Description
According to Strominger, Weinberg's soft graviton theorem is related to the fact that the BMS group is a symmetry of the S matrix. For this to even make sense, the BMS group must be promoted from two BMS groups acting on asymptotic states to a global BMS group. In this, spatial infinity plays a pivotal role as the link between the two. In this talk, we will introduce an extended boundary, called Spi, at spatial infinity. This boundary is canonically defined and possesses a strongly Carrollian structure. Preservation of this strongly Carrollian structure in a suitable way leads to the appearance of the BMS group at spatial infinity, up to some parity condition. We will show that this parity condition is related to regularity issues at Scri.