Cosmology: Measuring the expansion of the Universe with cosmic fireworks

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 LMU, TUM, and the Max Planck Institutes for Astrophysics (MPA) and for Extraterrestrial Physics (MPE) has now imaged and modelled an exceptionally rare supernova nicknamed SN Winny that could provide a new, independent way to measure how fast the Universe is expanding.

“The fact that this system is the only strongly lensed superluminous supernova known so far makes it particularly exciting”, says LMU-physicist Leon Ecker, the first author of a paper submitted to Astronomy & Astrophysics describing these results. “It is ideally suited to derive a precise value of the Hubble constant.”

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.”

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, team members from LMU and the Max Planck Institute for Extraterrestrial Physics (MPE) 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 results clearly confirm that the fifth image is real and has the same color as the other four,” says Roberto Saglia (LMU/MPE). “The positions of the five SN Winny images are now sufficiently precise to serve as the basis for a detailed mass model.”

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, PhD student Leon Ecker (LMU) and Master student Allan Schweinfurth (TUM), junior researchers in the team, built the first model of the lens mass distribution. It will help to get a precise value of the Hubble constant.

Two methods, two very different results

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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

A third, independent method now enters the picture: using a gravitationally lensed supernova. Stefan Taubenberger, a team member at TUM 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, among them also the Wendelstein Observatory of LMU and the Hobby Eberly Telescope in Texas, for which LMU is a partner institution. “It is especially exciting when one can observe something for the very first time, as for this gravitationally lensed superluminous supernova. Thankfully we have the telescopes needed to fully seize this opportunity,” says Daniel Gruen, Professor of Astrophysics, who is organizing LMU’s ongoing campaign. The results from these observations will provide crucial new insights and help clarify the long-standing Hubble tension.

Ecker, Schweinfurth et al: HOLISMOKES XX. Lens models of binary lens galaxies with five images of Supernova Winny - arXiv (arXiv: 2602.16620), eingereicht und noch zum PeerReview bei Astronomy & Astrophysics (A&A).

Taubenberger et al.: HOLISMOKES XIX: SN 2025wny at z = 2, the first strongly lensed superluminous supernova, akzeptiert zur Publikation in Astronomy & Astrophysics (A&A), December 2025. Preprint verfügbar auf arXiv (arXiv:2510.21694)