  Powerful extended radio galaxies like Cygnus A can be used for cosmological
tests. The characteristic, time-independent length $D_*$ for several radio
sources is calculated and compared to the average physical length for a sample
of radio galaxies. The ratio of these two lengths as a function of redshift is
used to constrain cosmological parameters. Cygnus A is the only low-redshift
($z<0.5$) radio galaxy for which we have an estimate for $D_*$. We comment on
the sensitivity to this one low-redshift point, and results including and
excluding Cygnus A are shown. A low density universe is favored, and the one
free model parameter is relatively well constrained.
