Scientists have known for nearly a century that the universe is expanding, but the exact rate of that expansion is still uncertain. This ongoing debate has even raised 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 how fast the universe is growing.
The object at the center of this discovery is a superluminous supernova located about 10 billion light-years away. It shines much brighter than typical stellar explosions. What makes it especially remarkable is how it appears in the sky. Instead of a single point of light, it shows up five separate times, creating a striking cosmic display caused by gravitational lensing.
As the supernova’s light travels toward Earth, it passes by two galaxies in the foreground. Their gravity bends the light and sends it along multiple paths. Because each path is slightly different in length, the light from each image arrives at different times. By carefully measuring these delays, scientists can calculate the current expansion rate of the universe, known as the Hubble constant.
Sherry Suyu, Associate Professor of Observational Cosmology at TUM and Fellow at the Max Planck Institute for Astrophysics, explains: “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.”
High-resolution imaging reveals a unique system
Gravitationally lensed supernovae are extremely uncommon, which means only a small number of these measurements have been made so far. Their reliability depends heavily on how accurately scientists can determine the masses of the galaxies bending the light, since those masses control the strength of the lensing effect.
To improve those measurements, researchers from MPE and LMU used the Large Binocular Telescope in Arizona, USA. Equipped with two 8.4-meter mirrors and an adaptive optics system that reduces atmospheric distortion, the telescope produced the first high-resolution color image of this system.
The image shows two lensing galaxies at the center, surrounded by five bluish points of light that represent the supernova’s multiple images. This configuration is unusual, since most similar systems produce only two or four images. By analyzing the positions of all five images, Allan Schweinfurth (TUM) and Leon Ecker (LMU), junior members of the team, created the first detailed model of how mass is distributed 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 Allan 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.”
Two methods, two very different results
Currently, astronomers rely on two main approaches to measure the Hubble constant, but they do not agree with each other. This disagreement is known as the Hubble tension.
One method focuses on nearby galaxies and builds up distance measurements step by step, similar to climbing a ladder. Because each step depends on the previous one, this approach is called the cosmic distance ladder. It uses objects with known brightness to estimate distances, then compares those distances to how fast galaxies are moving away. However, because it involves many calibration steps, small uncertainties can build up and affect the final result.
The second method looks at the early universe by studying the cosmic microwave background, the faint radiation left over from the Big Bang. Using models of how the universe evolved, scientists can calculate the current expansion rate. While this method is very precise, it depends heavily on assumptions about the universe’s history, which are still being examined and debated.
A new one-step method to measure the Hubble constant
A third technique is now emerging based on gravitationally lensed supernovae like SN Winny. Stefan Taubenberger, a key member of Professor Suyu’s team and lead author of the supernova identification study, explains that measuring the time delays between the multiple images, combined with knowledge of the lensing galaxies’ mass, allows scientists to directly determine the Hubble constant: “Unlike the cosmic distance ladder, this is a one-step method, with fewer and completely different sources of systematic uncertainties.”
Astronomers around the world are continuing 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 the long-standing disagreement over how fast the universe is expanding.
