  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.
