084497

The non-linear X-ray-UV relation in quasars can be used to build a Hubble diagram and test cosmological models at high redshift. To this aim comma we were awarded an XMM LP to observe 30 bright quasars at zvirgul3. We found a tighter-than- expected X-ray-UV relation comma implying that pointed observations can greatly reduce the uncertainties in the determination of quasar distances comma and a new population of X-ray weak quasars that represents virgul30% of the zvirgul3 sample. We propose an extensive survey of bright comma radio-quiet comma SDSS quasars at zvirgul4 with the goals of openPar1closePar detecting a 4sigma deviation from the LambdaCDM model using this zvirgul4 sample alone comma openPar2closePar measuring the X-ray weak fraction of quasars for the first time at zvirgul4 and its evolution with time.