A superluminous supernova nicknamed SN Winny, located 10 billion light-years away, appears in the sky five times due to gravitational lensing by two foreground galaxies. Researchers from the Technical University of Munich and other institutions have analyzed this rare event to measure the universe's expansion rate, known as the Hubble constant. The discovery could help resolve the ongoing Hubble tension between different measurement methods.
Sherry Suyu, associate professor of observational cosmology at the Technical University of Munich and fellow at the Max Planck Institute for Astrophysics, described SN Winny, officially SN 2025wny, as an extremely rare event. Discovered in August 2025 after six years of searching promising gravitational lenses, the supernova's light is bent by two foreground galaxies, creating five distinct images with slight time delays between arrivals on Earth. Suyu noted, “The chance of finding a superluminous supernova perfectly aligned with a suitable gravitational lens is lower than one in a million.” By measuring these delays and modeling the lensing galaxies' masses, scientists aim to calculate the Hubble constant directly in a one-step method, avoiding uncertainties from the cosmic distance ladder or cosmic microwave background analyses. Astronomers currently face the Hubble tension, where the distance ladder method yields a different expansion rate from early-universe measurements. Stefan Taubenberger, lead author on the supernova identification, explained, “Unlike the cosmic distance ladder, this is a one-step method, with fewer and completely different sources of systematic uncertainties.” High-resolution images from the Large Binocular Telescope in Arizona revealed the two central lensing galaxies surrounded by the five bluish supernova images. Allan Schweinfurth from TUM and Leon Ecker from Ludwig Maximilians University modeled the system's mass distribution, finding smooth profiles suggesting the galaxies have not collided. Schweinfurth said, “The overall simplicity of the system offers an exciting opportunity to measure the universe's expansion rate with high accuracy.” Observations continue with ground- and space-based telescopes to gather more data.