Extended-BMS Anomalies and Flat Space Holography

Abstract

We geometrically determine the BRST symmetry of a three-dimensional extended BMS field theory defined at the null boundary of asymptotically flat four-dimensional spaces. We then discuss its ghost number zero and one cocycles, independently of the four-dimensional bulk. The former determine a topological Lagrangian localized on the boundary of null infinity, while the latter classify the corresponding anomalies. This classification reproduces the eBMS anomalies known from the bulk, providing a rare non-perturbative test of flat space holography. While supertranslations are anomaly-free, superrotations admit independent central charges. This theory can thus be dual to Einstein gravity only if these central charges are dictated by the bulk.

arXiv commentv2, minor revisions and improved clarity; matches the version to be published in PLB