  We investigate graviton-photon oscillations sourced by cosmological magnetic
fields from Gertsenshtein effect. We adopt a robust perturbative approach and
we find that the conversion probability from graviton to photon can be
resonantly enhanced in monochromatic, multi-chromatic and scale invariant
spectrum models of stochastic magnetic field fluctuations. In addition, the
expansion of the Universe acts as a decoherence factor, which demands a natural
discretization scheme along the line of sight. Including also decoherence from
cosmic acceleration, we find that conversion probabilities for stochastic
magnetic fields are completely different than results predicted from existing
magnetic domain-like models in a wide range of magnetic strengths and coherence
lengths. Resonances can be tested by radio telescopes as a probe of high
frequency gravitational wave sources and primordial magnetogenesis mechanisms.
