Gravitational vacua in the Newman-Penrose formalism

In this paper, we derive the generic solution of the Newman-Penrose equations in the Newman-Unti gauge with vanishing curvature tensor. The obtained solutions are the vacua of the gravitational theory which are connected to the derivations in metric formalism from exponentiating the infinitesimal BMS generators in the Bondi-Metzner-Sachs (BMS) gauge in Compère and Long [ and ] by a radial transformation. The coordinate transformations in the Newman-Unti gauge connecting each vacuum are also obtained. We confirm that the supertranslation charge of the gravitational vacua with respect to global considerations vanishes exactly not only in the Einstein theory but also when including the Holst, Pontryagin, and Gauss-Bonnet terms, which verifies that the gravitational vacua are not affected by those trivial or boundary terms. Published by the American Physical Society 2024