Multipole expansion at the level of the action

TL;DR

This paper develops a comprehensive multipole expansion at the action level for scalar, electromagnetic, and gravitational fields, providing exact, gauge-invariant expressions for multipole moments and radiation, enhancing EFT methods in gravitational wave physics.

Contribution

It performs an all-orders multipole expansion at the action level, deriving exact multipole moments and gauge-invariant expressions for various fields within an EFT framework.

Findings

All-orders multipole expansion derived for scalar, EM, and gravity fields.

Exact expressions for multipole moments obtained.

Gauge invariance maintained in gravitational case.

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.