How could a lensed supernova measure expansion?

A rare natural experiment for cosmology

A gravitationally lensed supernova offers astronomers a unique way to probe the universe’s expansion. When an exploding star lies directly behind a massive galaxy or cluster from our line of sight, the foreground mass bends spacetime and splits the light into multiple images. Because the paths that light takes differ in length and gravitational potential, the images arrive at Earth at different times. Those time delays encode information about cosmological distances and the Hubble constant—the rate at which the universe expands.

Why this matters right now Cosmologists are wrestling with a persistent tension between measurements of the Hubble constant made from the nearby universe and those inferred from the early universe. A lensed supernova provides an independent, geometric route to H0 that relies on General Relativity and careful lens modelling rather than assumptions about early-universe physics. The rarity of such events makes every discovery valuable: with high-quality imaging, spectroscopic redshifts for the supernova and the lensing galaxy, and detailed mass maps of the lens, teams can convert measured time delays into a distance scale.

Key steps and limits - Obtain precise timing of each image’s light curve.
- Map the mass distribution in the lensing galaxy or cluster to reduce modelling uncertainties.
- Combine with redshift measurements to derive cosmological parameters.

Uncertainties remain. Lens mass models, substructure in the lens, and microlensing by stars can all bias results if not properly accounted for. Still, every well-observed lensed supernova tightens an alternative ladder for measuring expansion and sharpens constraints on dark energy’s behavior.