That the universe is expanding has been known for almost a hundred years now, but how fast? The exact rate of that expansion remains hotly debated, even challenging the standard model of cosmology. A research team at the Technical University of Munich (TUM), the Ludwig Maximilians University (LMU) and the Max Planck Institutes MPA and MPE has now imaged and modelled an exceptionally rare supernova that could provide a new, independent way to measure how fast the universe is expanding.

The supernova is a rare superluminous stellar explosion, 10 billion light-years away, and far brighter than typical supernovae. It is also special in another way: the single supernova appears five times in the night sky, like cosmic fireworks, due to a phenomenon known as gravitational lensing. 

Two foreground galaxies bend the supernova’s light as it travels toward Earth, forcing it to take different paths. Because these paths have slightly different lengths, the light arrives at different times. By measuring the time delays between the multiple copies of the supernova, researchers can determine the universe’s present-day expansion rate, 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 color image of unique supernova

Because gravitationally lensed supernovae are so rare, only a handful of such measurements have been attempted to date. Their accuracy depends strongly on how well one can determine the masses of the galaxies acting as a lens, because these masses control how strongly the supernova’s light is bent. To measure those masses, the team members from MPE and LMU obtained images with the Large Binocular Telescope in Arizona, USA, using its two 8.4-meter diameter mirrors and an adaptive optics system that corrects for atmospheric blurring. The result is the first high-resolution color image of this system published to date. 

The observations reveal the two foreground lens galaxies in the center and five bluish copies of the supernova - reminiscent of a firework exploding. This is quite unusual, since galaxy-scale lens systems normally produce only two or four copies. Using the positions of all five copies, Allan Schweinfurth (TUM) and Leon Ecker (LMU), junior researchers in the team, built the first model of the lens mass distribution.  

“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

So far, scientists have mostly relied on two methods to measure the Hubble constant, but these methods yield conflicting results. This puzzle is known as the Hubble tension.

The first is the local method, which measures distances to galaxies one step at a time, much like climbing a ladder, where each step depends on the previous one; hence, it is referred to as the cosmic distance ladder. It uses objects with well-known brightness to estimate distances and then compares those distances with how fast galaxies are moving away. Because this method involves many calibration steps, even small errors can accumulate and affect the final result.

The second method looks much farther back in time. It studies the cosmic microwave background, the faint afterglow of the Big Bang, and uses models of the early universe to calculate today’s expansion rate. This approach is highly precise, but it relies heavily on assumptions about how the universe evolved, and these assumptions are still subject to debate.

A new, one-step approach to Hubble constant

A third, independent method now enters the picture: using a gravitationally lensed supernova. Stefan Taubenberger, a leading member of Professor Suyu’s team and first author of the supernova-identification study, explains that by measuring the time delays between the multiple copies of the supernova and knowing the mass distribution of the lensing galaxy, scientists can directly calculate the Hubble constant: “Unlike the cosmic distance ladder, this is a one-step method, with fewer and completely different sources of systematic uncertainties.”

Astronomers worldwide are currently observing SN Winny in detail using both ground-based and space-based telescopes. Their results will provide crucial new insights and help clarify the long-standing Hubble tension.

Publications

• Taubenberger et al.: “HOLISMOKES XIX: SN 2025wny at z = 2, the first strongly lensed superluminous supernova”, accepted for publication in Astronomy & Astrophysics (A&A), December 2025. Preprint available on the arXiv (arXiv: https://arxiv.org/abs/2510.21694).
• Ecker, Schweinfurth et al: “HOLISMOKES XX. Lens models of binary lens galaxies with five images of Supernova Winny“ - submitted to Astronomy & Astrophysics (A&A) and already available on the arXiv (arXiv: http://arxiv.org/abs/2602.16620).

Further information and links

• Professorship for Observational Cosmology at the TUM School of Natural Sciences
• Observatory of the Ludwig Maximilians University Munich
• Max Planck Institute for Astrophysics (MPA)
• Max Planck Institute for Extraterrestrial Physics (MPE)
• Several scientists involved are members of the Excellence Cluster ORIGINS, founded by the Federal Ministry of Research, Technology and Space (BMFTR) and the Free State of Bavaria under the Excellence Strategy of the Federal Government and the states of Germany. www.exzellenz.tum.de/en

Contacts to this article:

Dr. Stefan Taubenberger
Technical University of Munich
Professorship for Observational Cosmology
TUM School of Natural Sciences
Max Planck Institute for Astrophysics (MPA)
stefan.taubenberger@tum.de

Allan Schweinfurth
Technical University of Munich
Professorship for Observational Cosmology
TUM School of Natural Sciences
Max Planck Institute for Astrophysics (MPA)
allan.g.schweinfurth@tum.de

Technical University of Munich
Corporate Communications Center

Ulrich Meyer
presse@tum.de
Teamwebsite

Original article: https://www.tum.de/en/news-and-events/all-news/press-releases/details/measuring-the-expansion-of-the-universe-with-cosmic-fireworks