Do reverberation-measured H$\beta$ quasars provide a useful test of cosmology?

TL;DR

This study evaluates whether reverberation-measured Hβ quasars can serve as reliable standard candles for testing cosmological models, finding limited current utility due to weak constraints and some tension with other measurements.

Contribution

It demonstrates that Hβ quasars have a cosmology-independent radius-luminosity relation, but their use as precise cosmological probes remains limited.

Findings

QSO R-L relation is independent of cosmology

Constraints from QSOs are weak and favor deceleration

Discrepancies persist even after extending the R-L relation

Abstract

We use 118 H$\beta$ quasar (QSO) observations in the redshift range $0.0023 \leq z \leq 0.89$ to simultaneously constrain cosmological model parameters and QSO 2-parameter radius-luminosity ($R-L$) relation parameters in six different cosmological models. We find that the $R-L$ relation parameters for these QSOs are independent of the assumed cosmology so these QSOs seem to be standardizable through the $R-L$ relation (although there is a complication that might render this untrue). Cosmological constraints obtained using these QSOs are weak, more favor currently decelerated cosmological expansion, and typically are in $\sim 2\sigma$ tension with those obtained from a joint analysis of baryon acoustic oscillation and Hubble parameter measurements. Extending the $R-L$ relation to a 3-parameter one to try to correct for the accretion rate effect does not result in a reduction of the cosmological constraints discrepancy nor does it result in the hoped-for significant reduction of the intrinsic scatter of the $R-L$ relation.