title: Symmetries of asymptotically flat four-dimensional spacetimes at null infinity revisited.
abstract: It is shown that the symmetry algebra of asymptotically flat spacetimes at null infinity in 4 dimensions should be taken as the semidirect sum of supertranslations with infinitesimal local conformal transformations and not, as usually done, with the Lorentz algebra. As a consequence, two-dimensional conformal field theory techniques will play as fundamental a role in this context of direct physical interest as they do in three-dimensional anti-de Sitter gravity.
authors.affiliation: Physique Théorique et Mathématique, Université Libre de Bruxelles and International Solvay Institutes, Campus Plaine C.P. 231, B-1050 Bruxelles, Belgium. gbarnich@ulb.ac.be
authors.forename: Glenn
authors.initials: G
authors.lastname: Barnich
authors.forename: Cédric
authors.initials: C
authors.lastname: Troessaert