Aberration of gravitational waveforms by peculiar velocity

TL;DR

This paper shows that the peculiar velocity of merging binaries causes apparent spin-1 polarisation in gravitational wave signals, which can be interpreted as aberration effects rather than new physics, and does not affect tests of General Relativity.

Contribution

It demonstrates that velocity-induced spin-1 polarisation in gravitational waves is a projection effect, not a deviation from General Relativity, and can be absorbed into orientation parameters.

Findings

Velocity causes apparent spin-1 polarisation in gravitational waves.

Aberration effects can be reinterpreted as pure spin-2 modes from a shifted direction.

Velocity bias does not impact tests of General Relativity.

Abstract

One key prediction of General Relativity is that gravitational waves are emitted with a pure spin-2 polarisation. Any extra polarisation mode, spin-1 or spin-0, is consequently considered a smoking gun for deviations from General Relativity. In this paper, we show that the velocity of merging binaries with respect to the observer gives rise to spin-1 polarisation in the observer frame even in the context of General Relativity. These are pure projection effects, proportional to the plus and cross polarisations in the source frame, hence they do not correspond to new degrees of freedom. We demonstrate that the spin-1 modes can always be rewritten as pure spin-2 modes coming from an aberrated direction. Since gravitational waves are not isotropically emitted around binary systems, this aberration modifies the apparent orientation of the binary system with respect to the observer: the system appears slightly rotated due to the source velocity. Fortunately, this bias does not propagate to other parameters of the system (and therefore does not spoil tests of General Relativity), since the impact of the velocity can be fully reabsorbed into new orientation angles.