On the Definition of the Spin Charge in Asymptotically-Flat Spacetimes

We propose a solution to a classic problem in gravitational physics consisting of defining the spin associated with asymptotically-flat spacetimes. We advocate that the correct asymptotic symmetry algebra to approach this problem is the generalized-BMS algebra $\textsf{gbms}$ instead of the BMS algebra used hitherto in the literature for which a notion of spin is generically unavailable. We approach the problem of defining the spin charges from the perspective of coadjoint orbits of $\textsf{gbms}$ and construct the complete set of Casimir invariants that determine $\textsf{gbms}$ coadjoint orbits, using the notion of vorticity for $\textsf{gbms}$. This allows us to introduce spin charges for $\textsf{gbms}$ as the generators of area-preserving diffeomorphisms forming its isotropy subalgebra. To elucidate the parallelism between our analysis and the Poincar\'e case, we clarify several features of the Poincar\'e embedding in $\textsf{gbms}$ and reveal the presence of condensate fields associated with the symmetry breaking from $\textsf{gbms}$ to Poincar\'e. We also introduce the notion of a rest frame available only for this extended algebra. This allows us to construct, from the spin generator, the gravitational analog of the Pauli--Luba\'nski pseudo-vector. Finally, we obtain the $\textsf{gbms}$ moment map, which we use to construct the gravitational spin charges and gravitational Casimirs from their dual algebra counterparts.