The Hubble constant determined through an inverse distance ladder including quasar time delays and Type Ia supernovae

Context. The precise determination of the present-day expansion rate of the Universe, expressed through the Hubble constant H 0 , is one of the most pressing challenges in modern cosmology. Assuming flat ΛCDM, H 0 inference at high redshift using cosmic microwave background data from Planck disagrees at the 4.4 σ level with measurements based on the local distance ladder made up of parallaxes, Cepheids, and Type Ia supernovae (SNe Ia), often referred to as Hubble tension. Independent cosmological-model-insensitive ways to infer H 0 are of critical importance. Aims. We apply an inverse distance ladder approach, combining strong-lensing time-delay distance measurements with SN Ia data. By themselves, SNe Ia are merely good indicators of relative distance, but by anchoring them to strong gravitational lenses we can obtain an H 0 measurement that is relatively insensitive to other cosmological parameters. Methods. A cosmological parameter estimate was performed for different cosmological background models, both for strong-lensing data alone and for the combined lensing + SNe Ia data sets. Results. The cosmological-model dependence of strong-lensing H 0 measurements is significantly mitigated through the inverse distance ladder. In combination with SN Ia data, the inferred H 0 consistently lies around 73–74 km s −1 Mpc −1 , regardless of the assumed cosmological background model. Our results agree closely with those from the local distance ladder, but there is a > 2 σ tension with Planck results, and a ∼1.5 σ discrepancy with results from an inverse distance ladder including Planck , baryon acoustic oscillations, and SNe Ia. Future strong-lensing distance measurements will reduce the uncertainties in H 0 from our inverse distance ladder.