Quantization of Gravity on Null Hypersurfaces
The initial value problem for general relativity on spacelike hypersurfaces is famously captured by the ADM formalism. Less well known is the Cauchy problem for general relativity on null hypersurfaces, which goes under the name of the Characteristic Initial Value Problem (CIVP). The CIVP is formulated on a pair of intersecting null hypersurfaces, encoding rich physics in the interplay between their respective initial data, and especially the gluing of these hypersurfaces at their shared codimension-two corner. In this work, we construct an operator-algebraic quantization of the CIVP. To do so, we first quantize each null hypersurface separately, using the corner symmetries as a guiding principle, and then glue them together at the joint initial cut. The resulting algebra admits an inner action of the full corner symmetry (semi)group consisting of superboosts, superrotations, and supertranslations. These are associated, respectively, with the quantization of the area, the H\'aji\v{c}ek one-form, and the expansion along both null directions. Supertranslations do not act as algebra preserving maps, but instead are quantized to quantum channels and included into the algebra via a generalized form of the Stinespring dilation theorem. The inclusion of superboosts and superrotations implements the gravitational constraints arising from Einstein's equations at the level of the quantum algebra. By virtue of the CIVP, in its local caustic-free domain of validity, our construction yields a candidate for the on-shell algebra of a gravitational subregion subtended by a pair of null hypersurfaces.