Cosmological constraints from the Hubble diagram of quasars at high
\nredshifts

Abstract

The concordance model (Lambda cold dark matter (Lambda CDM) model, where
\nLambda is the cosmological constant) reproduces the main current
\ncosmological observations(1-4) assuming the validity of general
\nrelativity at all scales and epochs and the presence of CDM and of
\nLambda, equivalent to dark energy with a constant density in space and
\ntime. However, the Lambda CDM model is poorly tested in the redshift
\ninterval between the farthest observed type Ia supernovae(5) and the
\ncosmic microwave background. We present measurements of the expansion
\nrate of the Universe based on a Hubble diagram of quasars. Quasars are
\nthe most luminous persistent sources in the Universe, observed up to
\nredshifts of z approximate to 7.5 (refs. (6,7)). We estimate their
\ndistances following a method developed by our group(8-10), based on the
\nX-ray and ultraviolet emission of the quasars. The distance
\nmodulus/redshift relation of quasars at z < 1.4 is in agreement with
\nthat of supernovae and with the concordance model. However, a deviation
\nfrom the Lambda CDM model emerges at higher redshift, with a statistical
\nsignificance of similar to 4 sigma. If an evolution of the dark energy
\nequation of state is allowed, the data suggest dark energy density
\nincreasing with time.