The BMS-like symmetry of extremal horizons

I revisit the calculation of infinite-dimensional symmetries that emerge in the vicinity of isolated horizons. I focus the attention on extremal black holes, for which the isometry algebra that preserves a sensible set of asymptotic boundary conditions at the horizon strictly includes the BMS algebra. The conserved charges that correspond to this BMS sector, however, reduce to those of superrotation, generating only two copies of Witt algebra. For more general horizon isometries, in contrast, the charge algebra does include both Witt and supertranslations, being similar to BMS but s.str. differing from it. This work has been prepared for the proceedings of the XXII Simposio Sofichi 2020, held in Chile in November 2020. The material herein is based on my work in collaboration with Laura Donnay, Hernan Gonzalez and Miguel Pino, and it is included in arXiv:1511.08687 and arXiv:1607.05703.