Multipole expansion at the level of the action

TL;DR

This work develops an all-order multipole expansion at the level of the action for a scalar field, electromagnetism, and linearized gravity, casting the source action in terms of STF multipole moments. By Taylor expanding the long-wavelength fields and decomposing tensor moments into irreducible $SO(3)$ representations, it derives explicit all-order expressions for the multipole moments and demonstrates gauge-invariant formulations for the EM and gravitational sectors. The paper also provides closed-form expressions for the radiated power and far-field fields, and discusses extending the framework to nonlinear gravity within NRGR through covariantization. These results streamline matching to the effective radiation theory and enable precise gravitational-wave observables in PN calculations.

Abstract

Sources of long wavelength radiation are naturally described by an effective field theory (EFT) which takes the form of a multipole expansion. Its action is given by a derivative expansion where higher order terms are suppressed by powers of the ratio of the size of the source over the wavelength. In order to determine the Wilson coefficients of the EFT, i.e. the multipole moments, one needs the mapping between a linear source term action and the multipole expansion form of the action of the EFT. In this paper we perform the multipole expansion to all orders by Taylor expanding the field in the source term and then decomposing the action into symmetric trace free tensors which form irreducible representations of the rotation group. We work at the level of the action, and we obtain the action to all orders in the multipole expansion and the exact expressions for the multipole moments for a scalar field, electromagnetism and linearized gravity. Our results for the latter two cases are manifestly gauge invariant. We also give expressions for the energy flux and the (gauge dependent) radiation field to all orders in the multipole expansion. The results for linearized gravity are a component of the EFT framework NRGR and will greatly simplify future calculations of gravitational wave observables in the radiation sector of NRGR.