Soft charges and zero modes at null boundaries
arXiv:2607.28543
The paper investigates how a global zero-mode ambiguity on null boundaries, including null infinity and black‑hole horizons, changes the canonical structure of field theories and introduces a new quasilocal edge observable that modifies the soft charge algebra.
Abstract
Asymptotic symmetries at null boundaries provide a powerful window into the soft sector of field theories, revealing conserved charges and infinite-dimensional symmetry algebras beyond the standard bulk dynamics. While these structures are usually associated with residual gauge transformations, null boundaries also exhibit a characteristic global zero-mode ambiguity, whose physical consequences are less explored. We discuss how this zero mode modifies the canonical structure of field theories at null boundaries. In particular, we show that it gives rise to a new quasilocal edge observable, with consequences to the charge algebra. It can arise on any kind of null boundaries, both at the null infinity and the finite distance such as a black hole horizon.
To appear in Proceedings of Science for the Conference School "Foundations of General-Relativistic Gauge Field Theory" held at Politecnico di Torino, Italy, in March 2026