The Hubble diagram of high redshift objects, QSOs and AGNs
  According to the Hubble law, high redshift objects such as Quasar (QSOs), X-ray Active Galactic Nuclei (AGN) together with the Gamma Ray Burst (GRBs) are the fastest and farthest objects. These characteristics provides strong motivations for they to be used to constrain the cosmological parameters, without the limitations found in the Ia supernovae study and which is restricted to redshift of up to 1.7. However, the variability and behavior in the QSOs and AGNs spectra tell us that they have very complex structures and the standard candle framework can not be applied. So far the available data of QSOs and AGNs have shown some anomalies observed in their brightness and metallicities, difficult to understand, under an orthodox point of view. Here, we show that their Hubble diagram flattens for $z\geq 3 $. The result need further confirmation, because the statistics of high redshift extragalactic objects is still poor. Details and some implications of these results are reported in this work. 
C. E. Navia, C. R. A. Augusto, and K. H. Tsui