve as holographic duals for cosmological (time-dependent) string backgrounds.

This talk is based on arXiv:2408.17406 (with Harry Goodhew and Aron Wall), arXiv:2501.06383, and arXiv:2510.21701 (with Aron Wall). See also the related work arXiv:2511.04511 on Cauchy Slice Holography for de Sitter (with Gonçalo Araújo-Regado and Aron Wall).

Dongsheng Ge (External speaker): How to get an interacting conformal line defect for free theories

Abstract: In this talk, I will argue that interacting conformal line defects in free quantum field theories can exist, provided that inversion symmetry is broken. Important for our demonstration is the existence of a special cross ratio for bulk-defect-defect three point functions that is invariant under the conformal group but picks up a sign under inversion. We examine the particular case of a free scalar field in detail, and provide a toy model example where this bulk field interacts via a Yukawa term with fermions on the line. We expect nontrivial line defects may also exist for free Maxwell theory in four dimensions and free bulk fermions. This talk is based on a recent work (2510.02871) with Chris Herzog and Sam Bartlett-Tisdall.

PGR Seminar Speaker: Orestis Papadopoulos
Title: Softening holographic nuclear matter

Abstract: The description of strongly interacting matter at large chemical potentials and/or temperatures is a difficult task. The AdS/CFT correspondence can provide insight into this regime. I will use a particular top-down construction, the Witten-Sakai-Sugimoto model, to describe nuclear matter at finite densities. Within this model, baryons can be described as instantonic solutions of a gauge theory living on D8 flavor branes in the bulk. At higher densities, multiple instantons can be approximated by a spatially homogeneous ansatz, with a topological baryon number being introduced via a discontinuity in the non abelian sector, along the holographic direction. I will present results of solutions with up to 4 discontinuities, which I check systematically, showcasing a new ground state, while also making the nuclear matter more realistic with a reduced incompressibility. I will also point out a connection between the homogeneous ansatz and pointlike instantons.

PGR Seminar Speaker: Rowan Wright

Title: Correlation functions of multiple light-like Wilson loops in maximally supersymmetric Yang-Mills theory

Abstract: Planar scattering amplitudes in N=4 Super Yang-Mills theory are famously dual to the expectation values of null polygonal Wilson loops. I will present ongoing work which shows that much of the same mathematical structure is also enjoyed by correlators of multiple light-like Wilson loops, such as a BCFW-like recursion relation and a simple generalisation of the Q-bar equation. I will also present a derivation of the leading singularities of one-loop scattering amplitudes/Wilson loops into products of tree level amplitudes from the perspective of twistor Wilson loop diagrams, and sketch out how the same procedure allows the one-loop leading singularities of multiple light-like Wilson loop correlators to be expressed in a similarly compact form.