Abstract

The holographic principle is a powerful tool in gaining insight into quantum gravity for spacetimes with a negative cosmological constant, but it is an outstanding question how generally it applies. I will review recent advances in celestial holography which conjectures that quantum gravity in spacetimes with flat asymptotics (or vanishing cosmological constant) is dual to a co-dimension two conformal field theory (CFT) on the celestial sphere at null infinity. Recasting the S-matrix, which is the basic observable in quantum gravity, in a basis of boost (rather than energy) eigenstates reveals similarities with CFT correlation functions. In particular, universal features that arise in energetically soft and collinear limits take a natural form in CFT language and spacetime (asymptotic) symmetries can be explained purely from CFT reasoning. I will highlight several intriguing and some puzzling features that arise in this program, and discuss some avenues for developing celestial holography for non-trivial asymptotically flat backgrounds and for any number of dimensions.