Throughout the last century, cosmology measured the universe's expansion rate using supernovae, redshifts, and the cosmic microwave background. But careful measurements led to drama: Hubble's law ceased to be a law — different methods yield Hubble constant values that diverge by almost 5σ. This is the 'Hubble tension,' and it hints at a crack in the very foundation of the ΛCDM cosmological model. Edwin Hubble discovered the expansion, Albert Einstein provided its geometry, and Fritz Zwicky proposed using gravitational lensing as a natural telescope. Now astronomers have applied this optical trick to catch echoes of distant supernovae, and from their time delays derive an independent expansion rate. It's like calculating the volume of a cathedral from the echoes of footsteps.

In the galaxy cluster MACS J0138, billions of light-years away, two Type Ia supernovae, SN Requiem and SN Encore, put on a remarkable show: the light from each split into multiple images, arriving with different delays. It's like a single thunderclap generating a series of rumbles as it echoes off canyon walls — the differences in sound arrival reveal the valley's geometry. Astronomers reconstructed a detailed map of the gravitational field from 23 multiple images of eight background sources, captured by Hubble and James Webb, along with MUSE spectroscopy.

The cluster mass was modeled using the Bayesian package Gravity.jl: dark matter was described by an elliptical profile, visible galaxies by scaling relations, and dark energy within ΛCDM set the global geometry. Light curves of the supernovae provided time delays accurate to within a few days. For Encore, one image leads the others by 38 days, while the slowest will arrive in 3177 days. For Requiem, the latest echo lags by nearly 11 years — expected by 2027. The joint analysis yielded H₀ = 67.0+9.3-7.8 km/s/Mpc — a value that nearly matches estimates from the cosmic microwave background and baryon acoustic oscillations.

This result not only confirms the early universe but also sharpens the Hubble tension: the independent lensing method sides with cosmological parameters, not local measurements. It's as if the same clock, viewed from different angles, runs at different speeds. The solution may lie in the properties of dark energy, unresolved systematics, or even a revision of the laws of gravity. Further monitoring and accounting for the surface brightness of arcs will refine the model. With the launch of the Rubin Observatory, the stream of lensed supernovae will grow by orders of magnitude, turning the method into a standard cosmic ruler. And in 2027, we will catch Requiem's last echo, and perhaps hear the quiet music of new physics, written in the light delays.