Nailing down the Hubble constant value through time delay cosmography: A 3% measurement from SN Refsdal

Abstract

We present the first inference of the Hubble constant and matter density parameter from a strongly lensed supernova (SN) by exploiting the mass model of a galaxy cluster enhanced by an extended surface brightness reconstruction. This has resulted in the most precise H0 measurement from a single strong lens so far, outperforming the 3.9% result from the lensed quasar DES J048-5354. Building upon the mass model of MACS J1149.5+2223 relying on image positions only, we incorporate the surface brightness distribution of the SN Refsdal host, to infer some cosmological parameter values within a flat LCDM framework. We consider two cosmological parameterizations, with either H0 and $Ω_m$ or H0 alone treated as free parameters. We explore four different mass-model realizations to assess some of the systematic uncertainties associated with the constraint weighting and source reconstruction. Although known as one of the main systematic effects, we deliberately keep the cluster total mass parametrization fixed to that of the previously published point-like mass model to enable a fair comparison. This reveals that incorporating 77,000 HST pixels reduces the uncertainty on the inferred value of H0 by a factor of 2.0. Our final measurements are $H_0=65.0^{+2.0}_{-1.8}$ km/s/Mpc and $Ω_m=0.30^{+0.08}_{-0.09}$, in agreement with previous results from lensed SNe. This corresponds to a substantially improved precision, reaching a remarkable 3.0% uncertainty on the value of H0. The contributions of the uncertainties on the lens total mass and on the measured time delays to the final H0 error are now comparable, indicating that future improvements will depend on more precise time-delay measurements and continued control of systematic effects. With sub-percent time-delay values reachable at cluster-scale, this mass model approach has the power to lead to unprecedentedly precise H0 measurements.