On rigidity of 3d asymptotic symmetry algebras

Abstract We study rigidity and stability of infinite dimensional algebras which are not subject to the Hochschild-Serre factorization theorem. In particular, we consider algebras appearing as asymptotic symmetries of three dimensional spacetimes, the b m s 3 $$ \mathfrak{b}\mathfrak{m}{\mathfrak{s}}_3 $$ , u 1 $$ \mathfrak{u}(1) $$ Kac-Moody and Virasoro algebras. We construct and classify the family of algebras which appear as deformations of b m s 3 $$ \mathfrak{b}\mathfrak{m}{\mathfrak{s}}_3 $$ , u 1 $$ \mathfrak{u}(1) $$ Kac-Moody and their central extensions by direct computations and also by cohomological analysis. The Virasoro algebra appears as a specific member in this family of rigid algebras; for this case stabilization procedure is inverse of the Inönü-Wigner contraction relating Virasoro to bms3 algebra. We comment on the physical meaning of deformation and stabilization of these algebras and relevance of the family of rigid algebras we obtain.