Nearly 14 billion years ago, the universe began with the Big Bang, according to the standard cosmological model. In the primordial nucleosynthetic wake of its birth, the first elements — hydrogen and helium — took shape within the first few minutes. Then, there were the dark ages.
“Use your imagination to transport yourself backwards in time, maybe 100 million years after the Big Bang, and the universe is just pitch black,” said Tucker Jones, a professor in the Department of Physics and Astronomy at the College of Letters and Science at UC Davis. “There is an infrared glow from the cosmic background radiation, but it appears dark to our eyes, totally dark. And then gradually, and then suddenly, the first star begins to shine.”
Jones, one of the speakers for the March 2026 Astronomy on Tap event at Sudwerk Brewing Co., was describing the cosmic dawn, an early period in the universe’s history when the first stars and galaxies formed.
The cosmic dawn is significant both for the first starlight and the formation of elements heavier than helium from stellar fusion.
“So in that beer you’re drinking, while the hydrogen came from the Big Bang, everything else came later, starting with the cosmic dawn,” Jones said.
“Nitrogen, carbon, oxygen, all that stuff in your beer, all that stuff in our bodies, it’s ‘star-stuff,’” he added, invoking a famous quote from Carl Sagan.
When exactly did the cosmic dawn occur?
While the jury is still out, scientists are getting closer to an answer thanks to the data being gathered by instruments aboard the James Webb Space Telescope, or JWST.
“What we are doing is searching for light from the early universe from very, very distant galaxies and stars,” Jones said. “Light that has been traveling for billions and billions of years to reach us.”
How the JWST is optimized to hunt for cosmic dawn
Orbiting the sun nearly 1.5 million kilometers away from Earth, the JWST is an infrared telescope uniquely designed to detect light from the earliest stars and galaxies.
“One weird thing about the universe is that it’s expanding all the time,” Jones said. “As the space expands, it stretches out the wavelengths of light with it.”
This phenomenon is called the cosmological redshift.
Take a galaxy from the early universe, for instance. Its billions of stars initially emitted blue light. As the light traveled across the universe, its wavelength stretched, reaching the red and infrared regions of the electromagnetic spectrum.
“What we are seeing with the JWST in these very distant galaxies is light that actually started out blue, or even ultraviolet, and it has been stretched out so much that it’s now infrared,” Jones said.
JWST’s mirrors, optics and instruments are optimized to detect infrared light and are thus specially tailored to observe the universe’s earliest stars and galaxies.
Detecting star-stuff
One of the first JWST images released by NASA shows the galaxy cluster SMACS 0723 as it appeared 4.6 billion years ago. In the image, a whorl of galaxies generously peppers the inky black of space.
Jones presented the image to the audience at Sudwerk Brewing.
“All this light, by the way, is completely invisible to our eyes,” he said. “This image to us is just black, total darkness, because it is all infrared.”
But were any of the imaged galaxies seen during their birth at the cosmic dawn?
To find the answer, astronomers analyze the chemical signatures of the galaxies. If they show traces of heavy elements such as nitrogen, carbon or oxygen, it’s a no-go.
“That is from stars that already were born and already died,” Jones said. “This tells us that we are seeing way back in the past, but we’re not quite seeing the cosmic dawn.”
As powerful telescopes like the JWST probe deeper into the cosmos, the frontier in the search for the cosmic dawn keeps getting pushed further back.
Jones was involved in one of the first research programs to use JWST data, which identified a galaxy seen only 360 million years after the Big Bang that wasn’t formed during the Cosmic Dawn.
“Now, we know of galaxies that are within 300 million years [of the Big Bang],” said Jones, referencing recent work in the field. “Best guess, give or take, is that the Cosmic Dawn is maybe 200 million years [after the Big Bang].”
“We can see we’re not quite there yet. We are still seeing star-stuff,” he added. “But gosh, we are marching back closer and closer. That’s the game. Will we get there? Yes, eventually! We’ll see when. I keep being surprised.”
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