Resonant photon-graviton conversion as a probe into inflationary universe

Inflationary cosmology predicts the generation of both density perturbation and relic gravitons. Tests of these predictions, especially the relic gravitons, will be very crucial to the verification of inflation. Efforts have been made to distinguish different inflation models using observed anisotropies of the cosmic microwave background radiation. However, the fact that both scalar and tensor modes contribute to the Sachs-Wolfe effect renders such determination difficult. We point out that, mediated by the primordial magnetic field, the relic gravitons can resonantly convert into photons at the same frequencies. A measurement of the spectrum of these long wavelength EM waves can help to determine the power law index directly, which will distinguish different inflationary models. This opens up a new window for a glance at ''gravity's rainbow''.