Detection of the cosmological time dilation of high-redshift quasars

A fundamental prediction of relativistic cosmologies is that, owing 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. While observations of cosmological supernovae unambiguously show 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 dismissing the claim that the apparent lack of the redshift dependence of quasar variability represents a substantial challenge to the standard cosmological model, this analysis further indicates that the properties of quasars are consistent with them being truly cosmologically distant sources.