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Perturbative gravity at null infinity
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.Comment: 14 pages, no figures. v2: typos corrected, references adde