Cosmological constraints from the Hubble diagram of quasars at high

The concordance model (Lambda cold dark matter (Lambda CDM) model, where is the cosmological constant) reproduces the main current observations (1-4) assuming the validity of general at all scales and epochs and the presence of CDM and of, equivalent to dark energy with a constant density in space and. However, the Lambda CDM model is poorly tested in the redshift between the farthest observed type Ia supernovae (5) and the microwave background. We present measurements of the expansion of the Universe based on a Hubble diagram of quasars. Quasars are most luminous persistent sources in the Universe, observed up to of z approximate to 7. 5 (refs. (6, 7) ). We estimate their following a method developed by our group (8-10), based on the -ray and ultraviolet emission of the quasars. The distance /redshift relation of quasars at z < 1. 4 is in agreement with of supernovae and with the concordance model. However, a deviation the Lambda CDM model emerges at higher redshift, with a statistical of similar to 4 sigma. If an evolution of the dark energy of state is allowed, the data suggest dark energy density with time.