Today, vast swaths of space are perfectly transparent, but that wasn’t always the case. In its early days, the universe was filled with a “fog” that made it opaque, obscuring the first stars and galaxies. NASA’s upcoming Nancy Grace Roman Space Telescope will study the universe’s later transition to the brilliant starscape we see today—an era known as the cosmic dawn.
“Something fundamental in the nature of the universe changed during this time,” said Michelle Thaller, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “With Roman’s big, sharp infrared image, we may finally understand what happened at a critical cosmic turning point.”
Lights out, lights on
Shortly after its birth, the cosmos was a sea of particles and radiation. As the universe expanded and cooled, positively charged protons were able to capture negatively charged electrons to form neutral atoms (mostly hydrogen, plus some helium). This was great news for the stars and galaxies these atoms would become, but bad news for light!
It probably took a long time for hydrogen and helium gas to coalesce into stars, which then orbited together to form the first galaxies. But even when stars began to shine, their light couldn’t travel very far before it hit neutral atoms and was absorbed by them. This period, known as the cosmic dark ages, lasted from 380,000 to 200 million years after the Big Bang.
Then the fog slowly lifted as more and more neutral atoms shattered over the next hundreds of millions of years: a period called the cosmic dawn.
“We’re very curious about how this process happened,” said Aaron Yung, a Giacconi Fellow at the Space Telescope Science Institute in Baltimore who is helping plan Roman’s observations of the early universe. “The wide, clear view of deep space that Roman has obtained will help us evaluate different explanations.”
Main suspects
It could be that the first galaxies were largely responsible for the energetic light that shattered the neutral atoms. The first black holes may have played a role as well. Roman searches far and wide to examine the two possible culprits.
“Roman will be an excellent tool for finding the building blocks of cosmic structures, such as galaxy clusters that will form later,” said Takahiro Morishita, an assistant scientist at Caltech/IPAC in Pasadena, California, who has studied the cosmic dawn. “It will quickly identify the densest regions, where the ‘fog’ is clearing further, making Roman a key mission for probing the evolution of early galaxies and the cosmic dawn.”
The first stars were probably very different from modern stars. When gravity began to pull matter together, the universe was very dense. Stars probably became hundreds or thousands of times more massive than the Sun and emitted a lot of high-energy radiation. Gravity pulled the young stars together to form galaxies, and their cumulative explosion may have once again ripped electrons from protons in the bubbles of space around them.
“You could call it the celebration of the beginning of the universe,” Thaller said. “We never saw the birth of the very first stars and galaxies, but it must have been spectacular!”
But these heavy stars had short lifespans. Scientists think they collapsed quickly, leaving behind black holes—objects with gravity so extreme that even light can’t escape their clutches. Since the young universe was also smaller because it hadn’t been expanding for very long, hordes of these black holes could have merged to form even larger ones—up to millions or even billions of times the mass of the Sun.
Supermassive black holes may have helped clear the hydrogen fog that permeated the early universe. The hot material that swirls around black holes at the centers of active galaxies, called quasars, before falling inward can generate extreme temperatures and emit huge jets of intense radiation. These jets can stretch hundreds of thousands of light-years across, stripping electrons from any atom in their path.
NASA’s James Webb Space Telescope is also exploring the cosmic dawn, using its narrower but deeper view to study the early universe. By combining Webb’s observations with Roman’s, scientists will produce a much more complete picture of that time.
So far, Webb has found more quasars than expected, given their rarity and Webb’s limited field of view. Roman’s magnified view will help astronomers understand what’s going on by seeing how common quasars really are, with tens of thousands fewer quasars than the handful Webb could find.
“With a more robust statistical sample, astronomers will be able to test a wide range of theories inspired by Webb’s observations,” Yung said.
By looking at the first few hundred million years of the universe through Roman’s wide eyes, scientists will also be able to determine whether a certain type of galaxy (such as the most massive ones) played a more important role in clearing the fog.
“It could be that young galaxies started the process and then quasars finished the job,” Yung said. The size of the bubbles carved out of the fog will give scientists an important clue. “Galaxies would create huge clusters of bubbles around themselves, while quasars would create large, spherical ones. We need a large field of view like Roman’s to measure their extent, because in both cases they are probably millions of light-years across, often larger than Webb’s field of view.”
Roman will work hand-in-hand with Webb to provide clues about how galaxies formed from the primordial gas that once filled the universe, and how their central supermassive black holes influenced the formation of galaxies and stars. The observations will help uncover the cosmic auroras that lit up our universe and ultimately made life on Earth possible.
The Nancy Grace Roman Space Telescope is operated from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with contributions from NASA’s Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team comprised of scientists from various research institutions. Key industry partners include BAE Systems, Inc. in Boulder, Colorado; L3Harris Technologies in Rochester, New York; and Teledyne Scientific & Imaging in Thousand Oaks, California.
Download high-resolution videos and images from NASA’s Scientific Visualization Studio
By Ashley Balzer
NASA Goddard Space Flight CenterGreenbelt, Maryland
Media contact:
Claire Andreoli
claire.andreoli@nasa.gov
NASA Goddard Space Flight CenterGreenbelt, Maryland
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