The physics of radio sources and cosmology

Malcolm Longair's first notable contribution to science was to point out that the radio source counts require not only that the source population should evolve, but that powerful sources should evolve much faster than weak sources. Ever since then he has been trying to define more quantitatively how one must fill up the P - z plane, and indeed much of this symposium has been devoted to that and closely related problems. The theoretician in each of us cannot help also wondering why. There are plenty of explanations for the greater abundance of radio sources in the past; all sorts of exciting things could have happened when the world was young and galaxies first shone forth out of the primaeval turbulence. There are fewer explanations of the fact that weaker radio sources weren't nearly as overabundant (relative to the present epoch) as powerful ones. However, there is one natural and elegant explanation, which depends on the idea that old, weak, diffuse sources are extinguished because of inverse Compton losses on the microwave background. This explanation is pursued in this paper. (Auth.)