How can a five-time lensed supernova measure cosmic expansion?

Multiple images give a geometric handle on the universe’s expansion

Gravitational lensing can split a single exploding star into multiple light paths when its light is deflected by a massive object between it and Earth. Each path has a different length and gravitational potential, so the same explosion appears as distinct images that brighten at different times. A supernova observed in five separate images is extraordinarily rare and valuable.

Timing those brightening events provides direct measurements of the differences in light‑travel time between the images. Those time delays depend on the geometry of the lensing system and on the expansion rate of space. By combining precise time delays with detailed models of the intervening mass that does the lensing, astronomers can derive an independent estimate of the Hubble constant—the rate at which the universe is expanding.

Why this matters

• Independent probe: Time‑delay measurements avoid some systematics that affect other Hubble‑constant methods, offering a cross‑check on persistent tensions between local and early‑universe estimates.
• Sensitivity to cosmology: Multiple-image systems constrain both the expansion rate and aspects of the mass distribution in lenses, improving cosmological inference.
• Rarity and precision: A five‑image event gives more independent time delays and redundancy, tightening uncertainties if the lens model is well constrained.

Caveats include the need for high‑cadence monitoring, deep imaging to map the lensing mass, and careful modeling. Still, multiply imaged supernovae are among the most promising new tools for resolving a century‑old question about how fast the universe expands.