Standardizing reverberation-measured Mg II time-lag quasars, by using the radius-luminosity relation, and constraining cosmological model parameters

We use 78 reverberation-measured Mg II time-lag quasars (QSOs) in the redshift range 0.0033 <= z <= 1.89 to constrain cosmological parameters in six different cosmological models. The basis of our method is the use of the radius-luminosity or R-L relation to standardize these 78 Mg II QSOs. In each cosmological model, we simultaneously determine R-L relation and cosmological model parameters, thus avoiding the circularity problem. We find that the R-L relation parameter values are independent of the cosmological model used in the analysis, thus, establishing that current Mg II QSOs are standardizable candles. Cosmological constraints obtained using these QSOs are significantly weaker than, but consistent with, those obtained from a joint analysis of baryon acoustic oscillation (BAO) observations and Hubble parameter [H(z)] measurements. So, we also analyse these QSOs in conjunction with the BAO + H(z) data and find cosmological constraints consistent with the standard spatially flat Lambda CDM model as well as with mild dark energy dynamics and a little spatial curvature. A larger sample of higher quality reverberation-measured QSOs should have a smaller intrinsic dispersion and so should provide tighter constraints on cosmological parameters.

Automated summary

Using 78 reverberation-measured Mg II QSOs in different cosmological models, the study found that these QSOs are standardizable candles and their constraints are consistent with BAO and Hubble parameter observations. Further analysis with BAO + H(z) data confirmed constraints consistent with the standard ΛCDM model, with the potential for tighter constraints from larger samples of higher quality QSOs in the future.