  Photon-axion conversion induced by intergalactic magnetic fields causes an
apparent dimming of distant sources, notably of cosmic standard candles such as
supernovae of type Ia (SNe Ia). We review the impact of this mechanism on the
luminosity-redshift relation of SNe Ia, on the dispersion of quasar spectra,
and on the spectrum of the cosmic microwave background. The original idea of
explaining the apparent dimming of distant SNe Ia without cosmic acceleration
is strongly constrained by these arguments. However, the cosmic equation of
state extracted from the SN Ia luminosity-redshift relation remains sensitive
to this mechanism. For example, it can mimic phantom energy.
