Double radio sources as standard rods of testing cosmological models

A new large sample of 425 double radio sources is studied. A strong inverse correlation between the absolute brightness and the linear size of a double radio source is shown to exist. Since there is a tendency of observing preferentially bright and thus small doubles at large redshifts and faint (and thus large) doubles at small redshifts, the expected minimum of the angular size is lost in the selection process. The standard rod with absolute luminosity is calculated: it turns out that the calibrated standard rod behaves as expected in standard cosmologies, especially if the mean density parameter of the Universe Omega is less than or equal to 1. The absolute luminosity of the largest double radio sources decays exponentially with the size in Omega less than or equal to 1 Friedman models. This is an agreement with the simple model of Jackson for the decay of brightness in double radio source components. However, it is not possible to exclude totally the Omega equal to 1 models or the static Euclidean Universe on the basis of the double radio source data.