Photon-axion conversion as a mechanism for supernova dimming: Limits from CMB spectral distortion

Axion-photon conversion induced by intergalactic magnetic fields has been proposed as an explanation for the dimming of distant supernovae of type Ia (SNe Ia) without cosmic acceleration. The effect depends on the intergalactic electron density ne as well as the B-field strength and domain size. We show that for ne < ∼ 10 cm the same mechanism would cause excessive spectral distortion of the cosmic microwave background (CMB). This small-ne parameter region had been left open by the most restrictive previous constraints based on the dispersion of quasar (QSO) spectra. The combination of CMB and QSO limits suggests that the photon-axion conversion mechanism can only play a subleading role for SN Ia dimming. A combined analysis of all the observables affected by the photon-axion oscillations would be required to give a final verdict on the viability of this model.