The universe is expanding – but how fast? There have been contradictory answers to this question so far. However, a cosmic stroke of luck could help clarify these contradictions. Astronomers have discovered a special measuring aid: one of the rare, super-luminous supernovae that stands directly behind a gravitational lens. Two foreground galaxies use their mass to amplify and distort the light of the galaxy, which is around ten billion light-years away. Using the resulting multiple images and their characteristics, astronomers can measure the rate of expansion of the cosmos. This means they have a method that is independent of previous procedures to determine this disputed value more precisely.
It has been known for almost a hundred years that the universe is expanding. However, the pace of this expansion and the exact value of the Hubble constant H0 that describes this behavior are controversial. Depending on the measurement method chosen, the measurements produce different values. The first measurement approach is based on the strange background radiation, a relic of the radiation that was first released around 380,000 years after the Big Bang. This faint microwave background is present everywhere in the cosmos. It contains features from which the expansion rate can be derived based on theoretical models of cosmic evolution. These measurements, based on the cosmological standard model, arrive at values of around 67 kilometers per second per megaparsec for the Hubble constant. The second approach uses the distances and redshifts of cosmic objects such as supernovae, variable stars or red giants to determine this value. These measurements resulted in Hubble constants averaging 73 kilometers per second per megaparsec – and thus a significantly higher value.
Looking for an independent measurement method
The reason for this so-called Hubble voltage is still unknown, as is which value is the correct one. In extreme cases, however, this could mean that the standard cosmological model is wrong and with it the basic assumptions about the structure and evolution of the universe. It would be correspondingly important to find a measurement method that works independently of the two existing approaches. Supernovae that are in a direct line behind a massive foreground object could provide an approach to this. This foreground object then acts like a gravitational lens that bends space-time. As a result, the light from the distant supernova is diffracted in a special way, and the rays travel different distances to the observer. As a result, the image of the supernova appears distorted and in multiple, slightly time-shifted copies. Astronomers can then determine the expansion rate of the universe from the time delays between the individual images. “In contrast to the cosmic distance ladder, this is a one-step measurement with fewer and completely different sources of systematic uncertainties,” explains lead author Stefan Taubenberger from the Technical University of Munich.
The problem, however, is that such cosmic strokes of luck are extremely rare. To date, fewer than ten such strongly lensed supernovae are known. That’s why the astronomers have now specifically searched survey data for such events. “We spent six years compiling a list of promising gravitational lenses and looking for such an event,” says Taubenberger’s colleague Sherry Suyu. In August 2025, the effort was rewarded – the astronomers found what they were looking for. The Zwicky Transient Facility (ZTF) in California detected the bright glow of a supernova on August 27, 2025. This stellar explosion, named SN 2025wny, occurred around ten billion light-years away and belonged to the class of superluminous supernovae. These release a particularly large amount of radiation, but are rare. The images also showed that the image of this supernova appears five times because it is behind a gravitational lens.
“Chance of one in a million”
This discovery is therefore a real stroke of luck in astronomy: “The probability of finding a super-luminous supernova that is located exactly behind a gravitational lens is less than one in a million,” explains Suyu. “It is an extremely rare event that could play a key role in our understanding of the cosmos.” Astronomers nicknamed this lensed supernova SN Winny – based on its official name, SN 2025wny. Closer analysis of the images revealed that SN Winny is special in a second respect: “When we have seen supernovae through gravitational lenses, the lenses were usually massive clusters of galaxies whose mass distributions are complex and difficult to model,” explains co-author Allan Schweinfurth from the Technical University of Munich. But when the team examined the lensed images of the SN Winny more closely using the Large Binocular Telescope in Arizona, something different became apparent: Five bluish images of the supernova could be seen as well as two other bright spots of light in the center of the ensemble.
The astronomers conclude that the gravitational lens in this case consists of only two individual foreground galaxies. “Overall, we find very smooth and regular light and mass distributions, which suggests that these galaxies have not yet collided with one another despite their apparent proximity to one another,” reports Schweinfurth. SN Winny thus offers the opportunity to map and measure the distorting masses and mass distribution far more precisely than is the case with more complex gravitational lenses. “This relative simplicity of the system offers an excellent opportunity to measure the expansion rate of the universe particularly precisely,” Schweinfurth continued. The rare combination of superluminous supernova and simple gravitational lensing consisting of well-resolved foreground galaxies has the potential to determine a value for the Hubble constant that is independent of previous methods and yet precise. “With the discovery of this first lensed supernova suitable for cosmography, we are entering a new and exciting era,” the astronomers write. If a few more such events are found, this could be enough to determine the expansion rate of the cosmos with less than one percent measurement uncertainty.
Source: Stefan Taubenberger (Technical University of Munich) et al., Astronomy & Astrophysics accepted, preprint doi: 10.48550/arXiv.2510.21694