2021MNRAS.503.2179Q

Although the Hubble constant H₀ and spatial curvature Ω_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 ultracompact structure in radio quasars and strong gravitational lensing with quasars acting as background sources to determine H₀ and Ω_K simultaneously. By applying the distance sum rule to the time-delay measurements of seven strong lensing systems and 120 intermediate-luminosity quasars calibrated as standard rulers, we obtain stringent constraints on the Hubble constant (H₀ = 78.3 ± 2.9 km s^–1 Mpc^–1) and the cosmic curvature (Ω_K = 0.49 ± 0.24). On the one hand, in the framework of a flat universe, the measured Hubble constant (H_0=73.6+1.8^_–1.6 kms^–1Mpc^–1) is strongly consistent with that derived from the local distance ladder, with a precision of 2 per cent. On the other hand, if we use the local H₀ measurement as a prior, our results are marginally compatible with zero spatial curvature (Ω_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 H₀ and Ω_K in the non-flat and flat Λ cold dark matter model, if the compact radio structure measurements are available from very long baseline interferometry observations.