The mystery of our transparent universe has been solved!
*This image captured by the James Webb Space Telescope contains roughly 20,000 galaxies. At the centre of this image — the pink, six-spiked point of light — is quasar J0100+2802, as seen when the universe was only 900 million years old. Credits: NASA, ESA, CSA, Simon Lilly (ETH Zurich), Daichi Kashino (Nagoya University), Jorryt Matthee (ETH Zurich), Christina Eilers (MIT), Rob Simcoe (MIT), Rongmon Bordoloi (NCSU), Ruari Mackenzie (ETH Zurich), Alyssa Pagan (STScI), Ruari Mackenzie (ETH Zurich)*
Long ago, when the universe was young, it was much harder to see. During the first billion years or so following the Big Bang, the space between the stars and galaxies was filled with clouds of cooled hydrogen gas, which absorbed all light. As a result, if any intelligent life existed at the time, they would have seen nothing but darkness out in space — no other stars or galaxies would have been visible to them.
Then came a time astronomers call the Era of Reionization, when all of that hydrogen gas transformed from opaque to transparent as it heated up and became ionized. Astronomers knew that this process occurred (we wouldn't even see the universe if it hadn't). Until now, though, they didn't have any solid evidence for how it happened.
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Now, a team of researchers — the Emission-line galaxies and Intergalactic Gas in the Epoch of Reionization or EIGER — led by Simon Lilly of ETH Zürich in Switzerland, has used the James Webb Space Telescope to solve this mystery.
Focusing Webb on a small region of space between the constellations Pisces and Andromeda, the team pointed the telescope directly at an ancient bright object known as quasar J0100+2802. A quasar is an active supermassive black hole at the core of a galaxy that is emitting immense amounts of energy as it consumes matter, and quasars are some of the brightest objects in the universe.
This artist's impression depicts what a distant quasar may look like up close. Credit: ESO/M. Kornmesser
However, the EIGER team wasn't solely interested in the quasar itself. They also used Webb to observe how the light from the quasar behaved as it passed through gases along the path from J0100+2802 to the telescope's instruments.
"As the quasar's light traveled toward us through different patches of gas, it was either absorbed by gas that was opaque or moved freely through transparent gas," NASA said.
Adding the data from Webb to similar observations collected by the W. M. Keck Observatory in Hawaii, plus the ESO's Very Large Telescope and the Magellan Telescope at Las Campanas Observatory, the EIGER team matched up the behaviour of the light with the locations of early galaxies visible along the line of sight to J0100+2802.
These six galaxies were imaged by NASA's James Webb Space Telescope, and appear as they did when the universe was only 900 million years old. Credits: NASA, ESA, CSA, Simon Lilly (ETH Zürich), Daichi Kashino (Nagoya University), Jorryt Matthee (ETH Zürich), Christina Eilers (MIT), Rob Simcoe (MIT), Rongmon Bordoloi (NCSU), Ruari Mackenzie (ETH Zürich); Image Processing: Alyssa Pagan (STScI), Ruari Macke
"[These early galaxies] are more chaotic than those in the nearby universe," Jorryt Matthee of ETH Zürich, who is the lead author of one of the team's three papers based on these observations, said in a press release. "Webb shows they were actively forming stars and must have been shooting off many supernovae. They had quite an adventurous youth!”
The results revealed that the galaxies were each surrounded by an envelope of hot, ionized, transparent gas up to 2 million light years in radius. For a sense of scale, our galaxy's nearest large neighbor, the Andromeda Galaxy, is about 2.5 million light years away.
"Not only does Webb clearly show that these transparent regions are found around galaxies, we've also measured how large they are," Daichi Kashino of Nagoya University, the lead author of another of the papers written by the team, explained in the press release. "With Webb's data, we are seeing galaxies reionize the gas around them."
As summarized in the infographic below, it was the universe's earliest galaxies, themselves, that were responsible for bringing about the Era of Reionization.
The four stages of the Era of Reionization. Credit: NASA, ESA, CSA, Joyce Kang (STScI)
As they went through bouts of star formation and stellar death through supernovae, the radiation emitted from these galaxies is what heated up and ionized the hydrogen gas, turning it from opaque to transparent. These bubbles of transparent, ionized gas then expanded and merged together, allowing light to then travel far throughout the universe.
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More mysteries to solve
Even as they answered the question of what was ultimately responsible for the transparent universe we see today, this team of researchers also added to another growing mystery about the early cosmos.
Given what we know about star formation and how stars gather together into galaxies, astronomers thought they had a pretty good idea of how many galaxies they'd find in the early universe. However, the numbers Webb is revealing far exceed those expectations.
"We expected to identify a few dozen galaxies that existed during the Era of Reionization – but were easily able to pick out 117," Kashino explained.
Also, the data collected by Webb allowed the team to measure the mass of the supermassive black hole powering quasar J0100+2802. They found that it tips the cosmic scales at around 10 billion times the mass of our Sun. That makes it the largest known supermassive black hole in the early universe.
"We still can't explain how quasars were able to grow so large so early in the history of the universe," Anna-Christina Eilers of MIT, the lead author of the team's third paper, told NASA.
"That's another puzzle to solve!"
