All around the world, radio antennae in remote landscapes are scanning the sky for the same faint signal from the “cosmic dawn,” a time when the first stars shone more than 12 billion years ago.
If detected, the signal will shed light on some of the most enduring mysteries about the origins of, well, everything—stars, galaxies, even the enigmatic dark matter and dark energy that scientists think makes up 95 percent of the universe’s mass. In fact, the discovery would be so significant that at least one of the many teams hunting for the signal thinks it would be a likely candidate for the Nobel Prize.
“There is a lot of competition about who will be there first, but on the other hand, there is also collaboration and knowledge that is shared,” said Anastasia Fialkov, a senior research fellow at the Kavli Institute for Cosmology in Cambridge, UK, in a call.
Slowly but surely, scientists are closing in on this momentous detection. In September, a team published a new timeframe of the era from which the signal originates that is about 10 times more precise than previous estimates. Last year, another team captured the most promising potential detection of the signal so far, though those results are still under review.
The signal is not a message from an alien civilization, or a glimpse of some exotic object at the edge of time. In fact, it comes from one of the universe’s simplest components: neutral hydrogen atoms. Because these atoms absorb and release photons with wavelengths of 21 centimeters, the signal is known alternately as the neutral hydrogen signal or the 21-centimeter signal.
The signature of this ancient hydrogen could open up the first observational window into the early Epoch of Reionization (EoR). This is the murkiest era of the universe’s history, and began a few hundred million years after the Big Bang.
“We know that neutral hydrogen is there, so the neutral hydrogen signal must also be there,” explained Leon Koopmans, a professor at the University of Groningen and principal investigator of the LOFAR Epoch of Reionization Key Science Project, which uses the LOFAR telescope to hunt for the neutral hydrogen signal.
Before the EoR, the universe was bereft of starlight, in a time known as the cosmic Dark Ages. After the EoR, the basic structure of the known universe we inhabit today, speckled with stars and galaxies and sculpted by dark matter and energy, had materialized. But scientists know next to nothing about the roughly 500-million-year stretch that separates the Dark Ages from the modern, light-filled universe.
The best bet for finally probing this inaccessible era is to capture that neutral hydrogen signal.
But detecting it has proved to be one of the most difficult pursuits in astronomy and cosmology. The 21-centimeter signal was already weak when it was created at cosmic dawn. After traversing extreme distances and timescales to reach us, the tiny signal is all but drowned out by louder interference from galaxies, stars, nebulae, and radio-emitting gadgets on Earth.
The signal is up to a million times fainter than all of this nearby radio noise, according to Koopmans.
“All the energy ever collected by a radio telescope, such as LOFAR, does not exceed that of a snowflake falling on Earth,” he said. “The energy emitted by the neutral hydrogen signal is still 100,000 less than that.”
The signal that could illuminate everything
For the first billion years of its life, the universe was drastically different from the place we live in today. In the aftermath of the Big Bang, it was so hot and energetic that protons and electrons were not able to combine to form stable neutral atoms, so the universe was basically a super-heated soup of opaque subatomic particles.
Cosmic conditions had cooled down by about 378,000 years after the Big Bang, enabling the formation of neutral hydrogen and ushering in what is called the Era of Recombination. When atoms started to form during this period, the universe became more transparent, enabling light to freely travel without being scattered by random subatomic particles. This radiation, called the cosmic microwave background, is the oldest light ever detected in the universe.
As the universe transitioned from hot plasma to cold condensing gas, it plunged into the cosmic Dark Ages. Scientists think there are only two observable forms of light from this time: the cosmic microwave background and the much sought-after 21-centimeter signal.
“When you tune your car radio between stations on the FM dial, 99.7% of the static you hear is radio noise from relativistic electrons spiraling around magnetic fields in our galaxy and other nearby galaxies, 0.3% is from the afterglow of the Big Bang, and only 0.01% is from the 21-centimeter signal,” said Judd Bowman, an experimental cosmologist at Arizona State University, in an email.
The signal was originally created when electrons in neutral hydrogen atoms changed energy states, before and during the EoR. Photons absorbed or released by these tiny electron shifts initially had the characteristic 21-centimeter wavelength, but the expansion of the universe is expected to have elongated them to anywhere from two to 20 meters by the time they reach Earth.
Once the first stars began to shine, flooding the universe with much more energetic radiation, the neutral hydrogen atoms gradually became ionized, which means they were stripped of electrons. This marked the beginning of the Epoch of Reionization, when the light from stars and galaxies converted much of the universe’s neutral hydrogen into ionized hydrogen. Most of the hydrogen in the universe remains ionized to this day.
As light from these luminous sources sprang forth and neutral hydrogen diminished as it became ionized, the signal weakened over the course of the EoR.
“The signal is sensitive to the light that the very first generation of stars would have produced,” Fialkov explained. “We can learn about the process of reionization from it: how efficient the first galaxies were at ionizing the gas and how this efficiency varies with the mass of galaxies and halos in which they sit.”
The process of reionization played out over the course of several hundred million years, but was completed by the time the universe reached its one billionth birthday. Because the universe was engulfed in darkness before reionization, it is challenging to detect anything from the early part of the EoR that could provide clues about the structure of the universe at that time.
Scientists have managed to spot some of the oldest stars and galaxies in the universe, but it is not yet possible to glimpse these radiant objects at cosmic dawn. That’s why neutral hydrogen is such a valuable means of indirectly detecting the first generation of stars and galaxies—provided scientists can capture it.
“One of the highest priorities in astrophysics is to understand the properties and evolution of the first stars and galaxies,” Bowman said. “These are the objects that transformed the early universe, altering nearly every atom with their radiation and seeding the universe with the elements that would ultimately make up the Earth and all of us.”
The planet-wide race to detect the 21-centimeter signal
The notion that the neutral hydrogen signal could be used to study some of the earliest days of the universe has been around for decades, but it is only within the past 10 years or so that technology has started to catch up to that vision.
LOFAR, which was completed in 2012, has an enormous collecting area with small antennae spanning the Netherlands, Germany, the United Kingdom, France, Sweden, and Ireland. This huge geographic range allows the team to hone in on the signal by correcting for errors in the instrument or perturbations in Earth’s atmosphere, Koopmans said.
The Murchison Widefield Array (MWA), also completed in 2012, is smaller than LOFAR, but has the benefit of low radio interference due to its remote location in the Western Australian outback. The Experiment to Detect the Global EoR Signature (EDGES), an instrument at the same site as the MWA, has already produced “the most promising evidence for a 21-centimeter detection so far,” said Bowman, who led the research, which was published in a 2018 Nature paper.
The team is now waiting for other measurements to confirm their findings, Bowman said. The need for verification is especially relevant to the 2018 study because it was full of surprises that challenge existing models of the early universe. The discrepancies between the predicted signal and what was actually detected suggest that “either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected,” Bowman’s team said in the study.
“We don’t know how to explain it with the standard astrophysics that we know and love,” said Fialkov. “Exotic models have to be added to explain it and it still doesn’t look natural.”
Some of those models suggest that dark matter may have been responsible for the colder-than-expected temperatures detected at the break of cosmic dawn. “We’ve learned from the explanations proposed for the depth of the EDGES profile that cosmic dawn may hold the secret to unlocking the nature of dark matter,” said Bowman.
The allure of such a cosmological treasure trove has motivated teams to build observatories to search for the neutral hydrogen signal in the Northern Cape of South Africa, the mountains of Tibet, and Antarctica, among other sites. There are even a few proposals to launch space observatories to hunt for even older signals, either from orbit or on the far side of the Moon.
“Signals from the Dark Ages, which precedes the formation of first stars, would be really interesting to observe, but those signals cannot be observed from the ground because they are blocked by the ionosphere,” Fialkov said, referring to a layer of Earth’s atmosphere. “It acts as a mirror to the signals coming from space and they don’t penetrate and cannot be observed from Earth.”
“So going to space would open up this observational regime, and of course, going behind the Moon would also allow us to avoid radio frequency interference,” she added.
For now, the race for the first detection of neutral hydrogen continues planetside, as teams around the world scan the skies for this ancient relic using hyper-precise radio arrays. A few more tools are set to join the search, too.
EDGES-3, a next-generation version of the instrument that detected the best signal candidate, is expected to be operational in 2020, according to Bowman. Another specialized telescope called the Hydrogen Epoch of Reionization Array (HERA), based in South Africa, is also poised to collect data. The Owens Valley Long Wavelength Array in California, which has searched for the signal for years, is being upgraded to target the era that yielded the 2018 detection.
Bowman said that he is hopeful one of these projects will detect the signal within the next few years. “We have learned so much about how to make these measurements,” he said. “Now, it is a matter of putting the lessons learned into practice.”
Along the same lines, observatories such as MWA, LOFAR, or South Africa’s MeerKAT are also helping to inform the construction of the mother of all radio telescopes—the Square Kilometre Array (SKA).
This facility will consist of millions of radio antennae in South Africa and Australia that will form an intercontinental observatory that is 50 times more sensitive than any modern observatory. It is currently on track to be operational sometime in the late 2020s, and one of its biggest missions is probing the EoR.
“I think a detection itself will already be wonderful, on par with the detection of the cosmic microwave background (in fact more difficult!),” Koopmans said. “The wonderful thing with nature is that it always surprises us!”
Regardless of which team is the first to claim that milestone detection, this growing army of radio observatories will collaboratively build the broader picture of the universe’s transition from the dark ages to the modern era of starlight.
“I’m surprised and amazed at what we can do from the ground,” Fialkov said. “We are confined on Earth, but we can still look way back and understand how the very first stars formed.”
Correction: An earlier version of this article said that the Owens Valley Long Wavelength Array in California would begin to look for the 21-centimeter signal soon, and conflated the EDGES and MWA projects. In fact, the Owens Valley Long Wavelength Array has been searching for the signal for several years, and EDGES and MWA are different projects at the same site. The article has been updated to reflect this.