Webb discovers supernova Hope, which may finally resolve a major controversy in astronomy

“The supernova was named Hope because it gives astronomers hope to better understand the changing rate of expansion of the Universe.”

A view of a gravitationally lensed supernova as seen by the James Webb Space Telescope.

Using the James Webb Space Telescope, astronomers have captured a stunning image of a distant supernova in a galaxy that looks like it's being stretched.

However, the golden patch hiding this gravitationally lensed supernova, dubbed the “Supernova of Hope,” is notable for more than just its aesthetic value. The supernova, which exploded when the 13.8 billion-year-old universe was only about 3.5 billion years old, tells us something about a huge problem in cosmology called “Hubble tension.”

The Hubble tension arises because scientists cannot agree on the exact rate of expansion of the Universe, as dictated by the Hubble constant. In principle, the speed can be measured starting in the local (and therefore recent) Universe and then moving further into the past – or it can be calculated starting in the distant (and therefore early) Universe and then working its way closer to us. The problem is that these two methods produce values that are inconsistent with each other. This is where Webb comes in.

Gravitational lensing supernovae in early space observed by Webb may provide a third way to measure this speed, potentially helping to solve the Hubble problem.

“The supernova was named 'Hope' because it gives astronomers hope to better understand the changing rate of expansion of the universe,” says Brenda Fry, team leader and postdoctoral researcher at the University of Arizona, in statement NASA.

The Hope supernova study began when Fry and her international team of scientists discovered three curious points of light in the Webb image of a distant, tightly packed galaxy cluster. These points of light weren't visible when the Hubble Space Telescope took an image of the same cluster, known as PLCK G165.7+67.0 or more simply G165, back in 2015.

“It all started with one question from the team: “What are these three dots that weren’t there before? Could it be a supernova?'” Fry said. “Initial analysis confirmed that these points correspond to an exploding star, and one with rare features.”

The area around G165 was chosen for the PEARLS program because it is in the midst of a “starburst,” a period of intense star formation in which 300 solar-mass stars are born per year. Such high rates of star formation correlate with a large number of supernova explosions.

A Type Ia supernova is depicted in this artist's photo.

Supernova Hope is a special type of supernova called a Type Ia supernova. Such supernovae occur in binary systems containing a main sequence star, such as the Sun, and a star that has exhausted its nuclear fusion fuel and has collapsed into a dead shell called a white dwarf.

If these stellar bodies are close enough, the dead star can act like a cosmic vampire, drawing plasma from the living “donor” star. As this continues, the material accumulates until it results in a thermonuclear explosion—an explosion we see as a Type Ia supernova. Because of the uniformity of their bursts of light, these supernovae are excellent tools that astronomers can use to measure cosmic distances. That's why astronomers call Type Ia supernovae “standard candles.”

One way to get the Hubble constant is to look at Type Ia supernovae in the local Universe, measure their distances from us and from each other, and then measure how fast they are moving away. The other main method for measuring the expansion of the universe is to observe the distant universe and then calculate the rate of expansion of the cosmos through deduction.

But, as already mentioned, these methods are not the same. Supernova Hope, however, may provide a bridge between these two methods.

Einstein lends a helping hand

Gravitational lensing is an effect predicted in Albert Einstein's 1915 theory of gravity, called the “general theory of relativity.”

General relativity suggests that objects with mass cause the curvature of spacetime, the four-dimensional union of space and time, and gravity arises from this curvature. The greater the mass of an object, the greater the curvature of space and, therefore, the greater the gravitational influence of the object. This is what causes moons to orbit planets, planets to orbit stars, and stars to orbit supermassive black holes.

This curvature of space-time has another interesting effect. When light passes by an object with a strong bending influence, which we will now call a “gravitational lens”, the path of the light is bent around the object of bending. The path of light depends on how close it is to the gravitational lens.

This means that light from the same object can follow paths that are curved to different degrees and have different lengths. So light can hit telescopes like Webb at different times. This is why a lensed background object can appear “smeared” like taffy, or appear in multiple places in the same image.

This is exactly what happens to the Hope supernova in this image as its light passes through G165's gravitational lens.

This illustration shows a phenomenon known as gravitational lensing, which is used by astronomers to study very distant and very faint galaxies.

“Gravitational lensing is very important for this experiment. The lens, made up of a cluster of galaxies located between the supernova and us, bends the light from the supernova, producing multiple images of it,” says Fry. “It’s like how a three-piece mirror gives three different images of the person sitting in front of it.”

The University of Arizona researcher explained that the effect was demonstrated right before the team's eyes in image G165 Webb, where the middle image of the supernova appeared upside down in relation to the other two.

“To get three images, the light traveled along three different paths. Because each path is a different length and light travels at the same speed, the supernova was imaged in this Webb image at three different times during its explosion,” Fry continues. In the tripartite mirror analogy, there was a time delay where the right mirror showed the person raising the comb, the left mirror showed the person combing the hair, and the middle mirror showed the person putting the comb down.”

“Three supernova images are special. Time delays, the distance to the supernova, and the properties of gravitational lensing provide the value of the Hubble constant.”

A closer look at the galactic taffy containing three instances of the Hope supernova.

The team tracked Supernova Hope using Webb as well as some ground-based instruments, including the 6.5-meter MMT Mount Hopkins Telescope and the Mount Graham Large Binocular Telescope, both located in Arizona.

This allowed the team to confirm that the Hope supernova is tied to a background galaxy located far behind the lensing cluster G165. Light from the cosmic explosion takes 10.3 billion years to reach Earth, meaning the white dwarf exploded just 3.5 billion years after the Big Bang.

Another team member took another time-lag measurement by analyzing the evolution of its light as it was scattered into component colors, or “spectrums,” by Webb, confirming the nature of Hope as a Type Ia supernova, Fry said. “Supernova Hope is one of the most distant Type Ia supernovae observed so far.”

Despite existing in the early Universe, the value of the Hubble constant obtained from observations of the Hope supernova appears to be consistent with measurements of other standard candles in the local Universe, thereby inconsistent with measurements of other objects in the early Universe.

“Our team’s results matter a lot,” concluded Fry. “The value of the Hubble constant matches other measurements in the local Universe and is somewhat inconsistent with values obtained when the Universe was young. Webb's third cycle observations will improve the uncertainty, allowing for a more accurate determination of the Hubble constant.”