The event, nicknamed SN Winny and officially designated SN 2025wny, is a superluminous stellar explosion about 10 billion light years away and far brighter than typical supernovae. It is also extraordinary because it appears five times in the sky as a result of gravitational lensing, in which the gravity of massive foreground objects bends and multiplies the light of a more distant source.
In this case, two foreground galaxies act as the gravitational lens, bending the supernova's light as it travels toward Earth and forcing it to follow different paths. Because these paths have slightly different lengths, the light from each image arrives at a different time. By measuring the time delays between the five images and combining that information with a model of the lens mass distribution, astronomers can derive a direct estimate of the present-day expansion rate of the universe, known as the Hubble constant.
Sherry Suyu, Associate Professor of Observational Cosmology at TUM and a Fellow at the Max Planck Institute for Astrophysics, explains the significance of the discovery: "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."
Because gravitationally lensed supernovae are so rare, only a handful of time-delay measurements have been attempted in the past, and those relied on more complex lensing situations. The precision of such measurements depends critically on how well the masses of the lensing galaxies can be determined, since these masses control how strongly the light is bent. To address this, team members from MPE and LMU used the Large Binocular Telescope (LBT) on Mount Graham in Arizona, USA, employing its twin 8.4 meter mirrors and an adaptive optics system to correct for atmospheric blurring.
These observations delivered the first published high resolution color image of the system. In the resulting image, the two lens galaxies appear in warm tones at the center, surrounded by five bluish images of SN Winny that resemble an exploding firework. This fivefold image configuration is unusual, as galaxy scale lenses more commonly produce two or four images. The positions of all five images provide strong constraints that junior researchers Allan Schweinfurth (TUM) and Leon Ecker (LMU) used to construct the first detailed model of the mass distribution in the lensing galaxies.
"Until now, most lensed supernovae were magnified by massive galaxy clusters, whose mass distributions are complex and hard to model," says Schweinfurth. "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."
The new work comes against the backdrop of the long-standing Hubble tension, in which two leading methods to determine the Hubble constant yield conflicting results. One approach, often called the local method, builds a "cosmic distance ladder" by calibrating the brightness of nearby objects and then using them to gauge ever more distant galaxies, combining those distances with measurements of how fast the galaxies recede. While powerful, this method involves many calibration steps, so small errors can accumulate along the ladder.
The second major approach looks deep into cosmic history by analyzing the cosmic microwave background, the faint afterglow of the Big Bang. Using a model of the early universe, scientists extrapolate from that relic radiation to infer today's expansion rate. This technique is highly precise but depends strongly on assumptions about the composition and evolution of the universe, and those assumptions are under active scrutiny.
Gravitationally lensed supernovae like SN Winny offer a third, independent route. By measuring the time delays between the multiple supernova images and combining them with a well constrained mass model for the lensing galaxies, researchers can infer the Hubble constant in a single step. Stefan Taubenberger, a key member of Suyu's team and first author of the supernova identification study, emphasizes that this approach has very different systematic uncertainties from both the distance ladder and cosmic microwave background methods, and therefore can help arbitrate the Hubble tension.
Astronomers around the world are now following SN Winny with both ground based and space based observatories to track its light curves in each of the five images and refine the lens model. As these observations continue, they are expected to yield a competitive measurement of the Hubble constant and provide fresh insight into whether the current cosmological framework needs to be revised.
Research Report:HOLISMOKES XIX: SN 2025wny at z = 2, the first strongly lensed superluminous supernova
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