Dimming Supernovae without Cosmic Acceleration

We present a simple model where photons propagating in extragalactic magnetic fields can oscillate into very light axions. The oscillations may convert some of the photons, departing a distant supernova, into axions, making the supernova appear dimmer and hence more distant than it really is. Averaging over different configurations of the magnetic field we find that the dimming saturates at about one-third of the light from the supernovae at very large redshifts. This results in a luminosity distance versus redshift curve almost indistinguishable from that produced by the accelerating Universe, if the axion mass and coupling scale are $m\sim {10}^{-16\mathrm{}}\mathrm{eV}$, $M\sim 4\times {10}^{11}\mathrm{GeV}$. This phenomenon may be an alternative to the accelerating Universe for explaining supernova observations.