By the time astronomers knew where to look, the star had been gone for billions of years. Its explosion was ancient history. Some of its light, however, was still on the way to Earth, and scientists believed they could predict where it would appear.
In December 2015, Hubble caught it. A calculation made by people on one small planet had anticipated the arrival of light from a catastrophe deep in the universe’s past.
The story began with an unusual discovery in November 2014. Hubble images revealed four appearances of the same supernova, arranged around a foreground galaxy. The dying star became known as Refsdal. Its light had traveled for roughly nine billion years before reaching our telescopes.
From NASA:
Refsdal’s story began in November 2014 when scientists spotted four separate images of the supernova in a rare arrangement known as an Einstein Cross around a galaxy within MACS J1149.5+2223. The cosmic optical illusion was due to the mass of a single galaxy within the cluster warping and magnifying the light from the distant stellar explosion in a process known as gravitational lensing.
"While studying the supernova, we realized that the galaxy in which it exploded is already known to be a galaxy that is being lensed by the cluster,” explains Steve Rodney, co-author, from the University of South Carolina. “The supernova's host galaxy appears to us in at least three distinct images caused by the warping mass of the galaxy cluster.”
A star hadn’t somehow exploded four times. Gravity had altered the journey of its light. A galaxy and the surrounding galaxy cluster stood between the explosion and Earth, bending and magnifying the light into separate images through gravitational lensing.
Mass curves spacetime, and light follows those curves. When the alignment is right, a distant object can appear in several places. The paths can also take different amounts of time, so images of the same event reach us at different moments. Looking toward one patch of sky can reveal an event whose other image hasn’t arrived yet.
I find that last part tough to leave alone. We’re accustomed to thinking that something has either happened or hasn’t. Astronomy asks us to consider a third question: Has the evidence reached us? An event can be long finished while its light continues crossing distances that stretch our understanding.
The researchers had another clue. Refsdal’s host galaxy also appeared in multiple images, produced by the cluster’s gravitational lens. By modeling the cluster’s mass, the team calculated that another view of the supernova should become visible about a year later, in a different location.
From NASA:
These multiple images of the galaxy presented a rare opportunity. As the matter in the cluster — both dark and visible — is distributed unevenly, the light creating each of these images takes a different path with a different length. Therefore, the images of the host galaxy of the supernova are visible at different times.
Using other lensed galaxies within the cluster and combining them with the discovery of the Einstein Cross event in 2014, astronomers were able to make precise predictions for the reappearance of the supernova. Their calculations also indicated that the supernova appeared once before in a third image of the host galaxy in 1998 — an event not observed by any telescope. To make these predictions, they had to use some very sophisticated modeling techniques.
Making that prediction required an explanation detailed enough to risk being wrong. Scientists had to specify an expected appearance before the observation existed. The sky would apply the test, with no obligation to accommodate their calculations.
Hubble began checking the field again in late October 2015. In December, the predicted image appeared. Researchers had successfully anticipated the time and location of a supernova’s reappearance, giving them a way to test competing models of the gravitational lens.
The original research paper explains that the timing and brightness matched some models’ advance predictions. Their differences were useful, too: The observation could reveal weaknesses and uncertainties in the calculations. A successful prediction opened further questions instead of ending the investigation.
The scientific value continued beyond that December discovery. Research published in 2023 used the delay between Refsdal’s images to constrain the Hubble constant, which measures how quickly the universe is expanding today. An extraordinary sight had become a tool for examining a fundamental question about the cosmos.
There’s something deeply encouraging about the work behind such an achievement. Telescopes, mathematics, patient observations, and people willing to compare predictions with evidence made an almost unimaginable event understandable. Each contribution depended on others. Wonder became a reason to investigate, and investigation gave us more to wonder about.
I keep going back to those months of watching. The light was already traveling. Nobody could hurry it, improve its timing, or make the universe offer a more convenient answer. All the scientists could do was prepare carefully, look where their calculations indicated, and wait.
We’re small beneath a night sky, but we can ask questions that reach astonishing distances. Refsdal gives me a welcome measure of humility and hope: A star died billions of years ago, and human beings learned enough to be waiting when another image of its light finally arrived.
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