Conserved currents, superpotentials and cosmological perturbations

Conserved vectors are divergencies of superpotentials. In field theory on curved backgrounds, they are useful in calculating global 'charges' in arbitrary coordinates and local conserved quantities for small perturbations with specific gauge conditions. Superpotentials are, however, ill-defined. A new criterion of Julia and Silva selects uniquely for Dirichlet boundary conditions the 'KBL superpotential' as proposed by Katz, Bicak and Lynden-Bell, which has remarkable properties. Here, we show that a Belinfante-type addition to the KBL superpotential in general relativity gives an expression that is independent of boundary conditions defined by a variational principle. The modified superpotential has the same global properties as the KBL one, except for angular momentum at null infinity, and it does not differ from the KBL superpotential in the linearized theory of gravitation. As an illustration in linearized theory on curved backgrounds, we calculate conserved quantities for small perturbations on a Friedmann-Robertson-Walker space-time associated with conformal Killing vectors. Our unifying view relates a number of applications in cosmology found in the literature. Globally conserved quantities have simple physical interpretations in the 'uniform Hubble expansion' gauge.