The ‘Beautiful Confusion’ of the First Billion Years Comes Into View

Susan Kassin, an astronomer at the Space Telescope Science Institute, showed images from previous observatories compared to JWST’s. It was like having the optometrist flip a lens so that the last lines on an eye chart come into focus. “Thank you, Webb — it’s a $10 billion difference,” she said. People chuckled and nodded.

JWST is singular in its ability to see the young cosmos, which has drifted far away from us in both space and time. Its infrared sensors, its ultracold location in space, and its sunshield — which blocks the light of the sun, moon and Earth — are uniquely suited to resolve the first galaxies and their stars. These objects are too faint and at the wrong wavelengths to be seen by previous observatories, like the Hubble Space Telescope.

For astronomers, the vibe is collegial. Many presentations at the KITP conference included pleas for collaborators and partners in brainstorming.

“It was crazy competitive when the first data dropped. Now it is about coming up with ideas,” said Caitlin Casey of the University of Texas, Austin. “There is a firehose of data, and everyone has enough.”

The astrophysicist Rachel Somerville, who co-organized the meeting, said the community is scrambling to absorb both JWST’s data and its implications. Observers see things that are not explained in current theories about the evolution of the young cosmos.

“Many presentations showed that there is a tension between theory and observation,” said Fabio Pacucci of Harvard University, in one example of a cosmic understatement. To punctuate the confusion astronomers feel about this once-in-a-lifetime telescope upending what we know of the young universe, he flashed up a tongue-in-cheek slide: a cartoon of a dog sitting at a table sipping coffee while its house is in flames, captioned “This is fine.”

The Biggest and the Brightest

Astronomers kept referring to one of the most consequential galaxies seen so far, an unexpectedly bright smear of light called, dryly, JADES-GS-z14-0. Hainline, at the University of Arizona, is part of the team that discovered it with JWST and confirmed its distance in May 2024. It is the earliest known galaxy, knocking down the previous record holder, which was found by the same team in 2023.

At the time the galaxy shone forth, sound waves from the tremendous clap that started the universe were still ringing through the void. The first stars had been born in a cataclysmic baby boom, and some had already died. The dark hearts of black holes lurked, too — regions of space where gravity is so strong that not even light can escape. And there was this cluster of stars, resolved as a fuzzy scorpion shape in JWST filters. Two instruments on JWST were able to distinguish JADES-GS-z14-0’s brightness and its distance from Earth. Because of the accelerating expansion of the universe, objects at great distances are very far back in time. Astronomers can tell their ages based on the stretching of their light into longer wavelengths, known as redshift. Based on the most recent measurements, the galaxy was determined to lie at a redshift of 14.18, which means we see it as it appeared 300 million years after the Big Bang — when the universe was about 2% of its current age.

Initially, astronomers speculated that such huge, bright things so early in the universe were at odds with the prevailing theoretical model of the cosmos. But people have softened on that claim. Our best model of the universe — a set of equations describing the evolution of matter and radiation along with dark energy and dark matter — is not dead yet.

“There was a lot of sensationalism” in JWST’s early days, said Alice Shapley of the University of California, Los Angeles. “There is no need for that. The data are so beautiful; let’s just study the universe we have.”

Astrophysicists are coalescing around three star-based theories for how galaxies grew so bright so fast. One holds that stars during the cosmic dawn were very different from stars today. The stars in JADES-GS-z14-0, for instance, might be ultrabright but not actually very massive. While this seems plausible, it is also tricky for theoretical modelers to deal with. The correlation between a star’s brightness and its mass is a key value entered into computer simulations. If this value — known as the initial mass function, or IMF — was different in the early universe, then researchers would have to rewrite their simulations to be able to accommodate an IMF that changes over time.

But nature doesn’t care about our computing issues, and a changing IMF is, in principle, one of the most logical ways to make sense of what we see. “The IMF is truly the house of cards on which we build everything. There are many reasons to believe it is quite different at very high redshift,” Casey said.

Another theory holds that the extra-bright early galaxies happened to be undergoing furious bursts of star formation. Over 10 million or 100 million years, galactic brightness could vary by a factor of 100 as star formation ramped up and down. That’s like a candle turning into a floodlight in the span of a few seconds. Relatedly, during these busy spells, supernova explosions might have made things look brighter than they would otherwise appear.

The third theory suggests that star formation was way more efficient then than now. In a typical galaxy today, a small fraction of the gas is forged into stars; the Milky Way builds between two and six sun-size stars a year. But maybe the smallness and compactness of the early universe made it a better stellar factory. Some calculations suggest an almost 100% rate of conversion from gas to star, meaning fast and furious stellar birth, said Pratika Dayal of the University of Groningen in the Netherlands.

All these alterations to existing theory come with side effects, like changes in how much dust there should be and puzzles about how stellar baby booms settled down. And they’re not even the only ideas out there. Andrea Ferrara, a cosmologist at the Scuola Normale Superiore in Pisa, Italy, showed his colleagues in Santa Barbara a new model that tries to explain the bright early galaxies by changing the amount of dust within them, which would typically block starlight. His model assumes that more dust used to be blown away by stellar winds. “Reducing dust attenuation is my favorite hypothesis, even though I am totally open to having the other two,” he told attendees. But, he acknowledged, his calculations might not hold up at a redshift of 14, meaning they might not work for galaxies like JADES-GS-z14-0.

“So please don’t discover other galaxies,” he concluded, to laughter.

Big Black Holes

Star-related theories are not the only ideas. Some astrophysicists point to active supermassive black holes, which they say might heat surrounding gas and cause galaxies like JADES-GS-z14-0 to appear extremely bright.

In a series of papers published in May, the JADES team argues that the galaxy is starry, and that its brightness cannot be explained by black holes. But other galaxies do have such dark hearts. We know supermassive black holes weighing hundreds of millions or billions of suns anchor the centers of modern galaxies. And JWST is seeing smeared light from many early galaxies, indicating that their gas, too, is being slung around by a central supermassive black hole. How, then, did the big black holes get there?

Since black holes were first predicted as a consequence of Albert Einstein’s theory of gravity, astrophysicists have imagined how they might form from the inward gravitational collapse of dying stars. They now know that the cosmos is filled with black holes formed in this way. But cosmologists have struggled to understand supermassive black holes. These black holes somehow grew large enough, and fast enough, to shape the galaxies that formed around them. If they began as collapsed stars, they would have had to grow at staggering rates that defy physical explanation.