hide
Free keywords:
General Relativity and Quantum Cosmology, gr-qc,Astrophysics, Cosmology and Extragalactic Astrophysics, astro-ph.CO
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.
