  A fundamental prediction of relativistic cosmologies is that, due to the
expansion of space, observations of the distant cosmos should be time dilated
and appear to run slower than events in the local universe. Whilst observations
of cosmological supernovae unambiguously display the expected
redshift-dependent time dilation, this has not been the case for other distant
sources. Here we present the identification of cosmic time dilation in a sample
of 190 quasars monitored for over two decades in multiple wavebands by
assessing various hypotheses through Bayesian analysis. This detection counters
previous claims that observed quasar variability lacked the expected
redshift-dependent time dilation. Hence, as well as demonstrating the claim
that the lack of the redshift dependence of quasar variability represents a
significant challenge to the standard cosmological model, this analysis further
indicates that the properties of quasars are consistent with them being truly
cosmologically distant sources.
