Determination of Cosmological Parameters with a Sample of Quasars

We use a sample of 808 quasars selected by Risaliti and Lusso to estimate two important cosmological parameters: the percentage of matter in the Universe Ωm and the Hubble constant. The method is based on an auxiliary experimental correlation between the luminosity of quasars in the Xray band and UV band in the form log L_X = β + γ log L_UV. For the flat ΛCDM model our fit gives Ω _m = 0.21 ± 0.12. Our main results are the following: firstly, the fraction of matter (baryonic + dark) contained in the Universe is 21% according to our estimate and is smaller than the one found by other authors using Type Ia Supernovae (though, considering the large error, our result is consistent with the supernova data). Secondly, the Einstein–de Sitter model is outside the 95% confidence interval of our best fit curve. In order to determine also the Hubble constant, we were compelled to fix one of the free parameters (β, γ,H₀,Ω _m ) and to determine the others with the non-linear least square method. We have proceeded in two different ways. Increasing h₀ = H₀/100 with a step of 0.01 in the range from 0.65 to 0.95, we obtain a Hubble constant H₀ = 74.6± 2.4 (km/s)/Mpc in agreement with the values found using CMB, supernovae and cepheids. On the other hand, increasing the parameter m = β/γ with a step of 0.03 in the range from 13.4 to 14.4, we obtain the same result (but with a greater statistical error), and hence a self-consistentmodel, only assuming β ≥ 8.21.