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Although the $\Lambda$ Cold Dark Matter model is the most accreditedcosmological model, information at high redshifts ($z$) between type Iasupernovae ($z=2.26$) and the Cosmic Microwave Background ($z=1100$) is crucialto validate this model further. To this end, we have discovered a sample of1132 quasars up to $z=7.54$ exhibiting a reduced intrinsic dispersion of therelation between ultraviolet and X-ray fluxes, $\delta_\mathrm{F}=0.22$ vs.$\delta_\mathrm{F}=0.29$ ($24\%$ less), than the original sample. This goldsample, once we correct the luminosities for selection biases and redshiftevolution, enables us to determine the matter density parameter $\Omega_M$ witha precision of 0.09. Unprecedentedly, this quasar sample is the only one that,as a standalone cosmological probe, yields such tight constraints on $\Omega_M$while being drawn from the same parent population of the initial sample.

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