Cosmological constraints from the Hubble diagram of quasars at high redshifts

The concordance (LambdaCDM) model reproduces the main current cosmological observations assuming the validity of general relativity at all scales and epochs, the presence of cold dark matter, and of a cosmological constant, equivalent to a dark energy with constant density in space and time. However, the LambdaCDM model is poorly tested in the redshift interval between the farthest observed Type Ia supernovae5 and that of the Cosmic Microwave background (CMB). We present new measurements of the expansion rate of the Universe in the range 0.5<z<5.5 based on a Hubble diagram of quasars. The quasar distances are estimated from their X-ray and ultraviolet emission, following a method developed by our group. The distance modulus-redshift relation of quasars at z<1.4 is in agreement with that of supernovae and with the concordance model. Yet, a deviation from the LambdaCDM model emerges at higher redshift, with a statistical significance of ~4 sigma. If an evolution of the dark energy equation of state is allowed, the data suggest a dark energy density increasing with time.