  The electromagnetic theory is, to a large extend, metric independent. Before
the metric is introduced, it is called premetric electrodynamics. Metric enters
the constitutive relation. We consider this relation for the Friedman model of
an expanding Universe and find that the magnitudes of the field quantities
depend on the scale factor. This factor, however, does not enter the
permeability and permittivity of the vacuum. A spatially uniform
electromagnetic field is obtained for spatially flat metric. Then plane
electromagnetic wave is found with uniform field playing the role of
amplitudes. It turns out that the magnitudes of the frequency and the wave
covector depend on the scale factor determining the redshift, but the phase
velocity of the wave is constant.
