After six years of persistent observation of a single region of the sky, an international research team from TUM, LMU, and the Max Planck Institutes has achieved a remarkable discovery. They detected a supernova, designated SN Winny, whose light appears not once but five separate times. The odds of finding such an alignment were less than one in a million.
The phenomenon responsible is gravitational lensing. Two massive foreground galaxies sit between Earth and the explosion, which occurred approximately 10 billion light-years away.
Their immense gravity bends spacetime, causing light from the supernova to travel along several distinct paths of varying lengths. Consequently, the same flash reaches Earth at five different moments, creating what scientists describe as cosmic echoes. This five-image alignment is exceptionally rare.
Superluminous supernovas are uncommon on their own, and a perfect alignment with a foreground galaxy cluster acting as a magnifying lens is vanishingly rare. The researchers were aware of the long odds but continued their search.
With the discovery confirmed, the critical work begins. By measuring the time delays between the five images—the gaps between those echoes—scientists can extract a crucial number: the Hubble constant, which describes the rate at which the universe expands. This measurement is significant because the Hubble constant is currently a subject of debate.
Two established methods for calculating it yield different results. One method, based on observations of the cosmic microwave background, produces one number.
Another, using standard candles like Type Ia supernovas, gives a higher figure. Astronomers refer to this discrepancy as the Hubble tension, and it remains unresolved. SN Winny offers a third, independent route. Gravitational lensing time delays allow scientists to calculate the expansion rate without relying on early universe data or a separate ladder of distance measurements.
The principle relies on geometry and gravity: light travels at a fixed speed, so knowing the difference in path lengths reveals the time delay. That delay indicates distance, and combining distance with the redshift of the supernova’s host galaxy yields the Hubble constant.
In practice, the process is painstaking. The delays must be measured with high precision, and the mass distribution of the lensing galaxies must be modeled accurately. A small error in either step can disrupt the entire calculation.
However, the payoff is a direct measurement that requires no assumptions about dark energy or calibration of Cepheid variables. The discovery was announced on June 3, with the accompanying paper and data made public.
Other teams will now attempt to replicate and refine the measurement. If the lensing result matches one of the existing methods, that method gains credibility. If it matches neither, the tension deepens.
Both outcomes would provide valuable information about where current models may be incorrect. SN Winny itself exploded 10 billion years ago, when the universe was about a quarter of its current age.
Its light has been traveling ever since, passing through the gravitational field of the two foreground galaxies, splitting into five beams, and continuing its journey. It reached Earth in 2024 and 2025, and the final image is expected to arrive in the coming years. After six years of searching, the researchers have found their one-in-a-million alignment.
Now they must interpret what the five images of a single dead star reveal about the cosmos.
