Resonant graviton–photon conversion with stochastic magnetic field in the expanding universe

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 correlation lengths. Resonances can be tested by radio telescopes as a probe of high frequency gravitational wave sources and primordial magnetogenesis mechanisms.