Resonant Photon-Graviton Conversion and Cosmic Microwave Background Fluctuations

We point out that the coupling between the cosmic microwave background radiation (CMBR) and the primordial magnetic field can resonantly convert the photons into gravitons, which induces a frequency-independent fluctuation in the photon flux. Using the observed CMBR fluctuation, we derive a bound on the primordial field strength. The effect can also convert the relic gravitons into photons. For the non-string-based inflation theories it provides a direct test via measurement of long-wavelength electromagnetic waves. For the string cosmology it gives a new bound on the Hubble parameter at the big bang.