Scientists have known for nearly a century that the universe is expanding, but the exact rate of expansion is still unknown. This ongoing debate even raises questions about the standard model of cosmology. Now, researchers from the Technical University of Munich (TUM), Ludwig-Maximilians-University (LMU), and the Max Planck Institutes MPA and MPE have identified and analyzed an extremely rare type of supernova that could provide a new and independent way to measure the growth rate of the universe.
The object at the center of this discovery is a superluminous supernova located about 10 billion light years away. It shines much brighter than a normal star explosion. Particularly noteworthy is the way it appears in the sky. Instead of a single point of light, it appears in five separate bursts, creating an impressive cosmic display caused by gravitational lensing.
As the supernova’s light travels toward Earth, it passes through two galaxies in the foreground. Gravity bends light and sends it along multiple paths. The length of each path is slightly different, so the light from each image arrives at a different time. By carefully measuring these delays, scientists can calculate the current rate of expansion of the universe, known as the Hubble constant.
Sherry Suyu, associate professor of observational cosmology at TUM and researcher at the Max Planck Institute for Astrophysics, said: “We named this supernova SN 2025wny after its official name. “This is an extremely rare event and could play an important role in improving our understanding of the universe. The odds of finding a superluminous supernova perfectly aligned with a suitable gravitational lens are less than one in a million.” We spent six years compiling a list of promising gravitational lenses and searching for such events, and in August 2025, SN Winny matched exactly one of them. ”
High-resolution imaging reveals unique system
Gravitational lensing supernovae are extremely rare, meaning that only a small number of these measurements have been made to date. Their reliability depends largely on how precisely scientists can determine the masses of the galaxies that bend the light. Because their mass controls the strength of the lensing effect.
To improve these measurements, MPE and LMU researchers used the Large Binocular Telescope located in Arizona, USA. The telescope, equipped with two 8.4-meter mirrors and an adaptive optics system to reduce atmospheric distortions, produced the system’s first high-resolution color images.
The image shows two lenticular galaxies in the center surrounded by five bluish dots of light representing multiple images of the supernova. This configuration is unusual since most similar systems only produce two or four images. By analyzing the positions of all five images, junior members of the team Alan Schweinfurth (TUM) and Leon Ecker (LMU) created the first detailed model of how mass is distributed within the lens galaxy.
“Until now, most lensing supernovae have been expanded by massive galaxy clusters, but their mass distributions have been complex and difficult to model,” says Alan Schweinferth. “However, SN Winny is lensed by just two separate galaxies. We find an overall smooth and regular distribution of light and mass for these galaxies, suggesting that despite their apparent closeness, they have not yet collided in the past. The overall simplicity of the system provides an exciting opportunity to measure the expansion rate of the Universe with high precision.”
Two methods, two completely different results
Currently, astronomers rely on two main approaches to measure the Hubble constant, but they do not agree with each other. This discrepancy is known as the Hubble tension.
One method focuses on nearby galaxies and builds up distance measurements step by step, like climbing a ladder. Because each step depends on the previous step, this approach is called a cosmic distance ladder. It uses objects of known brightness to estimate the distance and compares that distance to the speed at which the galaxy is moving away from us. However, because it involves many calibration steps, small uncertainties can accumulate and affect the final results.
The second method probes the early Universe by studying the cosmic microwave background radiation, the weak radiation left over from the Big Bang. Using models of how the universe evolved, scientists can calculate the current rate of expansion. Although this method is highly accurate, it relies heavily on assumptions about the history of the universe, which are still being investigated and debated.
A new one-step method to measure the Hubble constant
A third technique, based on gravitational lensing supernovae like SN Winny, is now emerging. Stefan Taubenberger, a key member of Professor Suyu’s team and lead author of the supernova identification study, explains that by measuring the time delay between multiple images, combined with knowledge of the lens galaxy’s mass, scientists can directly determine the Hubble constant. “The cosmic distance ladder is different, this is a one-step method, there are fewer sources of systematic uncertainty, and it’s something completely different.”
Astronomers around the world continue to observe SN Winny using both ground-based and space-based telescopes. These observations are expected to provide important new data that could help resolve long-standing disagreements about the rate of expansion of the universe.
