The Scavenger Hunt for Quasar Samples to Be Used as Cosmological Tools

Although the (Formula presented.) Cold Dark Matter model is the most accredited cosmological model, information at high redshifts (z) between type Ia supernovae ((Formula presented.)) and the Cosmic Microwave Background ((Formula presented.)) is crucial to validate this model further. To this end, we have discovered a sample of 1132 quasars up to (Formula presented.) exhibiting a reduced intrinsic dispersion of the relation between ultraviolet and X-ray fluxes, (Formula presented.) vs. (Formula presented.) ((Formula presented.) less), than the original sample. This gold sample, once we correct the luminosities for selection biases and redshift evolution, enables us to determine the matter density parameter (Formula presented.) with a precision of 0.09. Unprecedentedly, this quasar sample is the only one that, as a standalone cosmological probe, yields such tight constraints on (Formula presented.) while being drawn from the same parent population of the initial sample.