Null infinity, the BMS group and infrared issues

TL;DR

This review surveys the null-infinity framework for gravity, detailing how the BMS group extends Poincaré symmetry and how radiative data are encoded in intrinsic connections on $\mathcal{I}$ via the Bondi news tensor $N_{ab}$ and ${}^{\star}K^{ab}$. It explains the classical vacuum degeneracy induced by supertranslations, the memory effect, and the covariant phase space structure governing fluxes of BMS momenta. In quantum theory, it constructs the Fock representation from radiative data and shows soft charges vanish in this sector, highlighting infrared issues that demand non-Fock infrared sectors and dressed states, which connect to perturbative soft theorems and conserved supermomentum flux. The discussion clarifies two distinct infrared-sector formalisms, their relation to the soft theorems, and the limitations of perturbative Minkowski-space analyses, pointing toward both richer conservation laws and the need for non-perturbative GR insights to fully understand infrared gravity. $\,$

Abstract

There has been a recent resurgence of interest in the structure of the gravitational field at null infinity, sparked by new results on soft charges and infrared issues related to the S matrix theory in perturbative quantum gravity. We summarize these developments and put them in the broader context of research in the relativity community that dates back to several decades. In keeping with intent of this series, this overview is addressed to gravitational scientists who are not experts in this specific area.