An expected reappearance of the supernova SN Requiem, seen repeatedly due to gravitational lensing, may provide a third way to measure the Hubble constant.
Astronomers are expecting a particularly rare event in the coming months: the reappearance of a distant supernova that has already been seen several times before, not because it exploded again, but because the gravity of a huge galaxy cluster between the light source and us bends the light's paths and causes the flash to appear again and again at different times. If this prediction comes true, it could provide a new and precise measuring tool for a question that currently excites cosmology: exactly how fast the universe is expanding.
The supernova in question, SN Requiem, has already been observed three times in 2016. Researcher Sheri Soyo from the Max Planck Institute for Astrophysics and her colleagues calculated that the galaxy cluster serving as a gravitational lens should direct another portion of the light on a longer path, so a fourth flash is expected to appear again, probably during 2026 or 2027. Starting this June, the team plans to monitor the galaxy once a month using the Hubble Space Telescope, and if a reappearance is detected, the James Webb Space Telescope will also be enlisted for more precise observation.
The great interest in this event stems from the fact that the time difference between the different appearances of the same supernova is not only a spectacular phenomenon, but also a measuring tool. When gravitational lensing causes light to travel in several different paths, some of the rays travel a longer distance and some are also slowed down more in strong gravitational fields. If the delay between the flashes and their location on the sky are carefully measured, it is possible to calculate the absolute distance to the source – and from this to derive a value for the Hubble constant, that is, the rate of expansion of the universe.
Standard candles versus dark matter and energy
This is where one of the most heated controversies in modern cosmology comes into play. For decades, astronomers have measured the Hubble constant using “standard candles” – stars and supernovae whose luminous intensity allows us to infer their distance. This method gives a value of about 73 kilometers per second per megaparsec. In contrast, cosmologists who rely on the cosmic microwave background radiation, the remnant of light from the early universe, arrive at a lower value, about 67 kilometers per second per megaparsec, after taking into account the influence of dark matter and dark energy. Both measurements were made to an accuracy of about 1%, so the discrepancy between them no longer appears to be a random error, but rather a fundamental problem known as “Hubble tension.”
If the measurements based on standard candles are correct, it is possible that something in the standard cosmological method is missing or wrong. If the measurement from the early universe is correct, astronomers may still suffer from systematic bias in distance measurements. That is why the scientific community has been searching for years for a “third way” – an independent method that does not rely on the same assumptions and sources of error. Measuring time delays in gravitational lenses, known as time-delay cosmography, is currently considered one of the leading candidates for this role.
This method has an important advantage: it does not depend on the detailed physics of variable stars or Type Ia supernovae, and does not rely directly on assumptions about dark matter or dark energy. It requires only a well-varying light source, such as a supernova, and a gravitational lens whose mass distribution can be mapped with great precision. But this is also where the difficulty lies. Galaxy clusters are very strong gravitational lenses, capable of creating delays of years or even decades, but they are also incredibly complicated systems. They contain dozens or hundreds of galaxies, each with its own contribution to the gravitational field, along with halos of dark matter and hot gas. Therefore, building a reliable mass model is a complex task, combining spectroscopic observations, deflection measurements, and repeated analyses.
Seven independent teams built separate models
In the case of SN Requiem, Soyo and her colleagues recruited seven independent teams that built different models of the galaxy cluster. The results were then combined into an overall model, and the updated prediction determined that the fourth appearance is not expected until 2037, as initially thought, but within the next two years. The scientific significance of the appearance date is very large: If the supernova appears this year, the measurement is expected to tip the scales toward a higher value of the Hubble constant, close to the one that astronomers prefer. If the appearance is postponed until next year, it will actually reinforce the lower value, closer to the cosmologists’ camp.
The method has already yielded early successes. The supernova SN Refsdal, discovered in 2014 fifty years after Sjör Refsdal predicted such a phenomenon theoretically, provided a value of 64.8 for the Hubble constant to within 5.5%. Measurements based on excited quasars have also improved significantly, and in December 2025 an international group published a result of 71.6 with an accuracy of 4.6%, an intermediate value between the two camps. But this accuracy is not yet sufficient to be conclusive. Soyo estimates that SN Requiem alone could achieve an accuracy of 2%–3%, but to reduce the uncertainty to below 1% a larger population of excited supernovae would be needed.
This is where the next generation of astronomical surveys come in. The European Euclid Space Telescope, NASA’s Nancy Grace Roman Space Telescope, and the Vera C. Rubin Observatory in Chile are expected to discover more and more such events. Already, James Webb is discovering supernovae at a much faster rate than in the past. If the trend continues, what was until recently considered a rare, one-off event could become a routine measurement tool. Then, perhaps, the repeated flashes of exploding stars in the distant universe will finally provide an answer to the question of how fast our universe is really expanding.
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