Perturbative gravity at null infinity

TL;DR

This paper develops a boundary theory at null infinity that encodes gravitational radiation, reproduces soft graviton theorems via BMS symmetries, and computes the classical gravitational S-matrix.

Contribution

It introduces a boundary theory on null infinity capturing radiative modes and links BMS symmetries to soft graviton theorems, providing a new perspective on gravitational scattering.

Findings

BMS supertranslations relate to Weinberg's soft graviton theorem.

Extended BMS algebra includes superrotation Ward identities.

The theory reproduces the classical gravitational S-matrix.

Abstract

We describe a theory that lives on the null conformal boundary of asymptotically flat space-time, and whose states encode the radiative modes of (super)gravity. We study the induced action of the BMS group, verifying that the Ward identity for certain BMS supertranslations is equivalent to Weinberg's soft graviton theorem in the bulk. The subleading behaviour of soft gravitons may also be obtained from a Ward identity for certain superrotation generators in the extended BMS algebra proposed by Barnich & Troessaert. We show that the theory computes the complete classical gravitational S-matrix, perturbatively around the Minkowski vacuum.