Probing cosmological fields with gravitational wave oscillations
  Gravitational wave (GW) oscillations occur whenever there are additional
tensor modes interacting with the perturbations of the metric coupled to
matter. These extra modes can arise from new spin-2 fields (as in e.g.
bigravity theories) or from non-trivial realisations of the cosmological
principle induced by background vector fields with internal symmetries (e.g.
Yang-Mills, gaugids or multi-Proca). We develop a general cosmological
framework to study such novel features due to oscillations. The evolution of
the two tensor modes is described by a linear system of coupled second order
differential equations exhibiting friction, velocity, chirality and mass
mixing. We follow appropriate schemes to obtain approximate solutions for the
evolution of both modes and show the corresponding phenomenology for different
mixings. Observational signatures include modulations of the wave-form,
oscillations of the GW luminosity distance, anomalous GW speed and chirality.
We discuss the prospects of observing these effects with present and future GW
observatories such as LIGO/VIRGO and LISA.
