Superliminous supernovae are miraculous events. They are also an important tool for astronomers to measure cosmic distances and the rate at which the universe is expanding. 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 case, researchers at the University of Munich used the Large Binocular Telescope (LBT) in Arizona to observe a super-bright supernova 10 billion light-years away that was far brighter than most explosions of its kind.
What was special about this supernova was that it appeared five times in the night sky due to the gravitational lensing of 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 quickly the universe is expanding – also known as 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 EO 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), the Institute of Space Sciences (ICE, CSIC). Cluster of Excellence ORIGINS, the National Astronomical Observatory of Japan (NAOJ), the European Southern Observatory (ESO), the Space Telescope Science Institute (STScI) and several universities.
*Large binocular telescope on Mount Graham in Arizona, USA. Photo credit & ©: Dr. Christoph Saulder/MPE*
The paper describing their observations has been accepted for publication in Astronomy & Astrophysics
Few such measurements have been made so far because gravitational lensing supernovae are so rare. It is also a challenging process that requires astronomers to determine the masses of the lensing galaxies, as these determine how much the background object’s light is deflected. To determine the masses of the two galaxies, the team captured images with the LBT using its two 8.4-meter (27.5-foot) mirrors and an adaptive optics system. The observations revealed two foreground lensed galaxies in the center, surrounded by five bluish images of the supernova explosion, making them 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 name 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 super-bright supernova perfectly aligned with a suitable gravitational lens is less than one in a million. We have been looking for such an event for six years by compiling a list of promising gravitational lenses, and in August 2025 SN Winny exactly coincided with one of them.
The image surprised the team because galaxy-scale lens systems typically only produce two or four copies. The young researchers Allan Schweinfurth (TUM) and Leon Ecker (LMU) created the first model of the lens mass distribution from the positions of all five. Allan Schweinfurth said:
To date, most lensing supernovae have been magnified by massive galaxy clusters, whose mass distributions are complex and difficult to model. However, SN Winny is only blinded by two individual galaxies. We find overall smooth and regular light and mass distributions for these galaxies, suggesting that they have not collided in the past despite their apparent proximity. The overall simplicity of the system provides an exciting opportunity to measure the expansion rate of the Universe with high accuracy.
This, in turn, could help astronomers and cosmologists mitigate the ongoing problem of Hubble tension. Until now, scientists have relied primarily on two methods for measuring cosmic expansion: the Cosmic Distance Ladder and Cosmic Microwave Background (CMB) measurements. The former is the local method, which combines parallax, supernovae and redshift measurements of bright objects to determine distances step by step. Since each step depends on the previous one, even small mistakes can add up and affect the final result.
In contrast, CMB measurements look back to the beginning of cosmic time by studying 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 expansion rate. However, it relies heavily on assumptions about the evolution of the universe that are still a matter of debate. This study presents a third possible method in which astronomers use gravitational lensing supernovae and measure the time delays between multiple copies of the same image.
By calculating the mass distribution of the lensing galaxy, scientists can directly calculate the Hubble-Lemaitre constant. “In contrast to 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 of their study.
Astronomers around the world are now observing SN Winny in detail using ground- and space-based telescopes. Their results will provide new insights into cosmic expansion that could help resolve the Hubble tension.
Further reading: MPG
