Is the universe transparent or opaque

The author determines the maximal class of conformally flat cosmological models which eventually absorb all the electromagnetic radiation they contain. This is the requirement of the Wheeler-Feynman absorber theory of radiation. The condition for complete absorption is more or less written down immediately, and the various cosmological models tested by inspection. Some of the existing errors concerning the refractive index of the intergalactic medium and the past absorber are examined in some detail. Of the Friedmann cosmologies, only the oscillating models are opaque. A number of ever-expanding nonrelativistic cosmologies such as the Dirac and the latest Hoyle-Narlikar models, are also opaque. An assessment of the present value of the Wheeler-Feynman theory is given in the light of recent work on its quantization.