A Hubble diagram for quasars
  The cosmological model is at present not tested between the redshift of the
farthest observed supernovae (z ~ 1.4) and that of the Cosmic Microwave
Background (z ~ 1,100). Here we introduce a new method to measure the
cosmological parameters: we show that quasars can be used as "standard candles"
by employing the non-linear relation between their intrinsic UV and X-ray
emission as an absolute distance indicator. We built a sample of ~ 1,900
quasars with available UV and X-ray observations, and produced a Hubble Diagram
up to z ~ 5. The analysis of the quasar Hubble Diagram, when used in
combination with supernovae, provides robust constraints on the matter and
energy content in the cosmos. The application of this method to forthcoming,
larger quasar samples, will also provide tight constraints on the dark energy
equation of state and its possible evolution with time.
