Bi-orthogonal harmonics for the decomposition of gravitational radiation II: applications for extreme and comparable mass-ratio black hole binaries

The estimation of a physical system’s normal modes is a fundamental problem in physics. The quasi-normal modes of perturbed Kerr black holes, with their related spheroidal harmonics, are key examples, and have diverse applications in gravitational wave theory and data analysis. Recently, it has been shown that $\mathit{\text{adjoint}}$-spheroidal harmonics and the related spheroidal multipole moments may be used to estimate the radiative modes of arbitrary sources. In this paper, we investigate whether spheroidal multipole moments, relative to their spherical harmonic counterparts, better approximate the underlying modes of binary black hole spacetimes. We begin with a brief introduction to adjoint-spheroidal harmonics. We then detail a rudimentary kind of spheroidal harmonic decomposition, as well as its generalization which simultaneously estimates pro- and retrograde moments. Example applications to numerical waveforms from comparable and extreme mass-ratio binary black hole coalescences are provided. We discuss the morphology of related spheroidal moments during inspiral, merger, and ringdown. We conclude by discussing potential applications in gravitational wave theory and signal modeling.