A reemerging supernova offers a new dimension of the universe’s expansion

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The space camera helps astronomers determine the rate of expansion of the universe.

Images of the Refsdal supernova have repeatedly appeared in the constellation Leo thanks to the light from a stellar explosion passing through the gravitational field of the galaxy cluster. Analysis of the delays between Refsdal appearances provides a new measurement of the Hubble constant, researchers report online May 11 in Science . This constant describes how fast galaxies are moving away from each other as the universe expands. The new measurement is now contributing to the debate about how fast the universe is expanding.

Supernovae are powerful explosions that occur at the end of the life cycles of some stars. They belong to the most violent and bright events in the universe. In 2014, astronomer Patrick Kelly of the University of Minnesota in Minneapolis discovered the Refsdal supernova in Hubble Space Telescope images.

Instead of a single flash in the sky, Kelly says, “We saw four images.” They formed a pattern known as the Einstein cross. The cross is caused by the gravity of the cluster of galaxies between us and Refsdal distorting space and time, creating a gravitational lens that deflects light on its way to us.

The location and timing of the images we see depend on three things: the distribution of matter in the galaxy cluster that makes up the lens; distances between the Earth, the lens and the supernova; and became Hubble. The cumulative effect on the light from Refsdal is so strong that in 2014 Kelly predicted that another supernova image would be delayed for another year. Sure enough, the light from Refsdal reappeared in the sky in 2015.

Kelly and his colleagues’ new calculations significantly improved Refsdahl’s 2018 measurement of the Hubble constant, setting it at about 66.6 kilometers per second per megaparsec based on the gravitational lensing models that most closely matched their observations. “Thorough modeling of the system [лінзи] along with the gravitational forces it creates allowed [команді] reduce the error in the estimate of the Hubble constant by more than a factor of two,” says astronomer Vivian Miranda of Stony Brook University in New York, who was not involved in the new study.

Studies that estimate the Hubble constant in other ways are in poor agreement with each other. One method, which relies on ancient light left over from early cosmic times, suggests that the universe is expanding at about 67 km/s/Mpc. This is close to the value found by Kelly’s group. But the expansion estimate, which uses distances to supernovae based on their luminosity, is about 74 km/s/Mpc.

“The predicted value of the Hubble constant is very sensitive to the dynamics of the universe, both in the distant past and the recent present,” says Miranda. “If our understanding of the universe is accurate, all the different methods of measuring the Hubble constant should agree.”

Resolving discrepancies between different values ​​of the Hubble constant is critical to explaining things like dark energy, which appears to be accelerating the expansion of the universe.

It would take another supernova and a lens to make a big leap in precision over the Refsdal study. Repetitive image of a supernova star in the constellation of China , which is expected to appear in 2037, may help. Meanwhile, Kelly said, further refinements to the gravitational lensing model that gave rise to the multiplied Refsdal images could slightly improve estimates of the Hubble constant.

“This is the first example of such a measurement,” says Kelly. “This sets the stage for additional measurements and increased precision,” which could deepen astronomers’ understanding of our ever-expanding universe.

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