Superliminous supernovae are miraculous events. For astronomers, they also provide a vital tool for measuring cosmic distances and the rate at which the Universe is expanding. As part of the Cosmic Distance Ladder, these incredibly bright stellar explosions are the “standard candles” for objects billions of light-years away. In a rare event, researchers from the University of Munich, using the Large Binocular Telescope (LBT) in Arizona, witnessed a superluminous supernova 10 billion light-years away that was far brighter than most explosions of its kind.
What was especially amazing about this supernova was that it appeared five times in the night sky due to gravitational lensing by two foreground galaxies. These galaxies bent the path of the supernova’s light, causing it to take different paths. Because these paths have different lengths, the light appeared in different places around the galaxies at different times. By measuring the time delays between the multiple images, the researchers were able to obtain measurements of how fast the Universe is expanding – aka the Hubble-Lemaitre Constant.
The team consisted of researchers from the Technical University of Munich (TUM), the Max Planck Institute for Astrophysics (MPG), the Harvard & Smithsonian Center for Astrophysics (CfA), the E.O. Lawrence Berkeley National Laboratory, ETH Zurich, the Research Center for the Early Universe (RESCEU), the Cosmic Dawn Center (DAWN), the Ulugh Beg Astronomical Institute, the Chinese Academy of Sciences (CAS), Institute of Space Sciences (ICE, CSIC), the Excellence Cluster ORIGINS, the National Astronomical Observatory of Japan (NAOJ), the European Southern Observatory (ESO), the Space Telescope Science Institute (STScI), and multiple universities.
*Large Binocular Telescope on Mount Graham in Arizona, USA. Credit & ©: Dr. Christoph Saulder/MPE*
The paper describing their observations has been accepted for publication in Astronomy & Astrophysics
Only a few such measurements have been attempted to date because gravitationally lensed supernovae are so rare. It is also a challenging process, where astronomers must determine the masses of the lensing galaxies because this dictates how strongly the light from the background object is bent. To determine the masses of the two galaxies, the team obtained images with the LBT, using its two 8.4-meter (27.5 ft) mirrors and an adaptive optics system. The observations revealed two foreground lens galaxies at the center surrounded by five bluish images of the supernova explosion, making it look like fireworks!
Sherry Suyu, Associate Professor of Observational Cosmology at TUM and Fellow at the Max Planck Institute for Astrophysics, explained in an MPG press release:
We nicknamed this supernova SN Winny, inspired by its official designation SN 2025wny. It is an extremely rare event that could play a key role in improving our understanding of the cosmos. The chance of finding a superluminous supernova perfectly aligned with a suitable gravitational lens is lower than one in a million. We spent six years searching for such an event by compiling a list of promising gravitational lenses, and in August 2025, SN Winny matched exactly with one of them.
The image came as a surprise to the team since galaxy-scale lens systems normally produce only two or four copies. Using the positions of all five, junior researchers Allan Schweinfurth (TUM) and Leon Ecker (LMU) built the first model of the lens mass distribution. Said Allan Schweinfurth:
Until now, most lensed supernovae were magnified by massive galaxy clusters, whose mass distributions are complex and hard to model. SN Winny, however, is lensed by just two individual galaxies. We find overall smooth and regular light and mass distributions for these galaxies, suggesting that they have not yet collided in the past despite their close apparent proximity. The overall simplicity of the system offers an exciting opportunity to measure the Universe’s expansion rate with high accuracy.
This, in turn, could help astronomers and cosmologists relieve the ongoing issue of the Hubble Tension. To date, scientists have relied primarily on two methods to measure cosmic expansion: the Cosmic Distance Ladder and measurements of the Cosmic Microwave Background (CMB). The former is the local method, combining parallax, supernovae, and redshift measurements of bright objects to establish distances one step at a time. Since every step is dependent on the previous one, even small errors can add up and affect the final result.
In contrast, CMB measurements look back to the beginning of cosmic time by examining the “relic radiation” left over from the Big Bang. This approach is highly precise and relies on models of the early Universe to calculate its current rate of expansion. It relies heavily on assumptions about how the Universe evolved, however, which are still subject to debate. This study presents a third possible method in which astronomers use gravitationally lensed supernovae and measure the time delays between the multiple copies of the same image.
By calculating the mass distribution of the lensing galaxy, scientists can directly calculate the Hubble-Lemaitre Constant. “Unlike the cosmic distance ladder, this is a one-step method, with fewer and completely different sources of systematic uncertainties,” said Stefan Taubenberger, a leading member of Professor Suyu’s team and first author on their study.
Meanwhile, astronomers worldwide are observing SN Winny in detail with ground-based and space-based telescopes. Their results will provide new insights into cosmic expansion that could help resolve the Hubble Tension.
Further Reading: MPG