  We present a simple model where photons propagating in extra-galactic
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 1/3 of the light from the supernovae at very large
redshifts. This results in a luminosity-distance vs. redshift curve almost
indistinguishable from that produced by the accelerating Universe, if the axion
mass and coupling scale are m ~ 10^-16 eV, M ~ 4 10^11 GeV. This phenomenon may
be an alternative to the accelerating Universe for explaining supernova
observations.
