A statistical technique used to study the evolutionary history of the Universe. It involves counting the numbers of radio sources, N, above a threshold flux density, S, in a given area of sky. If the Universe is flat and the luminosity and space density of radio sources has not evolved with time, then a plot of N against S on logarithmic axes should be a straight line of slope −1.5. Deviations from this line for the faintest (and hence most distant) sources show that radio sources were both more powerful and more numerous in the early Universe, and are strong evidence that the Universe has evolved with time.
Subjects: Astronomy and Astrophysics.