SN H0pe: The First Measurement of H_0 from a Multiply-Imaged Type Ia Supernova, Discovered by JWST

The first James Webb Space Telescope (JWST) Near InfraRed Camera (NIRCam) imaging in the field of the galaxy cluster PLCK G165.7+67.0 (z=0.35) uncovered a Type Ia supernova (SN Ia) at z=1.78, called “SN H0pe." Three different images of this one SN were detected as a result of strong gravitational lensing, each one traversing a different path in spacetime, thereby inducing a relative delay in the arrival of each image. Follow-up JWST observations of all three SN images enabled photometric and rare spectroscopic measurements of the two relative time delays. Following strict blinding protocols which oversaw a live unblinding and regulated post-unblinding changes, these two measured time delays were compared to the predictions of seven independently constructed cluster lens models to measure a value for the Hubble constant, H_0=71.8^+9.8_-7.6 km s^-1 Mpc^-1. The range of admissible H_0 values predicted across the lens models limits further precision, reflecting the well-known degeneracies between lens model constraints and time delays. It has long been theorized that a way forward is to leverage a standard candle, however this has not been realized until now. For the first time, the lens models are evaluated by their agreement with the SN absolute magnification, breaking these degeneracies and producing our best estimate, H_0=75.4^+8.1_-5.5 km s^-1 Mpc^-1. This is the first precision measurement of H_0 from a multiply-imaged SN Ia, and provides a measurement in a rarely utilized redshift regime. This result agrees with other local universe measurements, yet exceeds the value of H_0 derived from the early Universe with ≳90% confidence, increasing evidence of the Hubble tension. With the precision provided by only four more events, this approach could solidify this disagreement to >3σ.