Evrim Yazgin
Cosmos science journalist
NASA’s James Webb Space Telescope has discovered a unique supernova which has helped measure the Hubble constant – the rate at which the universe is expanding.
Several research papers have been published from the observations of the supernova, dubbed Supernova H0pe. These papers can be accessed from this NASA blog post announcing the discovery.
Supernovae are the bright, powerful explosions caused by the violent death of a massive star.
“It all started with one question by the team: ‘What are those 3 dots that weren’t there before? Could that be a supernova?’” says University of Arizona astronomer Brenda Frye.
The points of light were not visible when the Hubble telescope imaged the same cluster in 2015.
“Initial analyses confirmed that these dots corresponded to an exploding star, one with rare qualities,” Frye adds. “First, it’s a Type Ia supernova, an explosion of a white dwarf star. This type of supernova is generally called a ‘standard candle,’ meaning that the supernova had a known intrinsic brightness. Second, it is gravitationally lensed.”
Gravitational lensing is a consequence of the characteristics of the universe described by Einstein’s general theory of relativity. Massive objects – like galaxies – warp space-time around them, bending the trajectory of light.
When objects line up, this creates a lensing effect which can magnify more distant objects.
In this case, the lens consists of 3 galaxies sitting between us and the supernova, bending the supernova’s light into 3 images.
“This is similar to how a trifold vanity mirror presents 3 different images of a person sitting in front of it,” Frye explains. “Since each path had a different length, and light travelled at the same speed, the supernova was imaged in this Webb observation at 3 different times during its explosion.”
“Trifold supernova images are special: The time delays, supernova distance, and gravitational lensing properties yield a value for the Hubble constant. The supernova was named SN H0pe since it gives astronomers hope to better understand the universe’s changing expansion rate.
SN H0pe is tethered to a galaxy which existed about 3.5 billion years after the Big Bang. It is the most distant Type Ia supernovae observed.
Researchers around the world made independent observations of SN H0pe using models of how the galaxies might have lensed the light from the supernovae.
“Since the Type Ia supernova is a standard candle, each lens model was ‘graded’ by its ability to predict the time delays and supernova brightnesses relative to the true measured values,” Frye says.
“The team reports the value for the Hubble constant as 75.4 kilometres per second per megaparsec, plus 8.1 or minus 5.5. [One parsec is equivalent to 3.26 light-years of distance.] This is only the second measurement of the Hubble constant by this method, and the first time using a standard candle,” Frye adds.
“This is one of the great Webb discoveries, and is leading to a better understanding of this fundamental parameter of our universe.”
“Our team’s results are impactful: The Hubble constant value matches other measurements in the local universe, and is somewhat in tension with values obtained when the universe was young,” she adds.
