  We study the propagation of light bundles in non-empty spacetime, as most of
the Universe is filled by baryonic matter in the form of a (dilute) plasma.
Here we restrict to the case of a cold (i.e., pressureless) and non-magnetised
plasma. Then the influence of the medium on the light rays is encoded in the
spacetime dependent plasma frequency. Our result for a general spacetime
generalises the Sachs equations to the case of a cold plasma Universe. We find
that the reciprocity law (Etherington theorem), the relation that connects area
distance with luminosity distance, is modified. Einstein's field equation is
not used, i.e., our results apply independently of whether or not the plasma is
self-gravitating. As an example, our findings are applied to a homogeneous
plasma in a Robertson-Walker spacetime. We find small modifications of the
cosmological redshift of frequencies and of the Hubble law.
