Measuring the expansion of the universe differently than usual
Researchers in Munich track the different light paths from a gravitationally-lensed supernova to measure the universe's expansion rate
To the point
- Expansion of the universe: An open question in cosmology is not whether the universe is expanding, but how. There are various methods for measuring this expansion. One of them uses star explosions, or supernovae. These are bright enough and their physics are well enough understood that the distances of various supernovae distributed throughout space can be measured over great distances.
- Gravitational lenses: Researchers in Munich have used the Large Binocular Telescope in Arizona to capture five images of one and the same supernova in a single picture. The gravity of two foreground galaxies has deflected the light from a supernova far in the background along different paths to Earth.
- Measurement of the Hubble parameter: The light from the five images traveled to Earth on different paths. This should allow the accelerated expansion rate of the universe to be measured directly, expressed by the so-called Hubble parameter.
- Resolution of a conflict: This measurement could resolve a conflict: In the past, two different values of the Hubble parameter were measured using two different groups of methods, which, for reasons unknown to date, differ only slightly but measurably.
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.
Measuring the expansion of the universe differently than usual | Science Snippet
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 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 (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
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.
