At the APS April Meeting, Scientists Debate the Latest Data from the James Webb Space Telescope

Nearly two years after it began its scientific operations in July 2022, the James Webb Space Telescope has been churning out data with exciting new implications for astronomy and cosmology. At the 2024 APS April Meeting in Sacramento, California, scientists gathered to present and debate the telescope’s latest observations.

“The most exciting sentence in physics is not ‘Eureka!’ but ‘That's funny,’” says astrophysicist Tommaso Treu of the University of California, Los Angeles, quoting Isaac Asimov. “I think that's what we're finding — something surprising.”

Situated 1.5 million kilometers from Earth and fitted with the largest mirror ever launched into space, the telescope is helping scientists study the early universe, the life cycle of stars, and exoplanets. JWST’s four instruments detect wavelengths spanning 600 nanometers (the visible red) to 28.5 microns (the mid-infrared).

The telescope’s sensitivity to infrared light lets it see through dust to the stars beyond, allowing researchers to glimpse distant objects and galaxies formed in the early universe. These older stars, born soon after the Big Bang, emitted a lot of visible and ultraviolet light, but those wavelengths stretch as the universe expands.

“Light that starts in the UV arrives at us in the infrared,” says Treu, who studies the earliest galaxies and who attended the April Meeting.

JWST’s recent data has added new fuel to an ongoing debate over how fast the universe is expanding. The heart of the issue: Different groups of researchers have for years measured different values for the Hubble constant, the universe’s current rate of expansion.

Researchers take two main approaches to measure the constant. In one method, they plug data on the cosmic microwave background, the oldest observable light in the universe, into the standard model of cosmology. Results vary, but researchers have found the Hubble constant to be around 68.3 kilometers per second per megaparsec, with 1 percent uncertainty.

In the other method, researchers estimate the distance from Earth of certain supernovae by using objects of known brightness, collectively known as “standard candles.” They also measure how quickly the supernovae are moving away from Earth, and thus calculate the Hubble constant. Different teams have gotten slightly different values using this method.

But the bigger problem is that the two approaches yield very different constants. The Supernova H0 for the Equation of State (SH0ES) team’s measurement of the supernovae offers the largest discrepancy — five standard deviations — from that of the cosmic microwave background method, at 73.3 with 2 percent uncertainty.

Many think the divergence between the two sets of results could signal that cosmology’s standard model is incomplete. In her talk at the April Meeting, Wendy Freedman of the University of Chicago presented new approaches to resolving the discrepancies.

In particular, observations using JWST could help researchers measure the distance to supernovae more accurately, Freedman noted. Before JWST’s launch, she and her colleagues developed a new method known as the “tip of the red-giant branch” method, which uses the luminosity of the brightest red stars in a galaxy to estimate supernovae distance. They found a Hubble constant value of 69.8 with an uncertainty of about 3 percent. Their new measurement is “consistent” with the results of the cosmic microwave background method, says Freedman.

Considering that value alone, “it doesn’t appear that we need fundamental new physics to incorporate into the standard model,” she says. But they’re refining that measurement further using JWST’s new observations of “tip of the red-giant branch” stars.

To further validate these methods, the team is working to measure the supernovae distances using a brand-new method that makes use of JWST’s observations of another star category, known as the J-region Asymptotic Giant Branch stars. They will also use JWST to refine an old method using pulsating stars known as Cepheids, a project Freedman has been working toward for decades.

Beyond the puzzle around the universe’s rate of expansion, JWST’s observations have raised questions about the early universe. Treu’s team at UCLA is one of several using the telescope’s data to study the earliest galaxies, formed when the universe was only about 500 million years old.

These galaxies are far more compact than younger galaxies — a hundred to a thousand times smaller in diameter, and a million to a billion times smaller in volume, than the Milky Way. These ancient galaxies also have a different chemical composition, containing less oxygen and other elements heavier than helium.

Using JWST, Treu’s team found ten to twenty more galaxies than expected. The galaxies were also brighter than anticipated, indicating they had formed stars quickly. “Galaxies seem to be forming earlier than we thought, and they form stars more rapidly and efficiently than we thought,” says Treu.

The next step is to use JWST to take higher resolution spectra of these objects to “really get into the details,” he says. “We don't understand how the universe formed stars from pristine gas well enough.”

Meanwhile, Tea Temim of Princeton University is using JWST to study closer objects: supernovae in the Milky Way and nearby galaxies. At the meeting, she presented analyses of JWST observations of supernova remnants, the high-speed, leftover material from stellar explosions.

JWST’s infrared camera allows researchers to see deeper into the layers of ejected material from a star. From these images, Temim and her colleagues aim to perform forensics — to understand how the supernova occurred, and how it interacts with the material surrounding it. “We're still trying to understand what types of systems produced the supernova explosions we observed,” says Temim.

In one study, they analyzed Cassiopeia A, located in our own galaxy. The images reveal details like a web-like network of material, as well as the supernova’s outermost shell colliding with circumstellar material. “It's the first time that we've seen the resolved interior of supernova-ejected material,” she says. One challenge, she notes, is determining which structures in the image represent material ejected during the explosion rather than material the star lost prior to the explosion.

Temim also discussed JWST’s observations of the remnants of Supernova 1987a, an explosion that occurred 37 years ago about 170,000 light-years away. It’s the closest supernova ever studied, says Temim. “We've been able to follow its evolution from the explosion until now,” she says. Studying the spectra of ejected material, they saw indirect evidence of a compact object inside, such as a neutron star or a pulsar wind nebula. In the future, Temim and her colleagues plan to use additional observations to better understand the explosion mechanisms.