How Were the Universe's First Stars Born, and How Can We Observe Them? Hayato Shimabukuro on the Cosmic Dark Ages and the 21cm Line

According to Shimabukuro, the universe is thought to have begun with a hot, dense, high-pressure Big Bang, and as it expanded, there followed an era in which no stars or galaxies existed. With no light-emitting objects, this pitch-dark era is called the cosmic dark ages. What existed during this period is thought to have been the hydrogen and helium produced in the Big Bang, along with dark matter.

Shimabukuro says the most important of these is hydrogen. Dark matter is also extremely important for the formation of cosmic structure, but from the standpoint of observing with electromagnetic waves, hydrogen will be the most promising tool going forward.

Looked at another way, the absence of stars and galaxies is also an advantage, Shimabukuro says. Because there is no need to account for the effects of stars and galaxies, this era offers a setting for probing purely cosmological properties, such as the structure and evolution of the universe. For example, it is hoped that it could yield information about dark matter and about inflation, which is thought to have occurred at the very beginning of the universe. Even if humans had existed in this era, there would have been no objects emitting visible light, so they would have seen nothing but a pitch-black universe, Shimabukuro said.

Dark matter plays an important role in thinking about how the first stars and galaxies formed. Because dark matter does not emit electromagnetic waves, it cannot be seen with light, but it does have gravitational properties.

Shimabukuro cited measurements of galaxy rotation speeds as what first pointed to its existence. Newtonian gravity predicted that rotation speeds would decrease with distance from a galaxy's center. But actual observations showed that rotation speeds barely dropped even in the outer regions. Explaining this requires some unknown matter beyond the visible matter of stars and galaxies. This is how the existence of dark matter came to be indicated.

Its true nature is still unknown, but there are several hypotheses: supersymmetric particles and axions predicted by particle physics, as well as very small primordial black holes thought to have existed in the early universe. Shimabukuro personally favors axions, though not for any particularly strong reason—the explanation was that Shimabukuro has written several papers on axions and so feels an attachment to them.

When dark matter, an unknown particle, clumps together, it forms structures called dark matter halos. Shimabukuro describes these as the "cradles" of stars and galaxies, explaining that the universe's first stars and galaxies are thought to have formed inside them.

A dark matter halo is a kind of "gravitational well," into which gas such as hydrogen and helium collects. The gas is compressed by its own gravity and shrinks, eventually leading to the formation of the first stars.

Here, Shimabukuro clarified the terminology. "First stars" does not refer to one specific star but is a collective term for stars formed from hydrogen and helium. Among specialists, it is more common to call them "Population III" stars, or "Pop III" for short, rather than "first stars."

Why "III"? There are also Population I (Pop I) and Population II (Pop II) stars, and the difference lies in the amount of heavy elements. Astronomy uses a rough classification in which all elements other than hydrogen and helium are lumped together as heavy elements. Stars like the Sun, which contain large amounts of heavy elements, are Population I, while stars that contain only hydrogen and helium and no heavy elements are Population III. Population II lies in between: these stars contain far fewer heavy elements than the Sun, but more than Population III.

Heavy elements matter because they are closely tied to how stars form, Shimabukuro explains. A star is born when gas collapses under its own gravity and, once it has collapsed far enough, nuclear fusion ignites in its interior and it begins to shine on its own. But gas does not collapse easily. Gas has pressure, which pushes back against gravity.

Pressure increases as the gas gets hotter, so for the gas to collapse, it needs to cool. This is where heavy elements come in. Heavy elements act as coolants, drawing energy out of the gas and lowering its temperature. The more heavy elements there are, the more easily the gas cools and the easier it is to form stars.

The material of the first stars, however, contained no heavy elements. Because the gas cools poorly and remains under strong pressure, the gas itself must be fairly massive to overcome that pressure and collapse. From this reasoning, the first stars are expected to have been fairly massive.

Shimabukuro added, however, that theoretical simulation studies suggest that, depending on conditions, relatively light first stars could also form. This debate has not yet been settled, and first stars of a wide range of masses, from light to heavy, may have formed.

More massive stars tend to have shorter lifespans. According to Shimabukuro, the Sun's lifespan is thought to be about 10 billion years, but first stars could be 10 to 100 times as massive as the Sun, and their lifespans are expected to be on the order of tens of millions of years down to a million years. For this reason, it is considered unlikely that any first stars have survived to the present-day universe.

On the other hand, if light first stars were born and have survived without exploding or the like, there may be a chance to observe them, Shimabukuro says. In that case, the fundamental prerequisite—and the most important check—would be to analyze the star's signal and confirm that it contains no heavy elements other than hydrogen and helium. There are other items to check as well, Shimabukuro noted.

Whether the first stars were born alone or alongside siblings is also a question of endless interest, Shimabukuro says. Because there is currently no Sun-like star around the Sun, the Sun is thought to have been born as an only child, but in star formation theory, being born as part of a binary is considered entirely ordinary.

It has also been suggested that the first stars may have been born as binaries. But this too is a view based on simulations, and since some results show them forming singly and others show them forming as binaries, no conclusion that everyone accepts has yet been reached. It remains a field of active debate.

Shimabukuro personally thinks it would be more interesting if they were born as binaries. If first stars orbiting each other exploded and died, becoming black holes, those black holes orbiting each other would produce gravitational waves. Gravitational waves are waves of gravity that propagate through spacetime, and if gravitational waves were produced by binary black holes originating from first stars, it might be possible to observe them. It would be interesting if something like that had happened, in Shimabukuro's view.

The birth of the first stars transformed the universe. Shimabukuro says the biggest change was, first of all, that light came on in the universe.

Furthermore, the first stars, shining on their own, emitted ultraviolet radiation that changed the state of the surrounding hydrogen. After the first stars died, new stars formed, and the first generation of galaxies containing them—the first galaxies—was born. Ultraviolet light from these early galaxies ionized the neutral hydrogen gas in the universe. This phenomenon is called cosmic reionization, and Shimabukuro describes it as an event that holds a great many mysteries.

The reason for the "re" is as follows. Right after the Big Bang, protons and electrons moved about separately because of the high temperature. Later, as the universe cooled, the two combined to form neutral hydrogen. During cosmic reionization, protons and electrons are separated once again, which is why it is called "re"ionization.

The general view is that reionization was driven by the first stars and first galaxies, but there is also the idea that objects called quasars contributed. On a more exotic note, some researchers consider scenarios in which dark matter contributed to reionization, Shimabukuro said.

During the cosmic dark ages, when there were no stars or galaxies, there is nothing of that kind to observe directly. But because the universe contains vast amounts of hydrogen, it is thought that observing signals from hydrogen would make it possible to observe even the dark ages. That signal is the 21cm line, and Shimabukuro says it is anticipated to be the only signal capable of observing the cosmic dark ages.

Neutral hydrogen consists of a proton and an electron, both of which have a physical quantity called spin. Shimabukuro likens this to an arrow. When the spins of the proton and electron are parallel, the energy is slightly higher; when they are antiparallel, it is slightly lower. Nature prefers lower-energy states, so the atom tries to move from the higher state to the lower one, shedding the leftover energy in the process. This shed energy is an electromagnetic wave with a wavelength of 21cm, and the name comes simply from that wavelength.

However, because the universe has been expanding ever since the Big Bang, the wavelength of the emitted 21cm line gets stretched. How much it stretches depends on the era in which it was emitted, and 21cm line emission from the epoch of reionization and the cosmic dawn is thought to measure several meters when observed today.

The 21cm line is thought to exist in every era—the epoch of reionization, the cosmic dawn, and the cosmic dark ages—but at present, the 21cm line from these eras has not been observed. Shimabukuro cautioned that the 21cm line from neutral hydrogen within the Milky Way has already been observed; what has not been observed is the signal from the early universe.

The biggest difficulty cited was foreground emission. Shimabukuro compared it to driving a car with the sun shining from behind a traffic light, so that the glare makes the signal impossible to see. The Milky Way shines very brightly with synchrotron radiation, and the 21cm signal lies behind that bright foreground. This bright light must first be removed, and the 21cm line found within what remains.

Because the 21cm line is a radio wave, observing it requires a radio telescope. There are radio telescopes resembling the dishes used at home to receive satellite broadcasts, and there are radio interferometers, which arrange many dipole antennas shaped like Christmas trees and combine them so they function as a single antenna.

As telescopes currently operating with the goal of observing the 21cm line, Shimabukuro named MWA in Australia, LOFAR in the Netherlands, uGMRT in India, and HERA in South Africa. Japan has no telescope for 21cm line observations. Radio interferometers require large numbers of antennas, which is costly, and they must be built in radio-quiet locations, so it is difficult for a single country to pursue one alone, and they sometimes become international projects.

21cm line observations aim at two types of signals. One is fluctuations in the signal, which seek to capture how the strength of the 21cm line varies from place to place in space; this is being attempted with radio interferometers. The other is the global signal, which examines the average strength of the signal; this is being attempted with relatively simple antennas shaped something like parabolic antennas.

In 2018, a group called EDGES published a paper reporting that it may have detected the global signal. Shimabukuro, who was doing research at the Paris Observatory at the time, read through it with excitement, thinking it might be a very exciting paper. But while reading, Shimabukuro sensed that something was "off." The reported signal was far stronger than theoretical predictions. This meant a signal had been detected that could not be explained by existing standard theories of physics and astrophysics, and many researchers, including Shimabukuro, were perplexed.

Meanwhile, the India-led SARAS 3 had also been observing with the aim of detecting the global signal, but it detected no signal and was unable to corroborate the EDGES result. This situation continues today: the EDGES result claiming a detection and the SARAS 3 result reporting no detection coexist, and the current state of affairs is that verification by a third party is awaited, Shimabukuro explained.

Personally, Shimabukuro says that if the EDGES result is real, it would be tremendously interesting because it cannot be explained by standard theory, but considers it more realistic, if conservative, to think there was an error in something such as the assessment of observational uncertainties.

Radio interferometers involve large-scale observations using many antennas. The planned SKA is a gigantic telescope that will attempt to detect the 21cm signal with about 130,000 antennas, and because no single country could fund it, it is being built with countries, centered on Europe, pooling their resources. SKA comprises SKA-Low and SKA-Mid, which differ in the frequencies they observe, and the one used for 21cm line observations is SKA-Low, located in Australia.

Shimabukuro serves as head of a science working group in the SKA Japan group. SKA's goals are not limited to the 21cm line; they range widely, from how cosmic magnetic fields originated, to the nature of dark matter and dark energy, and even the detection of signals from extraterrestrial life. The Japan group operates by having researchers interested in each goal come together to form teams.

Full completion is expected in the 2030s, but rather than waiting for completion, observations will be carried out in stages as antennas are added, and the resulting data will be verified. The first verification is expected around 2027, and Shimabukuro said initial observations are scheduled to begin the year after 2026, when viewers are watching. Once completed, it is expected to far outperform current instruments such as MWA, LOFAR, HERA, and uGMRT, becoming the most powerful telescope for 21cm line observations.

On one of SKA's goals, the detection of signals from extraterrestrial life, Shimabukuro explained as follows. The expectation is that an advanced civilization, like humans on Earth, might communicate using radio waves, and SKA might be able to pick them up. While noting that it is an optimistic estimate, Shimabukuro said a paper has been published suggesting that if extraterrestrial life existed relatively nearby—a few light-years to a few dozen light-years from Earth—and used radar like that used for aircraft, SKA might be able to observe a signal of that level.

What would Shimabukuro do if they personally detected such radio waves? Shimabukuro answered that the first step would be to doubt it and consider whether there could be an explanation that does not require extraterrestrial life. They would discuss it with many researchers, and even if it turned out to truly be radio waves from extraterrestrial life, announcing it suddenly might cause confusion, so they would proceed carefully.

It has been proposed that SKA could also be used to observe gravitational waves. According to Shimabukuro, gravitational waves come in a variety of frequencies and wavelengths, and detection methods using SKA can contribute in frequency bands different from conventional gravitational wave observations. Gravitational waves are thought to be emitted not only when ordinary-sized black holes orbit each other, but also when large objects such as galaxies merge, or when the supermassive black holes at the centers of galaxies collide. Because the frequency of gravitational waves differs depending on the mechanism, detecting gravitational waves at a variety of frequencies is important for investigating different physical mechanisms.

SKA has a weakness, too: it observes from Earth. On Earth, many people rely on radio waves in daily life, and that artificial radio emission interferes with 21cm line observations. In addition, Earth has an ionosphere, which deflects the 21cm line coming from space.

To avoid these problems, one option is to leave Earth to observe, and the top candidate is the far side of the Moon, Shimabukuro says. Artificial radio waves are extremely strong—even smartphone signals reach the Moon—but radio waves from Earth do not reach the Moon's far side. Moreover, the Moon has no atmosphere and therefore no ionosphere. It is an extremely radio-quiet environment, well suited to observing the 21cm line.

Several countries, including the United States and India, have proposed plans to build radio telescopes on the far side of the Moon. China is planning to send satellites into lunar orbit and arrange telescopes around the Moon to make observations, with launches scheduled for around 2028 to 2029, so it will likely be carried out within the next few years. In Japan, too, the Tsukuyomi project is underway, led by JAXA.

Looking ahead, Shimabukuro said that since construction of SKA is already underway, it will be completed and begin observing within a few years, and there is no doubt that radio astronomy with SKA will flourish in the 2030s. Because radio astronomy as a whole, not just the 21cm line, will advance greatly, young students who enter the field will be able to do research in an era when the course of astronomy changes significantly. Shimabukuro personally thinks that in the 2040s and 2050s, radio astronomy on the lunar surface will become the next goal, bringing another major turning point.

At the same time, observing the 21cm line is no simple matter. In addition to powerful instruments like SKA and lunar telescopes, it is also an arena that tests human ingenuity and resourcefulness, including the techniques for analyzing the signal. Shimabukuro said it would be extremely exciting to actually carry out 21cm line observations within their own research career and give the resulting knowledge back to society. No one has yet captured the 21cm line from the cosmic dark ages, and the validity of the EDGES result is still awaiting third-party verification.