Soft Algebras via Bulk Double Soft Limits

Abstract

Soft and collinear limits of "celestial amplitudes" give rise to infinite dimensional symmetry algebras for two-dimensional (2D) "celestial" conformal field theories (CFTs). A small subset of these operators generates the action of the entire set recursively via the commutation relations. Insertion of this subset into celestial CFT correlators gives the boundary version of the first three terms in a soft expansion of the corresponding 4D bulk gravitational scattering amplitude. In this paper, we find that a bulk analog of this, which would allow the entire soft expansion of a gravitational amplitude to be determined via just the first three terms, does not follow trivially from the celestial algebras. We show how the interplay of soft and collinear limits results in subtleties for bulk amplitudes that do not show up in the boundary description. We also briefly comment on this construction in gauge theory.