Journal club
Academic year 2022-2023
Journal club organised by Arnaud Delfante and Noémie Parrini.
Guillaume Lhost | EMRI’s and gravitational waves; motivations, adiabatic inspirals and matching with the transition to plunge | 1 June 2023
The 2-body problem in General Relativity is a challenging task and involves a wide variety of research programs in physics. Great progress has been made in the development of Post Newtonian/Minkoskian expansion, effective one-body and numerical relativity in order to study the dynamics of a two compact object system and its associated wavefront. In a decade (let’s hope), the new generation of gravitational wave detectors will be launched. Therefore, it will be possible to detect small frequency gravitational waves which are characteristic of EMRI’s (extrem mass ratio inspirals). These latter can be seen as a secondary black hole evolving around a supermassive primary black hole by following forced geodescis. This takes place in the context of gravitational self-force and adiabatic inspiral motion. In this talk, I will walk through different aspects of the two-body problem. I will talk about the motivations to study EMRI’s, and the way we compute them. The quasi-circular and equatorial case will be described, from the early/late adiabatic inspiral to the matching with transition to plunge that occurs close to the last stable orbit. Finally, I will discuss about how to treat EMRI’s in the case of bound ellipitical orbits.
Matthieu Vilatte (Ecole Polytechnique and Thessaloniki U.) | Carrollian fluids and applications to flat fluid/gravity | 8 May 2023
The fluid/gravity duality is a spin-off of the general AdS/CFT correspondence in which a solution of Einstein’s equations is mapped to a fluid living on the timelike conformal boundary of AdS. The bulk spacetime is then reconstructed order by order in derivatives of the fluid velocity (or equivalently of the holographic coordinate). In this talk I want to discuss the flat limit of this paradigm, limit in which the conformal boundary becomes null, inheriting then Carrollian symmetries. After reviewing the physics of Carrollian fluid, explaining how one can get such a system from the c->0 limit of a relativistic one, I will delve into the construction of a gauge, covariant with respect to the Carrollian boundary in which the bulk spacetime is described as an expansion whose coefficients depends on the dual fluid data. Finally, I will go through the conditions for the expansion to be resummable and present how one can, in the case of time independent Ricci-flat spacetimes, recover the gravitational multipoles from a pure Carrollian boundary viewpoint.
Arghya Chattopadhyay | A relevant story of irrelevance | 24 April 2023
In this relevant talk I will talk about a special Irrelavant deformation which seems to be of recent relevance. I will go through a basic review of the topic and touch upon two recent relevant paper that came out last month. The details of the papers are the following:
- To be reviewed as a single short story: hep-th/0401146, 1602.05534, 1611.03470.
- Quick explainer: 2303.04836, 2303.01247.
Victor Lekeu | Supersymmetry and Morse theory | 13 and 27 March 2023
The journal club will be about the classic 1982 paper “Supersymmetry and Morse theory” by Witten, which uses physics techniques (supersymmetric quantum mechanics, the path integral, instantons…) to prove theorems of differential geometry and topology. More specifically, I will talk a bit about Morse theory, the Euler characteristic, and their link with the quantisation of a specific N=2 supersymmetric non-linear sigma model with smooth Riemannian manifold as target space.
Josh O’Connor | Dual gravity from A_1^{+++} | 6 March 2023
In this talk, we will study the non-linear realisation based on the $A_1^{+++}$ Kac-Moody algebra. We will see that this construction directly leads to the equations of motion for gravity as well as the duality relation between the graviton and the dual graviton in four dimensions.
Academic year 2021-2022
Journal club organised by Yannick Herfray and Simon Pekar.
Nicolas Boulanger | Scientific exchanges between Einstein, Weyl and Cartan | 13 June 2022
From a historical point of view (sometimes, there will be some formulae), we discuss the emergence of Gauge theory. This will give us the opportunity to appreciate how some great minds of the 20th centuries interacted and (mis)understood each others.
Josh O’Connor | Kac-Moody Algebras and Physics | 23 May 2022
The first half of this talk will review the structure of Kac-Moody algebras. The second half will focus on the original E11 paper from 2001 (hep-th/0104081).
Taehwan Oh (Kyunghee University) | Brief Idea about Worldline Particle Action | 9 May 2022
Coadjoint orbits of isometry group provide the phase spaces of the corresponding particle actions. In this construction, it is crucial to find appropriate coordinate systems for the coadjoint orbits. We are trying to choose the coordinate from the dual pair correspondence so a worldline action can be obtained in a simple manner by realising the coadjoint orbit as a constraint surface. Also we have investigated that a worldline action of a spinning particle can be expressed in terms of dual pair correspondence.
Mattia Serrani | Massive Spinor-Helicity, On-Shell Amplitudes and some results | 7 March 2022
Amplitudes are the most important observables in QFTs and therefore they need particular attentions. The usual way to compute them is by Feynman diagrams, but is not the most convenient one. Spinor-Helicity formalism allow us to work only with objects that transform under the Little Group of the Poincaré group. In particular I will focus mostly on the massive Spinor Helicity formalism they introduced and on some interesting results. If time permits, I will also give a brief comment about BH-scattering.
Tung Tran | (Ambi)-twistor and massless fields | 21 February 2022
In this talk, I will give a short introduction to (Ambi)-twistor theory. In particular, the relationship between (Ambi)-twistor representatives and massless fields on spacetime will be the main focus of this talk. In other words, we will learn how to do the Penrose transform in practice.
Chrysoula Markou | Chaos in string scattering | 7 February 2022
A classical system is often said to be exhibiting chaotic behaviour if its time evolution is sensitive to even the tiniest of changes in its initial conditions. There exist several examples of the kind, in fields ranging from condensed-matter physics to mathematical biology, with weather prediction being perhaps the most famous example associated with chaos. While it is usually nonlinearities, at the level of the equations of motion, that serve as a sign of chaos, the subject of this talk will be a series of recent papers, notably 2103.15301, that identify chaotic traits at the level of scattering amplitudes in the context of string theory for the first time.
Yannick Herfray | Cartan geometry of null-infinity | 24 January 2022
I will review my recent results on the geometry of null-infinity. Generically characteristic data for gravity are equivalent to having a curved Cartan geometry at null-infinity. This geometry corresponds in, physics terminology, to the gauging of the 4D Poincaré group and can be understood as a “conformally Carrollian”. In fact the curvature of this geometry is an invariant characterisation of the presence of gravitational radiation and this gives a mathematically and conceptually precise content to the well-known fact that “gravitational radiation is the obstruction to having a preferred Poincaré group acting at null-infinity, asymptotic symmetries being given by the BMS group instead”.
Paolo Pichini (Uppsala U.) | Compton Black-Hole Scattering for s≤5/2 | 10 January 2022
Quantum scattering amplitudes for massive matter have received new attention in connection to classical calculations relevant to gravitational-wave physics. Amplitude methods and insights are now employed for precision computations of observables needed for describing the gravitational dynamics of bound massive objects such as black holes. An important direction is the inclusion of spin effects needed to accurately describe rotating (Kerr) black holes. Higher-spin amplitudes introduced by Arkani-Hamed, Huang and Huang at three points have by now a firm connection to the effective description of Kerr black-hole physics. The corresponding Compton higher-spin amplitudes remain however an elusive open problem. Here we draw from results of the higher-spin literature and show that physical insights can be used to uniquely fix the Compton amplitudes up to spin 5/2, by imposing a constraint on a three-point higher-spin current that is a necessary condition for the existence of an underlying unitary theory. We give the unique effective Lagrangians up to spin 5/2, and show that they reproduce the previously-known amplitudes. For the multi-graviton amplitudes analogous to the Compton amplitude, no further corrections to our Lagrangians are expected, and hence such amplitudes are uniquely predicted. As an essential tool, we introduce a modified version of the massive spinor-helicity formalism which allows us to conveniently obtain higher-spin states, propagators and compact expressions for the amplitudes.
Simon Pekar | Perturbatively exact w_{1+\infty} | 13 December 2021
I will explain how to recover the wedge part of the w_{1+\infty} asymptotic symmetry algebra of self-dual gravity from the point of view of collinear limit of MHV amplitudes. I will then move to one-loop in order to see that the algebra is undeformed by quantum corrections. If time allows, I will also present a way to deform the algebra by introducing non-minimal coupling.
Ivano Basile | Allowable complex metrics | 6 December 2021
I will present the latest paper by Witten. It proposes a criterion to exclude unphysical saddle points in the semiclassical expansion of the gravitational path integral. I will focus on the criterion itself and some basic examples including spheres and black holes.
Yannick Herfray | Celestial w_{1+\infty} from twistors | 22 November 2021
I will present this recent paper by L. Mason, T. Adamo and A. Sharma on twistors and w-infinity algebra. I will focus on the first part which is a review of what appears to be a classical result in twistor theory: the w-infinity algebra is a group of symmetry for the self-dual sector of gravity. This makes the emergence of this group in the context of celestial holography a lot less mysterious and begs the question whether chiral higher-spin gravity could possess a similar symmetry extending it.
Tung Tran | Introduction to the IKKT matrix model | 8 November 2021
I will give a short review of the matrix IKKT model on a fuzzy 4-sphere. It is argued that the IKKT model can give rise to a higher-spin theory (HS-IKKT). I will try to explain the main features of this HS-IKKT in this talk.
Akshay Bedhotiya | HS symmetry from CFT perspective | 25 October 2021
CFTs in three dimensions provide an interesting class of theories with higher spin currents. We can construct two separate classes of theories with fundamental matter, namely N free bosons and fermions each allowing an infinite set of conserved spin currents. By including quartic interactions, tuning the coupling and flowing to the IR one can generate critical models of the two theories. The higher spin symmetry can be “slightly broken” by including interactions with a U(N) Chern Simons (CS) gauge field. The resulting ‘quasi -bosonic’ and ‘quasi-fermionic’ family of theories are characterized by a single parameter and are conjectured to possess an astonishing bosonization duality linking the free limits of both theories to the corresponding strong coupling limits of the other theory where they reduce to the corresponding critical counterparts. This allows us to deduce correlators in free and critical models as limits of a single interacting theory.
Yannick Herfray | The Lambda-BMS group | 11 October 2021
Based on 1905.00971.