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 like hot fudge.
However, the golden spot that hides this gravitational lensing supernova, nicknamed "supernova Hope," is remarkable not only for 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 about a huge problem in cosmology called the "Hubble Voltage."
The Hubble tension stems from the fact that scientists cannot agree on the exact expansion rate of the universe, dictated by the Hubble constant. In short, the speed can be measured from the local (and therefore recent) universe, and then further back in time - or it can be calculated from the distant (and therefore early) universe, and then working upwards. The problem is that both methods return values that do not match each other. This is where the James Web Space Telescope (JWST) comes into the picture.
Gravitational lensing supernovae in the early cosmos observed by the JWST could provide a third way to measure velocity, and possibly help solve this 'Hubble problem'.
"The supernova was named 'supernova Hope' because it gives astronomers hope to better understand the changing expansion rate of the universe," said Brenda Frye, research team leader and researcher at the University of Arizona, in a NASA statement.
This investigation into supernova Hope began when Frye and her global team of scientists found three curious points of light in a JWST image of a distant, dense cluster of galaxies. Those bright spots in the image were not visible when the Hubble Space Telescope imaged the same cluster, known as PLCK G165.7+67.0 or, more simply, G165, in 2015.
Related: Strange physics at the edges of black holes could help solve persistent 'Hubble problems'"It all started with one question from the team: 'What are those three points that weren't there before? Could that be a supernova?'" Frye said. "Initial analyzes confirmed that these dots corresponded to an exploding star, a star with rare properties."
The space surrounding G165 was selected for the PEARLS program because it is in the midst of a starburst, a period of intense star formation that produces stars with a mass of 300 solar masses per year. Such high star formation rates are correlated with higher rates of supernova explosions.
Supernova Hope is a specific type of supernova, called a Type Ia supernova. These supernovae occur in binary stars that contain a main sequence star, such as the Sun, and a star that has used up its nuclear fusion fuel and has become 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 or 'donor' star. As this continues, the material builds up until it causes a thermonuclear explosion - explosions we think of as Type Ia supernovae. Because of the uniformity of their flashes of light, these supernovae are an excellent tool that astronomers can use to measure cosmic distances. Astronomers therefore call type Ia supernovae 'standard candles'.
One way to get a value for the Hubble constant is to look at Type Ia supernovae in the local universe and measure their distances to us and to each other, then measure how quickly they recede. The other main technique for measuring the expansion of the universe is to take observations of the distant universe and then calculate how fast the cosmos is expanding through deduction.
But again, these methods are not consistent with each other. However, Supernova Hope could act as a bridge between the two techniques.
Einstein lends a hand
Gravitational lensing is an effect predicted in Albert Einstein's magnum opus theory of gravity, formulated in 1915 and called "general relativity."
General relativity suggests that objects with mass cause the warping of spacetime, the four-dimensional unification of space and time, with gravity arising from this warping. The greater the mass of the object, the more extreme the curvature of space and therefore the greater the gravitational influence of that object. This is why moons orbit planets, planets orbit stars, and stars orbit supermassive black holes.
This curvature of spacetime has another interesting effect. When light passes through an object with a strong warping influence, an object we will now call a 'gravity lens', the path of the light is bent around the curvature of the object. The path the light takes depends on how close it comes to the gravitational lens.
This means that light from the same object can follow paths that are bent to different degrees and of different lengths. Therefore, that light can arrive at telescopes like the JWST at different times. This is how a background object with a lens can look "smeared" like toffee or appear in multiple places in the same image.
That's what happens to supernova Hope in this image as its light passes through the gravitational lens G165.
"Gravitational lenses are important for this experiment. The lens, made up of a cluster of galaxies located between the supernova and us, bends the supernova's light into multiple images," Frye said. "This is similar to how a trifold vanity mirror presents three different images of a person sitting in front of it."
The University of Arizona researcher explained that the effect was demonstrated right before the team's eyes in the G165 JWST image, where the center supernova image appeared to be flipped from the other two images.
"To obtain three images, the light traveled along three different paths. Because each path was a different length and the light traveled at the same speed, the supernova in this JWST observation was imaged at three different times during its explosion" , Frye continued. "In the three-fold mirror analogy, a time delay followed where the right mirror depicted a person lifting a comb, the left mirror showed hair being combed, and the middle mirror showed the person putting the comb down.
"Trifold supernova images are special. The time delays, supernova distance and gravitational lens properties provide a value for the Hubble constant."
The team monitored the supernova Hope with the JWST and with several instruments on Earth, including the MMT 6.5-meter telescope on Mount Hopkins and the Large Binocular Telescope on Mount Graham, both located in Arizona.
This led the team to confirm that supernova Hope is anchored in a background galaxy far behind the lens cluster G165. The light from the cosmic explosion has been traveling to Earth for 10.3 billion years, meaning this white dwarf blew its top just 3.5 billion years after the Big Bang.
"Another team member performed another time-delay measurement by analyzing the evolution of its light dispersed in the constituent colors or 'spectrum' of the JWST, which confirmed the Type Ia nature of supernova Hope," Frye said. "Supernova Hope is one of the most distant Type Ia supernovae observed to date,"
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Despite its existence in the early universe, the value of the Hubble constant provided by the observations of supernova Hope appears to agree with measurements of other standard candles in the local universe, and thus not consistent with measurements of other objects in the early universe.
"Our team's results are impactful," Frye concluded. "The value of the Hubble constant is consistent with other measurements in the local universe and is somewhat in tension with values obtained when the universe was young. JWST observations in cycle 3 will improve the uncertainties, allowing more sensitive constraints on the Hubble constant become possible."
The team's research is peer-reviewed before publication.
