Measuring the expansion of the universe with cosmic fireworks

High-resolution image taken with the Large Binocular Telescope on Mount Graham in Arizona, USA, displaying the two lens galaxies in a warm tone, and the five lensed copies of SN Winny in blue. © Credit: SN Winny Research Group

Munich astronomers image and model extremely rare gravitationally lensed supernova

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) as well as the Max Planck Institutes for Astrophysics (MPA) and Extraterrestrial Physics (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.

• An image that could solve a long lasting cosmic mystery


• Unprecedented chance to measure the growth of the universe


• Collaboration between TUM, LMU and Max Planck Institutes

The supernova is a rare superluminous stellar explosion, 10 billion lightyears 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.”

Large Binocular Telescope auf dem Mount Graham in Arizona, USA
© Credit: Dr. Christoph Saulder / MPE

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 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 comes as a surprise, since galaxy-scale lens systems normally produce only two or four copies. Using the positions of all five copies, Allan Schweinfurth and Leon Ecker, 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.”

Members of the SN Winny Research Group at Research Campus Garching (from left): Stefan Taubenberger, Allan Schweinfurth, Alejandra Melo, Elias Mamuzic, Sherry Suyu, Christoph Saulder, Roberto Saglia, Leon Ecker, Limeng Deng. © Credit: Dr. Robert Reich / TUM

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.

SN Winny
Credit: Elias Mamuzic / MPA / TUM

A new, one-step approach

Animation (available in several languages) showing the gravitational lensing effect of the pair of foreground galaxies on the host galaxy of SN Winny. The host galaxy is lensed into multiple images, which are distorted and stretched out to form a bluish ring around the lens. The explosion of SN Winny itself and the time-delayed arrival of its multiple lensed copies on Earth are also simulated. Ultimately, the animation fades to a real observation of SN Winny, captured at the Large Binocular Telescope in Arizona.

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.

Source: Max Planck Institute for Astrophysics/Press Releases

---

Contacts:

Prof. Dr. Sherry Suyu
Scientific Staff
Tel: 2015

Stefan Taubenberger
Tel: 2019
tauben@mpa-garching.mpg.de

---

Original publication

1. Taubenberger et al.
HOLISMOKES XIX: SN 2025wny at z = 2, the first strongly lensed superluminous supernova
accepted by Astronomy & Astrophysics (A&A), December 2025

Source

2. Ecker, Schweinfurth et al.
HOLISMOKES XX. Lens models of binary lens galaxies with five images of Supernova Winny
submitted to Astronomy & Astrophysics (A&A)

Source

---

Astronomers Discover the First Gravitationally Lensed Superluminous Supernova

SN 2025wny

Gravitationally lensed superluminous supernova
An artist’s interpretation of light from a supernova passing through a gravitational lens, reaching Earth at different times.
Credit: Oskar Klein Center, University of Stockholm / Samuel Avraham & Joel Johansson.

---

Maunakea, Hawaiʻi – An international team of astronomers using a combination of ground-based telescopes, including the W. M. Keck Observatory on Maunakea, Hawaiʻi Island, has discovered the first-ever spatially resolved, gravitationally lensed superluminous supernova. The object, dubbed SN 2025wny, offers a rare look at a stellar cataclysm from the early Universe and provides a striking confirmation of Einstein’s theory of general relativity.

SN 2025wny lies so far away that its light has traveled 10 billion years to reach Earth; the Universe was just 4 billion years old when the explosion occurred. Normally, a supernova at this distance would be far too faint to detect from the ground. But two foreground galaxies act as a natural gravitational “magnifying glass,” boosting the supernova’s brightness by a factor of 50 and splitting it into distinct, spatially separated images.

“This is nature’s own telescope,” says Joel Johansson, lead author from the Oskar Klein Centre, Stockholm University. “The magnification lets us study a supernova at a distance where detailed observations would otherwise be impossible.”

The study, led by Stockholm University, is published in The Astrophysical Journal Letters.

An artist’s interpretation of light from a supernova passing through a gravitational lens, reaching Earth at different times.
Credit: Oskar Klein Center, University of Stockholm / Samuel Avraham & Joel Johansson.

A new method to probe the expansion of the universe

Because each of the multiple lensed images takes a slightly different path around the intervening galaxies, their arrival times differ. Measuring these time delays provides a powerful, independent method to determine the Hubble constant—the rate at which the Universe is expanding.

A major unsolved problem in modern cosmology is the Hubble tension—the growing mismatch between measurements of the Universe’s expansion rate made from the early Universe versus those made from nearby objects. The disagreement suggests that our current cosmological model may be incomplete. Strongly lensed supernovae like SN 2025wny offer a new, independent way to measure this expansion rate through time-delay differences between the lensed images, helping determine whether the tension reflects new physics or limitations in existing methods.

“A lensed supernova with multiple, well-resolved images provides one of the cleanest ways to measure the expansion rate of the Universe,” says Ariel Goobar of the Oskar Klein Centre. “SN 2025wny is an important step toward resolving one of cosmology’s most significant challenges.”

A surprising and exceptionally hot explosion

Superluminous supernovae are extremely bright, rare explosions. SN 2025wny stands out even in this elite category: its early ultraviolet light, stretched into optical wavelengths by cosmic expansion, revealed an exceptionally hot, brilliant event.

The supernova’s intense brightness illuminated its host galaxy, allowing astronomers to identify narrow absorption lines from elements such as carbon, iron, and silicon. These fingerprints point to a low-metallicity, star-forming dwarf galaxy—exactly the kind of environment thought to produce superluminous supernovae during the Universe’s youth.

How the Discovery Was Made

The discovery relied on a chain of cutting-edge observatories working together on scientific breakthroughs. The Zwicky Transient Facility (ZTF) at Palomar Observatory in California first detected the explosion during its nightly monitoring of the sky. The Nordic Optical Telescope (NOT) on La Palma in the Canary Islands provided early spectroscopy of the transient, Liverpool Telescope (LT) also on La palma provided four separate images of SN 2025wny, and Keck Observatory ultiumately provided the decisive spectra that confirmed both the supernova type and its extreme distance.

Yu-Jing Qin, a postdoctoral researcher at Caltech, led a series of spectroscopic observations using Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS), targeting each of the individual supernova images and the lensing galaxies.

The Keck spectra revealed a forest of narrow absorption lines from the supernova’s host galaxy – the fingerprints of elements such as carbon, iron and silicon – which nailed down the redshift and nature of the event.

“The spectrum taken with LRIS provides the most convincing measurement of its distance/redshift and pinpointed its classification as a superluminous supernova, which is a rare subclass. We were really impressed by the data quality and are pursuing further observations using other Keck instruments,” said Qin.

These rapid-response observations were enabled by Keck Observatory’s Target of Opportunity (ToO) policy, which allows scientists to request immediate access for short-lived cosmic events.

“It’s always exciting to get a request for a very rapid response to a transient event like this,” said John O’Meara, Chief Scientist and Deputy Director for Keck Observatory. “Keck was ready to respond, and we were happy to deliver and participate in this breakthrough.”

What Comes Next?

SN 2025wny demonstrates that strongly lensed supernovae at very high redshifts can be discovered and resolved with today’s surveys—a crucial proof of concept ahead of the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), which is expected to uncover hundreds more.

Follow-up observations with the Hubble Space Telescope and James Webb Space Telescope are already underway. These data will refine the gravitational lens model, map the multiple images with exceptional precision, and ultimately measure the time delays needed for a new, independent determination of the Hubble constant.

The extraordinary magnification also offers an unprecedented view into how such extreme explosions work and how stars evolved in the early Universe.

Source: W.M. Keck Observatory/Science News

---

About LRIS

The Low Resolution Imaging Spectrometer (LRIS) is a very versatile and ultra-sensitive visible-wavelength imager and spectrograph built at the California Institute of Technology by a team led by Prof. Bev Oke and Prof. Judy Cohen and commissioned in 1993. Since then it has seen two major upgrades to further enhance its capabilities: the addition of a second, blue arm optimized for shorter wavelengths of light and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe. LRIS was used in observing distant supernovae by astronomers who received the Nobel Prize in Physics in 2011 for research determining that the universe was speeding up in its expansion.

About W. M. Keck Observatory

The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaiʻi feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. For more information, visit: www.keckobservatory.org

---