Past seminars (2023-2024):

• 16 July 2024 – Bin Zhu (Edinburgh)
• Eikonal amplitudes on the celestial sphere
• Celestial scattering amplitudes for massless particles are Mellin transforms of momentum-space scattering amplitudes with respect to the energies of the external particles, and behave as conformal correlators on the celestial sphere. However, there are few explicit cases of well-defined celestial amplitudes, particularly for gravitational theories: the mixing between low- and high-energy scales induced by the Mellin transform generically yields divergent integrals. In this talk, we argue that the most natural object to consider is the gravitational amplitude dressed by an oscillating phase arising from semi-classical effects known as eikonal exponentiation. This leads to gravitational celestial amplitudes which are analytic, apart from a set of poles at integer negative conformal dimensions, whose degree and residues we characterize. We also study the large conformal dimension limits, and provide an asymptotic series representation for these celestial eikonal amplitudes. Our investigation covers two different frameworks, related by eikonal exponentiation: 2-2 scattering of scalars in flat spacetime and 1-1 scattering of a probe scalar particle in a curved, stationary spacetime. These provide data which any putative celestial dual for Minkowski, shockwave or black hole spacetimes must reproduce. We also derive dispersion and monodromy relations for these celestial amplitudes and discuss Carrollian eikonal-probe amplitudes in curved spacetimes. This talk is based on the recent work with T. Adamo, W. Bu, and P. Tourkine.
  

• • 2 July 2024 – Akshay Yelleshpur Srikant (Oxford)
  • Celestial OPEs from S-Matrix unitarity
  • OPE coefficients are essential data in any CFT. In the context of CCFT, they are computed by examining the collinear behavior of scattering amplitudes. However, recent results indicate that the OPE contains more singularities beyond those associated with the collinear limit. In this talk, I will introduce a new method for computing OPEs which is inspired by Cutkosky’s cutting rules and is closely linked to S-Matrix unitarity. I will demonstrate this method can be used to derive compact formulas for OPEs and multi-OPEs for any masses and spins directly from scattering amplitudes.
    

• • 18 Jun 2024 – Sabrina Pasterski (Perimeter)
  • Radiation in Holography
  • We show how to encode the radiative degrees of freedom in 4-dimensional asymptotically AdS spacetimes, using the boundary Cotton and stress tensors. Background radiation leads to a reduction of the asymptotic symmetry group, in contrast to asymptotically flat spacetimes, where a non-vanishing news tensor does not restrict the asymptotic symmetries. Null gauges, such as Λ-BMS, provide a framework for AdS spacetimes that include radiation in the flat limit. We use this to check that the flat limit of the radiative data matches the expected definition in intrinsically asymptotically flat spacetimes. We further dimensionally reduce our construction to the celestial sphere, and show how the 2-dimensional celestial currents can be extracted from the 3-dimensional boundary data.
    

• • 21 May 2024 – Simone Speziale (CPT, Marseille)
  • Different Universal Structures at Scri, Covariance and 2-Cocycles
  • In the first part of the talk, I will draw a parallel between
    different symmetry groups that arise on null surfaces at a finite
    distance, and extensions of the BMS symmetry at future null infinity.
    In the second part I will discuss how ambiguities in the charges and
    fluxes can be entirely removed thanks to the key property of
    covariance, one side of which is the absence of field-dependent
    cocycles in the algebra.
    

• • 7 May 2024 – Laurent Freidel (Perimeter)
  • Null Raychauduri: Canonical Structure, Dressing Time and Quantisation
  • In this talk, I will review our recent work with L. Ciambelli and R. Leigh, focusing on the non-perturbative characterization of the gravitational phase space along null boundaries. I will present the construction of the Carrollian curved geometry, the Poisson bracket and discuss the construction of  a field-dependent time called dressing time that allows the construction of a positive definite boost generator for expanding null surfaces. If time permits, I will discuss some elements of the quantization and the appearance of a crucial Quantum central charge. I will also discuss how these results connect to the celestial holography program when one treats the null boundary as an asymptotically null one.
    

• • 23 April 2024 – Temple He (Caltech)
  • 
    A synergy of shockwaves, edge modes, and infrared physics
  • In this talk, I will focus on two interesting directions of research in the infrared sector of gauge theories and gravity. First, we study the relationship between shockwave geometries and the gravitational memory effect in four-dimensional asymptotically flat spacetime. In particular, we show the ‘t Hooft commutation relations of shockwave operators are equivalent to the commutation relation between soft and Goldstone modes parametrizing a sector of the gravitational Hilbert space. Second, we present a concrete connection between soft modes on the celestial sphere and entanglement edge modes in QED, paving the way to study entanglement properties of soft modes.
    

• • 2 April 2024 – Walker Melton (Harvard)
  • Celestial Leaf Amplitudes
  • Celestial amplitudes of massless particles are forced to be distributional at low points due to bulk translation invariance, which is an unusual property for a 2D CFT. However, these distributional amplitudes can be extracted from smooth leaf amplitudes, where a bulk interaction vertex is integrated only over a hyperbolic slice of spacetime. In this talk, I describe the construction of gluon MHV leaf amplitudes, explain how the full scattering amplitude can be extracted from these leaf amplitudes, and describe some properties of their soft and collinear limits. Finally, I will describe how these leaf amplitudes can be generated by a simple 2D system of free fermions and Liouville Theory in the large central charge limit. This talk is based on work done with Andrew Strominger and Atul Sharma [2312.07820, 2402.04150] and [2403.18896] with Andrew Strominger, Atul Sharma, and Tianli Wang.
    

• • 19 March 2024 – Massimo Tarrona (U Napoli & INFN)
  • Celestial Holography Revisited
  • I will present some recent works in collaboration with Charlotte Sleight where we propose a new extrapolate dictionary for celestial correlators. This new definition is motivated by the AdS/CFT correspondence where the central object under study is not the S-matrix but the boundary limit of time ordered correlation functions. We shall discuss various examples, both perturbatively and non-perturbatively. I will conclude mentioning possible connections with previous proposals based on the S-matrix.

• • 5 March 2024 – Yannick Herfray (IDP, Tours)
  • Carrollian $Lw_{1+\infty}$ Representation from Twistor Space
  • As explained by Tim Adamo, Lionel Mason and Atul Sharma the action of the Lw1+infinity algebra is local and geometrical in twistor space. I will present the main outcome of our recent work with Laurent Freidel and Laura Donnay which is an explicit realization of this action on Carrollian fields (i.e. on scri). The action is derived by direct Penrose transform, and this ensures that the resulting set of differential operators on scri form a representation of the algebra. We also show that this representation, derived from twistor space, coincides with the canonical action of Freidel–Pranzetti–Raclariu.
    

• 20 Feb 2024 – Sruthi Narayanan (Perimeter)
• Celestial Amplitudes from the Outside In
• Thus far, most of the advances in celestial holography have utilized a mapping between bulk S-matrix elements and boundary conformal correlators to understand more about the structure of celestial CFTs. In this talk I will give some background on what has been done thus far and then discuss how we can study this duality in the opposite way: by considering specific conformal field theories and deducing what bulk theories they should correspond to. This talk is largely based on the paper “Celestial Gluon Amplitudes from the Outside In” from this past December.
  

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• 6 Feb 2024 – Yangrui Hu (Perimeter)
• Celestial Quantum Error Correction
• Quantum gravity in 4D asymptotically flat spacetimes features spontaneous symmetry breaking due to soft radiation hair, intimately tied to the proliferation of IR divergences. A holographic description via a putative 2D CFT is expected free of such redundancies. In this talk, we address this issue by initiating the study of Quantum Error Correction in Celestial CFT (CCFT). We start by constructing a toy model with finite degrees of freedom by revisiting noncommutative geometry in Kleinian hyperkähler spacetimes. The model obeys a Wick algebra that renormalizes in the radial direction and admits an isometric embedding à la Gottesman-Kitaev-Preskill. Then we promote qubits to qunits and construct a toy model of CCFT from the perspective of quantum error-correcting codes. In our code, the hard states with quantized BMS soft hair form the logical subspace. This allows us to reverse errors induced by soft radiation. Technically, the construction relies on the recently studied $w_{1+\infty}$ hierarchy of soft currents and its realization in twistor space.
  

• 23 Jan 2024 – Francisco Rojas (Adolfo Ibanez U)
• Celestial loop amplitudes in field and string theory
• Due to their exponentially soft behavior in the UV, string amplitudes are an excellent testing ground for studying their corresponding celestial correlators. In this talk, we will review celestial amplitudes in the Type I superstring at one-loop, for the case of external gluons. We will analyze some of its properties and analytic structure. After toroidal compactification to 4 spacetime dimensions, its field theory limit describes gluon amplitudes in N=4 SYM whose planar amplitude is known to all loops in the ‘t Hooft coupling.  We will see that, even though each term in the loop expansion produces divergent celestial amplitudes (after the usual Mellin transforms), their all-loop resummation (the BDS formula) yields a convergent expression which is directly related with the known positivity of the cusp anomalous dimension. To conclude, we will briefly comment on ongoing work on how to extract the correct field theory limit from string amplitudes in the conformal basis.
  

• 9 Jan 2024 – Romain Ruzziconi (Oxford)
• Carrollian Avenue to Celestial Holography
• Carrollian holography aims to express gravity in four dimensional asymptotically flat space-time in terms of a dual three dimensional Carrollian CFT living at null infinity. In this talk, I will review some aspects of Carrollian holography and compare this approach with the AdS/CFT correspondence. I will introduce the notion of Carrollian amplitude and explain how this relates to celestial amplitudes. Finally, I will present recent results concerning Carrollian OPEs and deduce how soft symmetries act at null infinity.
  

• 12 Dec 2023 – Diksha Jain (TIFR, Mumbai)
• The S-Matrix and Boundary Correlators in Flat Space
• In this talk, I will define and study the boundary correlation functions obtained from the path integral as a functional of boundary values in flat space. The flat space S-matrix can be extracted directly from these boundary correlation functions after smearing. Next, I will derive the constraints on this path-integral that follow from the unitarity of the S-matrix. We then study the locality structure of boundary correlation functions. In the massive case, we find that the boundary correlation functions for generic locations of boundary points are dominated by a saddle point which has the interpretation of particles scattering in a small elevator in the bulk, where the location of the elevator is determined dynamically, and the S-matrix can be recovered after stripping off some dynamically determined but non-local “renormalization” factors. In the massless case, we find that while the boundary correlation functions are generically analytic as a function on the whole manifold of locations of boundary points, they have special singularities on a sub-manifold, points on which correspond to light-like scattering in the bulk. This analysis parallels the analysis of bulk-point singularities in AdS/CFT and generalizes it to the case of multi-bulk point singularities.
  

• 28 Nov 2023 – Niklas Garner (U Washington)
• Twistorial Monopoles & Chiral Algebras
• Twistor methods are remarkably powerful for studying four-dimensional scattering problems, but a description in terms of a local QFT on six-dimensional twistor space is often obstructed by an anomaly. As observed by Costello and Paquette, when such a description exists the chiral algebra of asymptotic symmetries can also be described as the degrees of freedom living on the boundary of a three-dimensional QFT obtained by reduction from twistor space. In this talk I will describe some recent work with N. Paquette studying non-perturbative aspects of these celestial chiral algebras by combining the above observation with techniques available to three-dimensional, holomorphic-topological gauge theory.
  

• 14 Nov 2023 – Adam Ball (Perimeter)
• Multicollinear Singularities in Celestial CFT
• After reviewing the celestial OPE, soft current algebras, and the Jacobi identity, I will introduce the holomorphic multicollinear limit and its celestial avatar, the multi-OPE. This new tool provides more information than the usual 2-OPE, and I will use it to show that from the perspective of hard celestial amplitudes the failure of Jacobi is due to the presence of certain branch cuts in the z-dependence. This puts to rest some concerns in the literature that the hard OPE is sick in theories failing Jacobi. Finally, I will use the multi-OPE to show the existence of a new singular term in the OPE between two massless particles, and I will argue it is associated with multi-particle operators.
  

• 17 Oct 2023 – Hofie Hannesdottir (IAS)
• What can be measured asymptotically?
• In this talk, we consider asymptotic observables in quantum field theories in which the S-matrix makes sense. We argue that in addition to scattering amplitudes, a whole compendium of inclusive observables exists where the time ordering is relaxed. These include expectation values of electromagnetic or gravitational radiation fields as well as out-of-time-order amplitudes. We explain how to calculate them in different ways: by relating them to amplitudes and products of amplitudes and by using a generalization of the LSZ reduction formula. Finally, we discuss how to relate them to one another through new versions of crossing symmetry. As an application, we discuss one-loop contributions to gravitational radiation in the post-Minkowski expansion, emphasizing the role of classical cut contributions and highlighting the infrared physics of in-in observables.
  

• 3 Oct 2023 – Noah Miller (Harvard)
• Integer modes and Bulk $w_{1+\infty}$ symmetry
• In celestial holography, integer modes with Δ ∈ ℤ have the special importance of generating soft symmetries. In this talk (based on the paper arxiv.org/abs/2302.04905) we prove the completeness of these modes and their descendants (and their shadow pairs) for the massless scalar by reconstructing the positive frequency Wightman function as a sum of these modes. Interestingly, this proof utilizes a novel CFT-like inner product called the RSW inner product.We then discuss how integer modes and descendant wave functions can be used to the bulk spacetime symmetry which satisfies the commutation relations of the (loop algebra of) the S algebra and the w_(1+infinity) algebra in Self Dual Yang Mills and Self Dual Gravity. In these self dual theories, we will find that there is essentially a complete correspondence between radiative modes and “gauge transformations,” once interpreted appropriately.
  

• 19 Sept 2023 – Wei Bu (Edinburgh)
• Infra-red structures of scattering on self-dual radiative backgrounds
• The scattering of gluons and gravitons in trivial backgrounds is endowed with many surprising infrared features which have interesting conformal interpretations on the two-dimensional celestial sphere. However, the fate of these structures in more general asymptotically flat backgrounds is far from clear. In this paper, we consider holomorphic infrared structures in the presence of non-perturbative, self-dual background gauge and gravitational fields which are determined by freely specified radiative data. We make use of explicit formulae for tree-level gluon and graviton scattering in these self-dual radiative backgrounds, as well as chiral twistor sigma model descriptions of the classical dynamics. Remarkably, we find that the leading holomorphic part of tree-level collinear splitting functions – or celestial OPEs – and infinite-dimensional chiral soft algebras are undeformed by the background. We also compute all-order holomorphic celestial OPEs in the MHV sectors of gauge theory and gravity.