For millions of years following the Big Bang, after the universe’s roiling soup of particles had cooled, the cosmos was a dark and boring place. There were no stars to make light. No familiar swirls of galaxies. Certainly no planets. And the entire universe was shrouded in neutral hydrogen gas.
Then, perhaps 100 million years or so in, everything started to change. Over the next billion-odd years, the universe went from a bland, unimpressive landscape to a rich and dynamic one. This profound shift began when the first stars lit up. As they burned, generating heat and forging new matter, their intense light began tearing apart the hydrogen that pervaded the universe. Everywhere electrons were ripped from these atoms, leaving the bulk of hydrogen — the most abundant element in the universe — in the ionized state it remains in today.
The Big Bang created a hot, ionized soup of subatomic particles. Hundreds of thousands of years in, in an event known as recombination, neutral atoms formed. The period known as the dark ages followed; the universe was suffused with neutral hydrogen gas. But when the first stars turned on, sometime more than 100 million years after the Big Bang, they ripped electrons from the hydrogen, gradually reionizing the cosmos.
CREDIT: NAOJ
This pivotal period — when all that hydrogen went from one form to another — is known as the epoch of reionization. It began with our cosmic dawn and ushered in the modern era with all its marvelous textures and features. It serves as the backdrop for when the universe grew up.
“It’s the last major shift that happens to our universe,” says theoretical astrophysicist Julian Muñoz of the University of Texas at Austin. Everything changed over that billion years or so and nothing much has changed in the billions of years since.
While there are models that describe how this great transition might have happened, giant gaps in our picture remain. When did the first stars form and when did light, escaping their host galaxies, kick off reionization? What kinds of galaxies were most responsible and what was the role of black holes? How did reionization proceed across time and space? And what clues might it hold to other cosmic mysteries, like the nature of dark matter?
“We don’t understand how the universe came to be what it is today,” Muñoz says.
Some answers are now within reach, thanks to new tools that allow scientists to look back deep into the universe’s first billion years. The James Webb Space Telescope (JWST), launched in 2021, is peering at the galaxies that existed only hundreds of millions of years after the Big Bang and is already turning up surprises. At the same time, next-generation radio telescopes are focusing not on the galaxies but on the neutral hydrogen that once pervaded all of space. That hydrogen provides clues to how the epoch of reionization unfolded, and other characteristics of the cosmos.
“The tools that we can bring to bear now on studying this epoch of cosmic history are unlike anything we’ve had before,” says astrophysicist Rob Simcoe of MIT.
Light galore
Our current understanding of the early universe’s development goes something like this: After the Big Bang, 13.8 billion years ago, the cosmos expanded and the primordial soup of subatomic particles cooled. Within the first second, protons and neutrons formed. Within the first few minutes, they joined up into atomic nuclei. About 380,000 years in, those nuclei began capturing electrons to form the first atoms. This milestone, in which the ionized soup became neutral atoms, is known as recombination (a misnomer, since nuclei and electrons had never combined before).
Until they were captured into atoms, the unfettered electrons scattered light like a dense fog in a car’s headlights. But with electrons reined in, photons could shoot out through the cosmos. Today, those particles of light arrive to us in the form of a faint glow known as the cosmic microwave background.
Then the universe entered what are known as the dark ages. With hydrogen and some helium gas pervading the cosmos, there was nothing much around to make light. Yet blobs of dark matter were busy pulling in the surrounding gas, some of it condensing enough to set off nuclear fusion. A hundred million years or more after the Big Bang, the first stars lit up in our cosmic dawn. As these early stars burned, their ionizing ultraviolet light began escaping from their galaxies. This created bubbles of ionized hydrogen that grew until they merged, eventually filling the cosmos.
JWST is poised to answer many questions about early galaxies and how their light drove the process of reionization. For now, though, the telescope is turning up more questions than answers. There were many more galaxies in early times than scientists had thought — and these galaxies were producing far more than enough of the type of light needed to reionize the universe.
Early images released by the telescope were overflowing with galaxies that dated to less than 600 million years after the Big Bang. Then, in late 2022, came confirmation of the earliest galaxy yet; it existed just 350 million years after the Big Bang. That record was then busted when UC Santa Cruz astrophysicist Brant Robertson and colleagues announced a galaxy that dated to just 290 million years after the Big Bang.
Many of these galaxies are brighter and more massive than expected: In 2023, six galaxies dating to within 700 million years of the Big Bang made headlines for how mature they already appeared. Despite the early epoch, their stellar masses rival that of today’s Milky Way, which has 60 billion solar masses worth of stars.
Standard theory can’t explain so much star formation so early, so these galaxies were dubbed the “universe breakers.”
“It’s just absolutely wild,” says astrophysicist Erica Nelson of the University of Colorado Boulder, a coauthor on the paper. “It implies an early universe that is either more chaotic and bursty than we thought, or a universe in which things can evolve more quickly.”
The galaxy in this James Webb Space Telescope image, dubbed JADES-GS-z14-0, broke the record for the most distant known galaxy. It dates to 290 million years after the Big Bang.
CREDIT: NASA, ESA, CSA, STSCI, B. ROBERTSON (UC SANTA CRUZ), B. JOHNSON (CFA), S. TACCHELLA (CAMBRIDGE), P. CARGILE (CFA)
The discoveries may force a reexamination of galaxy evolution. And they raise big questions about reionization.
Even the faintest early galaxies that JWST has spotted are producing loads of reionizing light, four times as much as expected, astrophysicist Hakim Atek of the Institut Astrophysique de Paris and colleagues have found. Despite their dimness, there are enough of these galaxies to reionize the universe mostly on their own.
And JWST is also turning up hints that supermassive black holes formed much earlier in cosmic history than thought; the high-energy emissions they generate as they feed on surrounding matter would also have contributed to reionization.
With all that light, the universe should have been reionized sooner than we know it was, Muñoz and colleagues suggest in a 2024 paper titled “Reionization after JWST: a photon budget crisis?”
It’s not really a crisis, Muñoz says. Existing research has established that reionization ended 1.1 billion years after the Big Bang. But the seeming overabundance of reionizing light is a clear sign that something is missing in our picture of the early universe. “We don’t know all the pieces of the puzzle,” he says.
Seeking clues in hydrogen
Other efforts hope to track reionization by using next-generation radio telescopes to see how much neutral hydrogen existed across time in the early universe.
Scientists have probed this hydrogen in other ways. The scattering of the light of the cosmic microwave background, for example, offers clues to the total amount of reionization since that light was emitted, roughly 380,000 years after the Big Bang. Quasars, the bright beacons of radiation produced by massive, feeding black holes, offer another probe. Neutral hydrogen absorbs specific wavelengths of light from quasars on its path to an observer, providing a sign of the hydrogen’s presence. But as you approach earlier epochs, there are fewer quasars.
In this simulation of the epoch of reionization, regions of ionized hydrogen gas (blue and translucent) expand over time, overtaking regions of neutral hydrogen (dark and opaque).
CREDIT: M. ALVAREZ, R. KAEHLER AND T. ABEL
So scientists now aim to detect a radio signal from the neutral hydrogen itself, before it was ionized, back through cosmic dawn and even into the dark ages. This signal, known as the 21 cm line, has been detected since the 1950s and is used widely in astronomy, but it hasn’t been definitively spotted from the early universe.
The radio signal arises because of a quantum transition in neutral hydrogen’s electron. The transition, which emits a bit of electromagnetic radiation at a wavelength of 21 centimeters, doesn’t happen often. But when neutral hydrogen is abundant, it’s possible to spot.
And the signal can do more than track neutral hydrogen’s whereabouts. It also serves as a sort of thermometer. Scientists can use it to better understand the cosmic temperature, including clues to when energy is injected into the intergalactic medium in the form of light or heat.
Such blasts of energy could come from the first stars and feeding black holes. Or the energy could hint at something more exotic: interactions between dark matter and itself, or unknown interactions between dark matter and more familiar matter. Such interactions, Muñoz notes, could heat up or cool down the intergalactic medium. The 21cm line offers a way to probe the processes at play, including any spurred by unexpected physics. “It can give you information you won’t otherwise get,” he says.
One telescope looking for this fingerprint is known as the Hydrogen Epoch of Reionization Array, or HERA. If JWST is known for its complexity and cost, HERA is more off-the-shelf. It’s “made of PVC pipe and wire mesh and telephone poles,” says astrophysicist Josh Dillon of the University of California, Berkeley.
HERA consists of 350 radio antennas spread across 5 percent of a square kilometer in the Northern Cape province of South Africa. While the telescope itself is low-tech, its observations require the most advanced signal processing and data analysis available. That’s because the inherently faint signal has to be spotted amid booming radio noise from our galaxy and others.
Dillon compares spotting the 21 cm signal to listening for the treble at a concert when the bass is 100,000 times stronger. “That is why it hasn’t been done yet,” he says.
The HERA telescope, an array of 350 radio antennas in South Africa, aims to detect fluctuations in a signal from the neutral hydrogen that pervaded the early universe.
CREDIT: SOUTH AFRICAN RADIO ASTRONOMY OBSERVATORY (SARAO)
HERA seeks a statistical measure of the spatial fluctuations in the 21 cm signal. Those fluctuations arise from variations in the distribution of neutral hydrogen across the sky and so offer a sense of how the gas, as well as the stars and galaxies, were arranged. Other teams instead aim to make a bulk measurement that captures an average signal across the sky. Since the techniques differ, one could help verify the other.
Dark matter has already been invoked to explain one claimed detection. In 2018, researchers with the Experiment to Detect the Global Epoch of Reionization Signature, or EDGES, reported a detection of the averaged 21 cm signal that corresponds to when the light from the first stars started interacting with the surrounding hydrogen.
The signal is stronger than expected, suggesting colder than predicted hydrogen gas, which has fueled a lot of skepticism around the claim. Some researchers have pointed to interactions between the hydrogen gas and dark matter as a possible explanation, but such an explanation would require unexpected physics.
“There are a lot of fanciful theories,” says observational cosmologist Sarah Bosman of Heidelberg University in Germany. “It has to be fanciful,” she notes, because no ordinary physics would give the strength that EDGES saw.
Bosman admits to being one of the few people enthusiastic about the claim, which she says has motivated researchers working on other experiments that might confirm or refute it. “It’s given the field a really good boost,” she says.
HERA and other telescopes are forerunners of the Square Kilometer Array, which will attempt to map the 21 cm signal across the entire sky. This array will connect radio antennas in South Africa and Australia into the largest radio telescope ever built. Though still under construction, the telescope connected two of its stations to take its first data in 2024.
Better tools, deeper knowledge
No one really knows what to expect from the 21 cm signal, Bosman notes. It could demand only minor tweaks to the existing picture of cosmic evolution, or it might uncover new physics that rewrite our understanding entirely. It’s just too soon to tell.
But Dillon says that the 21 cm line could one day offer “the biggest possible dataset.” The ultimate aim is to probe the time frame from roughly 100 million years after the Big Bang to a billion years after. That time frame represents less than 10 percent of the total life of the universe, but because of the continued expansion of the universe, the time frame covers roughly half the volume of the visible universe.
Future instruments will help reach all the way back. There are various proposals for new radio telescopes in space and even on the Moon, where they would be free from Earth-based interference. The most ancient 21 cm signal would arrive to us at wavelengths that are reflected off Earth’s ionosphere, notes Anastasia Fialkov, a cosmologist and astrophysicist at the Institute of Astronomy in Cambridge, England. Telescopes in space, or on the Moon, could get around that problem.
Any 21 cm clues would be studied alongside JWST’s observations of early galaxies, as well as observations from its successor, the Nancy Grace Roman Space Telescope, and future ground-based observatories like the European Extremely Large Telescope currently under construction in Chile.
Studies of quasars also have plenty more to say, notes Simcoe of MIT, who wrote with colleagues about quasars in the early universe in the 2023 Annual Review of Astronomy and Astrophysics.
Quasars are particularly useful, Simcoe says, for identifying “the last regions of the universe that are still holding on to their neutral hydrogen gas.” It’s within these pockets that the youngest stars and galaxies — or the material that birthed them — must reside.
These early stars could be producing trace elements different from what we see produced by today’s stars. If light from quasars reveals those trace elements in an ancient cloud of gas, it’s a clue that we’re reaching an ancient population — perhaps the first stars.
“It will mean we have finally gotten there,” Simcoe says. “And that’s really what the quest is: To find out, when did complexity emerge in the universe? When did the universe really start to look the way it does today?”
No one knows when we’ll know, but Simcoe thinks the present tools, or perhaps the next ones on deck, are capable: “We’re knocking at the door.”
