Measurements of the Hubble constant and cosmic curvature with quasars: ultra-compact radio structure and strong gravitational lensing

TL;DR

This study combines radio quasar structures and strong gravitational lensing to simultaneously measure the Hubble constant and cosmic curvature, providing constraints consistent with local measurements and exploring the potential of quasars as cosmological probes.

Contribution

It introduces an improved method using quasars and lensing to jointly determine $H_0$ and $\,Omega_K$, enhancing precision and consistency checks with existing cosmological data.

Findings

Measured $H_0$ consistent with local distance ladder

Constrained cosmic curvature near flat universe

Demonstrated quasars as effective standard rulers

Abstract

Although the Hubble constant $H_0$ and spatial curvature $\Omega_{K}$ have been measured with very high precision, they still suffer from some tensions. In this paper, we propose an improved method to combine the observations of ultra-compact structure in radio quasars and strong gravitational lensing with quasars acting as background sources to determine $H_0$ and $\Omega_{K}$ simultaneously. By applying the distance sum rule to the time-delay measurements of 7 strong lensing systems and 120 intermediate-luminosity quasars calibrated as standard rulers, we obtain stringent constraints on the Hubble constant ($H_0=78.3\pm2.9 \mathrm{~km~s^{-1}~Mpc^{-1}}$) and the cosmic curvature ($\Omega_K=0.49\pm0.24$). On the one hand, in the framework of a flat universe, the measured Hubble constant ($H_0=73.6^{+1.8}_{-1.6} \mathrm{~km~s^{-1}~Mpc^{-1}}$) is strongly consistent with that derived from the local distance ladder, with a precision of 2\%. On the other hand, if we use the local $H_0$ measurement as a prior, our results are marginally compatible with zero spatial curvature ($\Omega_K=0.23^{+0.15}_{-0.17}$) and there is no significant deviation from a flat universe. Finally, we also evaluate whether strongly lensed quasars would produce robust constraints on $H_0$ and $\Omega_{K}$ in the non-flat and flat $\Lambda$CDM model if the compact radio structure measurements are available from VLBI observations.