  Graviton-photon oscillation is the conversion of gravitational waves to
electromagnetic waves and vice versa in the presence of a background
electromagnetic field. We investigate this phenomenon in a cosmological
scenario considering a background cosmic magnetic field and assuming different
gravitational frameworks. We obtain the damping term that characterizes the
attenuation of the conversion probability in cosmological backgrounds. This is
a general feature that is present even for standard General Relativity.
Furthermore, we show that the effects of decoherence, which are due to the
interaction with the cosmological expansion and with the additional degrees of
freedom of alternative theories of gravity, can be relevant to the phenomenon
of graviton-photon mixing.
