BERLIN — For six years, a team of astronomers hunted for something most experts said they would never find. A single supernova, perfectly aligned behind two massive galaxies, its light bent by gravity into five separate images arriving at Earth at different times. They called it SN Winny.
The chance of this happening, the researchers calculated, is less than one in a million.
SN Winny sits roughly 10 billion light-years away. Between it and Earth lie two foreground galaxies so massive that their gravity warps the fabric of spacetime. That warping acts like a cosmic lens. Light from the supernova travels along several different paths, each slightly longer than the last. So the same explosion reaches us not as one flash, but as five, spread out over time.
Think of it as five echoes of a single bang.
The astronomers who found SN Winny work at the Technical University of Munich, Ludwig Maximilian University of Munich, and the Max Planck Institutes. They spent half a decade scanning the sky. Most searches for gravitationally lensed supernovae turn up nothing. This one turned up a superluminous event — a star that died with extraordinary brightness — and it was perfectly placed.
Why does any of this matter? Because of a problem called the Hubble tension.
The Hubble constant is the number that describes how fast the universe is expanding. Two established ways of measuring it give two different answers. One method, based on observations of the early universe, produces one number. Another, based on observations of nearby stars and galaxies, produces a different number. Neither side has been able to explain the gap. It has become one of the most stubborn puzzles in cosmology.
SN Winny offers a third route.
By measuring the time delays between each of the five images of the supernova, scientists can calculate the Hubble constant independently. The math is straightforward in principle: the longer the delay, the more the light has been stretched by the expansion of space. The geometry of the lens — the two foreground galaxies — is known. Plug in the delays, and out comes a number.
If that number matches one of the two existing methods, the Hubble tension gets resolved. If it matches the other, the tension gets deeper. Either way, the result will be a data point that cannot be ignored.
The supernova itself is remarkable. A superluminous supernova is already rare. One that is also gravitationally lensed into multiple images is vanishingly so. The alignment required — Earth, the two foreground galaxies, and the distant explosion all in a near-perfect straight line — is the kind of cosmic accident that astronomers spend careers hoping for.
The researchers found SN Winny after six years of searching. They did not stumble onto it. They looked, systematically, for a needle in a haystack the size of the observable universe.
Now they have it. And the clock is ticking.
The time delays between the five images are not infinite. The supernova will fade. The echoes will stop. The window for measuring those delays precisely is finite. The team is racing to gather the data before the light goes dark.
If they succeed, the payoff is a new, independent measurement of how fast the universe is expanding. That measurement could settle a long-standing dispute. Or it could deepen the mystery. Either way, it will come from a single star that died 10 billion years ago, whose death cry reached us in five separate waves.
That is the kind of story the universe tells, if you are patient enough to listen.
