Compact radio sources and the determination of the cosmological parameters

Nuclei of Cygnus A type radio sources are characterized by relativistic ejections. Observations of two-sided VLBI jets allow the determination of the cosmological parameters. If the bulk Lorentz factor is >or=3 and the ratio of the apparent velocities of the two opposite jets is <or=2, the mean of their velocities is approximately=c within 6%. From existing data, the authors obtain H₀ approximately=100 km s^-1 Mpc^-1 and q₀<or=1. If the age of the globular clusters is tau approximately=15 Gyr, the cosmological constant Lambda ₀ cannot be equal to 0.