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Oct. 26, 2023 – Oct. 29, 2023

Jefferson Laboratory, Harvard University

*17 Oxford St, Cambridge, MA 02138*

Confirmed Speakers: Tim Adamo* (University of Edinburgh), Nima Arkani-Hamed* (IAS), Shamik Banerjee (NISER, Bhubaneswar, India), Roland Bittleston (Perimeter Institute), Wei Bu (University of Edinburgh), Miguel Campiglia (Universidad de la República), Eduardo Casali (Harvard University), Luca Ciambelli (Perimeter Institute), Sangmin Choi (Ecole Polytechnique), Kevin Costello* (Perimeter Institute), Jordan Cotler* (Harvard), Lance Dixon (SLAC/Stanford), Laura Donnay (SISSA), Roberto Emparan (ICREA and University of Barcelona), Laurent Freidel* (Perimeter Institute), Daniel Grumiller (TU Wien), Temple He (Caltech), Elizabeth Himwich (Princeton University), Yangrui Hu (Perimeter Institute), Dan Kapec (Harvard University), Alok Laddha (Chennai Mathematical Institute), Lorenzo Magnea (University of Torino), Juan Maldacena (IAS), Lionel Mason* (University of Oxford), Prahar Mitra (University of Cambridge), Rob Myers (Perimeter Institute), Natalie Paquette* (University of Washington), Sabrina Pasterski* (Perimeter Institute), Monica Pate* (New York University), Malcolm John Perry (University of Cambridge), Andrea Puhm* (University of Amsterdam), Francisco Rojas (Universidad Adolfo Ibáñez), John H. Schwarz (Caltech), Ali Seraj (Queen Mary University of London), Atul Sharma (Harvard University), Eva Silverstein (Stanford), David Skinner* (DAMTP, Cambridge), Marcus Spradlin (Brown University), Andrew Strominger* (Harvard), Tadashi Takayanagi (YITP), Tomasz Taylor* (Northeastern University), Anastasia Volovich (Brown University), Robert Wald (University of Chicago), Diandian Wang (Harvard University), Alexander Zhiboedov (CERN)

*Collaboration PIs

The kickoff meeting of the Simons Collaboration in Celestial Holography was attended by the collaboration PIs, their postdocs and students, and many other faculty and junior researchers working in the field. Attendance was capped at 207 attendees, due to the venue’s recorded maximum capacity.

Collaboration PIs and postdocs gave short 10+5 minute talks and other speakers gave 20+5 minute talks. There were 41 talks in all (not counting a small number of cancellations due to illness/visa issues); we can give only a lightning summary of these many thrilling contributions below. Talk categorizations by topic are approximate.

**Bottom-Up Holography/Celestial Symmetries:** Andrew Strominger leveraged the fact that Minkowski space has regions which can be foliated by lower dimensional de Sitter spacetimes; this fact motivates the use of special states called alpha-vacua, which can ameliorate puzzling aspects of the CCFT correspondence. Alfredo Guevara proposed a toy model for a Hilbert space in celestial holography that is protected by quantum error correction. Andrea Puhm used CFT technology to clarify the difference between CCFT symmetries and asymptotic symmetries in spacetime. Akshay Yelleshpur discussed conditions for associativity of celestial OPEs, and simplifications that arise in the presence of supersymmetry. Sangmin Choi explained how divergent 1-loop corrections to the subleading soft theorem correspond to certain large gauge transformations with long-range interactions. Atul Sharma elucidated functional forms of CFT correlators that live on the boundary of the Klein spacetime of (2, 2) signature, “the celestial torus”. Daniel Grumiller presented general arguments that a CCFT will satisfy the mathematical axioms of a log-CFT. Diandian Wang used on-shell recursion relations to fix the expansion of a classical celestial OPE. Laura Donnay explained that the logarithmic soft theorems in gravity arise from Ward identities associated to supperotations. Shamik Banerjee derived the most general OPE governing +-helicity gravitons compatible with the infinite-dimensional classical symmetry algebra. John Schwarz discussed suggestive properties of celestial OPEs for a variety of theories. Prahar Mitra presented higher-dimensional generalizations of the BMS group. Yangrui Hui proposed that integrated multi-particle states should be included in the celestial OPEs and are related to mulit-collinear limits. Laurent Friedel employed the classical Poisson bracket approach to characterizing asymptotic symmetries in GR. Sabrina Pasterski emphasized important open questions in CCFT. Robert Myers initiated the study of entanglement entropy in CCFT and its dual. Temple He studied the phase space of higher-dimensional Yang-Mills theory. Miguel Campiglia extended the usual gravitational phase space at null infinity to achieve Diff(S^2) covariance. Dan Kapec connected non-commuting soft limits with the existence of a CCFT conformal manifold.

**Top-Down Holography/Twistors:** Francisco Rojas explained how string theory amplitudes in the celestial basis can recover field theory amplitudes in a certain limit. Kevin Costello discussed a new top-down celestial holography construction for self-dual Yang-Mills with matter, using intersecting stacks of branes in the topological string. Roland Bittleston described an extended celestial chiral algebra based on self-dual gravity, motivated from twistor space, that characterizes special deformations of SDGR. Wei Bu constructed a 2d CFT from large gauge transformations of SDYM using scaling reductions on twistor space. Lionel Mason recalled twistorial approaches to reconstruct flat spacetime from asymptotic data. Tim Adamo explored scattering in the presence of strong background fields with twistorial methods. Eduardo Casali described how 4d Goldstone modes appear as generalized Wilson lines in twistor space and derived their OPEs. David Skinner reported on progress constructing twisted holographic celestial duals for gravitational theories on Ricci-flat Kahler metrics.

**Amplitudes & Bootstrap:** Lance Dixon reported on an astonishing and still-mysterious duality relating different high-loop planar amplitudes in N=4 super Yang-Mills. Lorenzo Magnea mapped the factorization properties of IR divergences in gauge theory amplitudes to the celestial sphere, and explained how that suggests a representation of the soft scattering as a free gauge-valued bosonic CCFT. Leonardo Rastelli presented new bounds on pion scattering in large-N QCD using bootstrap techniques. Sam Gralla derived formulas for BMS charges at all infinities and determined BMS-covariant notions of impact parameter and spin for classical gravitational scattering. Mina Himwich delineated nonlocal properties of celestial scattering amplitudes including massive particles.

**Memory Effects & Soft Modes:** Robert Wald clarified numerous formal consequences of asymptotic symmetries and memory for QFT scattering. Luca Ciambelli presented novel solutions and conservation laws related to the null Raychaudhuri equation, and interpreted it as a stress-tensor inequality. Ali Seraj introduced a new gyroscopic gravitational memory effect and discussed its prospects for detection. Eanna Flanagan examined lore about the behavior of classical soft modes and the consequences for black hole soft hair and information content.

**Related Advances (AdS, dS, & more):** Eva Silverstein reported on a trove of recent progress constructing holographic duals for patches of 3d de Sitter (dS) spacetimes, including computations of the dS entropy, by applying a combination of controlled deformations from familiar AdS/CFT examples. Jordan Cotler revisited other approaches to dS holography such as dS/CFT, as well as new nonperturbative tools for studying gravity in dS. Juan Maldacena discussed aspects of a universal double-cone wormhole solution in AdS/CFT which explains the ramp in the spectral form factor that arises in general chaotic systems. Tadashi Takayanagi proposed the concept of pseudo-entropy as the appropriate holographic version of entanglement entropy for celestial and dS holography. Roberto Emparan resolved old puzzles about how spinning string states interpolate to higher-dimensional black objects as they become strongly interacting.