2025 Simons Collaboration on Celestial Holography Annual Meeting

The 2025 annual meeting of the Simons Collaboration on Celestial Holography was held April 17–18th at the Simons Foundation, with 133 in-person participants representing a broad range of interests and perspectives.

A hallmark of the collaboration’s second year, and this annual meeting, has been an accelerating dialogue between Collaboration members and the theoretical physics community at large. In addition to Collaboration PI talks, invited speakers outside of the collaboration presented pioneering research congruent to Collaboration objectives.

The primary foci of the Collaboration are: (1) to find and understand universal quantum gravitational physics, including asymptotic symmetries, and their experimental signatures; (2) to find examples of celestial holography from the top down using string theory, and concomitant connections to mathematics; and (3) to advance our understanding of holography in general spacetimes, particularly asymptotically flat spacetimes. Significant progress was reported on all of these fronts at the meeting. Organizing meeting contributions by these three broad goals, a brief summary follows.

1. Ashoke Sen presented landmark results which determine gravitational waveforms at early and late times from soft theorems, including logarithmic corrections. This remarkable universal result is independent of the intractable details of the collision producing the waveform. Nima Arkani-Hamed presented work in progress exploring dramatic, emergent simplifications of scattering amplitudes in the limit of large numbers of external scattering states, organized by chord diagrams reminiscent of a discretization of the Minkowski Penrose diagram.
2. Kevin Costello and David Skinner reported on advances in self-dual limits of gauge and gravitational theories, respectively. Costello presented a new index theoretic derivation of the QCD beta function, and presented new results in non-perturbative effects in self-dual Yang-Mills, a system which also has applications to aspects of Bridgeland stability conditions in the derived category of coherent sheaves on twistor space. Skinner discussed a new top-down celestial dual in the topological string, incorporating self-dual Einstein gravity for the first time (rather than self-dual conformal gravity, which had been a limit of earlier constructions). Wei Song developed a complementary top-down approach utilizing dual, well-controlled deformations of AdS/CFT dual pairs to arrive at an asymptotically flat holographic duality in a linear dilaton background.
3. Raphael Bousso presented novel definitions of the hologram (formally, the entanglement wedge) for general spacetimes, including asymptotically flat spacetimes, with an emphasis on the novel issues of complementarity that emerge in these cases. Sabrina Pasterski explored recent proposals for defining entanglement entropy in flat spacetimes with BMS symmetry, and benchmarking in detail their compatibility with entanglement entropy in AdS/CFT via uplift. Edward Witten presented a framework for understanding algebras in quantum gravity in general spacetimes by incorporating an explicit observer in spacetime, and the appropriate notions of entropy therein.

In brief, there was an influx of novel perspectives on the question of flat space holography, both from within and without the Collaboration. This stimulated further discussions and nascent collaborations among senior and junior scholars. Meal and coffee breaks were characterized by active discussions blending participants at all career stages, most of whom also attended the preceding two-day satellite meeting. Further, many results built on or were intimately connected to recent Collaboration achievements reported on at the previous annual meeting. For example, the work by Sen explored subleading logarithmic memory effects whose asymptotic symmetry interpretation was emphasized earlier by Andrea Puhm. The meeting reflected the rapid progress, stimulated by Collaboration activity, in the physics of holography for asymptotically flat spacetimes.

Kevin Costello
Perimeter Institute

Non-Perturbative Aspects of Self-Dual Gauge Theory
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Self-dual gauge theory is conformal in perturbation theory but has a non-trivial beta-function when instanton effects are included. Kevin Costello will give two computations of this beta-function, one based on the Grothendieck-Riemann-Roch formula and one using holography in the topological string. This leads to two new ways to compute the standard QCD beta-function at one loop, without using Feynman diagrams. If time permits, Costello will also discuss how instantons effect scattering amplitudes.
 

Wei Song
Tsinghua University

The TsT/TTbar Correspondence: A Top-Down Approach of Non-AdS Holography
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The TsT/TTbar correspondence is an example of non-AdS holography constructed from string theory. Beginning with Type IIB string theory on AdS₃ × N with NS-NS three-form flux, and applying TsT transformations (comprising T-duality, a shift, and another T-duality), one can obtain an asymptotically flat spacetime featuring a linear dilaton in the string frame. The holographic dual of this configuration is closely connected to a single-trace version of the TTbar-deformed conformal field theory (CFT) in two dimensions. Wei Song will discuss various aspects of this model, such as the spectrum, black holes, symmetries, and correlation functions.
 

Raphael Bousso
University of California, Berkeley

Holograms in Asymptotically Flat Space
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[Based on joint works with Geoff Penington and with Sami Kaya.] Raphael Bousso will review and refine the construction of generalized entanglement wedges. Bousso will then specialize to the case of asymptotically flat spacetimes and classify the asymptotic boundaries of wedges and entanglement wedges. Raphael will recover the notion of entanglement wedge complementarity, which are richer and more subtle than in AdS.
 

David Skinner
University of Cambridge

Celestial Holography and Self-Dual Einstein Gravity
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To date, top-down constructions of celestial holography involve a combination of twisted holography and twistor theory. In the original (Burns) model of Costello-Paquette-Sharma, the gravitational theory is the B model living in a suitable twistor space, and corresponds to a theory of self-dual conformal gravity in four dimensions. David Skinner will discuss a variant of this model coming from a CY 5-fold that admits a K3 fibration over twistor space and yields a theory of self-dual Einstein gravity in four dimensions. This is based on work in progress with Roland Bittleston, Kevin Costello, and Atul Sharma.
 

Nima Arkani-Hamed
Institute for Advanced Study

1/n

TBA
 

Edward Witten
Institute for Advanced Study

A Background Independent Algebra in Quantum Gravity

Edward Witten will consider the algebra of observables along the worldline of an observer as a background independent algebra in quantum gravity.
 

Sabrina Pasterski
Perimeter Institute

Lifting Swing Surfaces to AdS
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The entanglement entropy for regions in a BMS field theory living at null infinity has been proposed to be holographically dual to certain ‘swing surfaces’ in flat space. Sabrina Pasterski will lift this construction to AdS/CFT and revisit both bulk and boundary aspects of this proposal.
 

Ashoke Sen
International Center for Theoretical Sciences

Classical Gravitational Wave Tails from Soft Theorem
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If a set of massive objects collide in space, and the fragments disperse, possibly forming black holes, then this process will emit gravitational waves. Computing the detailed gravitational wave-form associated with this process is a complicated problem, not only due to the non-linearity of gravity but also due to the fact that during the collision and subsequent fragmentation the objects could undergo complicated non-gravitational interactions. Nevertheless, the classical soft graviton theorem determines the power law fall-off of the wave-form at late and early times, including logarithmic corrections, in terms of only the momenta of the incoming and outgoing objects without any reference to what transpired during the collision. Ashoke will explain the results, briefly outline the derivation of these results and discuss possible generalizations and applications.