Five Copies of One Supernova Could Finally Resolve the Hubble Tension

For nearly a century, astronomers have known the universe is expanding. The argument has never been about whether it grows, but how fast. Two of the best methods for measuring that rate - the cosmic distance ladder and readings from the cosmic microwave background - keep returning different answers, a disagreement now known as the Hubble tension. A supernova discovered in August 2025 may offer the cleanest test yet of where the truth lies.

The object, designated SN 2025wny and nicknamed SN Winny by the team that found it, sits roughly 10 billion light-years from Earth. It is a superluminous supernova - a class of stellar explosion far brighter than ordinary supernovae, making it detectable across cosmological distances. What makes SN Winny truly unusual is not its brightness alone. As its light travels toward Earth, it passes through two foreground galaxies whose gravity bends and splits the beam into five separate images, each arriving at a slightly different time.

Five images, one clock

The phenomenon responsible is gravitational lensing, predicted by general relativity and observed for decades, though rarely in such a clean configuration. Most gravitationally lensed supernovae found to date were magnified by massive galaxy clusters, whose irregular mass distributions make modeling difficult. SN Winny is lensed by just two individual galaxies with comparatively smooth, regular structures.

That simplicity matters enormously for what comes next. By measuring the time delays between the five copies of the explosion - the differences in when each image's light reaches a telescope - researchers can calculate the Hubble constant directly. The calculation depends on knowing how the lensing galaxies distribute their mass, because mass governs how sharply the light bends and how long each path takes.

To build that mass model, team members from the Max Planck Institute for Extraterrestrial Physics and Ludwig Maximilians University used the Large Binocular Telescope in Arizona, whose twin 8.4-meter mirrors and adaptive optics system correct for atmospheric distortion. The resulting image - the first high-resolution color view of the system - shows two warm-toned foreground galaxies at center and five blue copies of the supernova arranged around them, resembling a firework mid-burst.

Allan Schweinfurth, a PhD student at the Technical University of Munich who led the lens modeling work alongside Leon Ecker at LMU, notes that the galaxies appear not to have collided despite their close angular proximity in the sky. Their overall smooth structure means the mass model contains fewer free parameters, making the eventual Hubble constant measurement less susceptible to modeling errors.

Why existing methods disagree

The Hubble tension - currently sitting at roughly a 5-sigma discrepancy between the two leading methods - has been debated for more than a decade. The cosmic distance ladder, which chains together measurements of stellar distances step by step, gives a present-day expansion rate of around 73 kilometers per second per megaparsec. The cosmic microwave background method, which extrapolates forward from the physics of the early universe, returns a value closer to 67. Neither result has crumbled under scrutiny, which has led some physicists to wonder whether the standard cosmological model itself needs revision.

Stefan Taubenberger, first author of the supernova identification study and a researcher in Sherry Suyu's group at TUM and the Max Planck Institute for Astrophysics, explains why a lensed supernova measurement is valuable precisely because it avoids the assumptions embedded in both existing methods: "Unlike the cosmic distance ladder, this is a one-step method, with fewer and completely different sources of systematic uncertainties."

Suyu, who spent six years building a catalog of candidate gravitational lenses in anticipation of an event like this, describes the odds of finding it: "The chance of finding a superluminous supernova perfectly aligned with a suitable gravitational lens is lower than one in a million." SN Winny appeared in August 2025 matched precisely against one of the catalog entries.

What the time delays will reveal - and what they won't

The measurement is not yet complete. Astronomers worldwide are currently monitoring SN Winny with both ground-based and space-based telescopes to track how each of the five images brightens and fades over time. Those light curves, combined with the lens mass model already in hand, will yield the time delays and ultimately the Hubble constant.

There are real limitations to acknowledge. Gravitationally lensed supernova measurements remain rare - fewer than ten have been attempted - and the technique is still young enough that systematic errors are not fully characterized. The accuracy of the final result will depend heavily on how well the lens model captures the true mass distribution of the two foreground galaxies. Even smooth-looking galaxies contain dark matter halos whose profiles require careful treatment. The team's current model is a first version; the uncertainty will shrink as more imaging data arrives.

A five-image lensed configuration is also unusually constraining compared to the more common two- or four-image systems. Each additional image adds a geometric anchor point that the mass model must satisfy, reducing the space of valid solutions. That structural advantage is one reason the team considers SN Winny a particularly promising case.

The two published studies - Taubenberger et al. identifying the supernova, and Ecker and Schweinfurth et al. presenting the lens model - are both submitted to Astronomy and Astrophysics, with preprints available on arXiv. The Hubble constant result will require continued monitoring as the supernova fades over the coming months.