source count

A graph used as a test of cosmological models, in which the numbers of cosmic radio sources seen are plotted against their apparent luminosities. Source counts are important for determining the structure of the universe. William Herschel (1738-1822) used star counts in an attempt to elucidate the structure of the Milky Way. He failed because he was unaware of the degree of obscuration by interstellar matter. The technique was revived in the 1950s and 1960s by radio astronomers, particularly Martin Ryle at Cambridge. Plots were made of the numbers of extragalactic radio sources found within given magnitude ranges over the whole sky. All these sources are radio galaxies and quasars. The way the number of sources counted increases as the telescope probes to fainter detection limits can in principle be used to distinguish between different cosmological theories. From the outset, Ryle asserted that source counts favoured the Big Bang model, in contrast to the steady-state theory advocated by the Cambridge theorist, Fred Hoyle. It is now accepted that it is difficult to disentangle the results of the evolution of radio sources from cosmological effects.