The Frontiers of Observational Astronomy and Japan’s Contributions
Masanori Iye
Professor Emeritus, National Astronomical Observatory of Japan
Professor Emeritus, The Graduate University for Advanced Studies (SOKENDAI)
Member, The Japan Academy
Today, I will talk about recent advances in observational astronomy, focusing in particular on the achievements of Japan’s Subaru Telescope and related projects. Since ancient times, humanity has gazed beyond the horizon and contemplated the vastness of the night sky. Every civilization has passed down its own view of the universe, which has served as a spiritual foundation for its people.
Evolution of Our View of the Universe
Modern astronomy began in the early seventeenth century, when Johannes Kepler discovered his three laws of planetary motion from observational data, and Isaac Newton later explained them through the law of universal gravitation. Around the same time, Galileo Galilei, using a telescope of his own making, recognized that the Milky Way is composed of innumerable stars. He also argued that the Earth is one of the planets orbiting the Sun, for which he was put on trial by the religious authorities.
In 1785, William Herschel proposed the “galactic universe” model, asserting that the very existence of the Milky Way indicates that we reside within a flattened, disk-like system—often likened to a “fried-egg” shape.
Then, about a century ago, in 1924, Edwin Hubble discovered that the Andromeda Nebula lies outside our Milky Way and is in fact another spiral galaxy, revealing that the universe is far larger than previously thought. From observations showing that all 24 spiral galaxies he studied were receding, Hubble further demonstrated in 1929 that the universe is expanding. Although astronomy was not eligible for the Nobel Prize at that time, Hubble’s discoveries were so revolutionary that they have often been said to be worthy of two Nobel Prizes, fundamentally transforming humanity’s view of the universe.
Considering the past state of the expanding universe discovered by Hubble, George Gamow arrived in 1947 at the idea that the universe began with a Big Bang in an extremely hot, dense state. Although this fiery Big Bang universe cooled rapidly as it expanded, Gamow predicted that its residual heat should still be observable today in the form of microwave radiation. Indeed, this remnant heat—known as the cosmic microwave background radiation—was discovered in the 1960s by Arno Penzias and his collaborators. In 1992, it was confirmed that this radiation exhibits faint fluctuations in its spatial distribution. Today, these tiny primordial irregularities are thought to have grown over time, eventually giving rise to the large-scale structure of the universe, including the present-day distribution of galaxies.
The Subaru Telescope
As astronomy advanced through the twentieth century in this way, Japan completed in 1960 a 188-centimeter-diameter telescope at the Okayama Observatory, built by the Tokyo Astronomical Observatory of the University of Tokyo. At the time, this telescope ranked as the fifth largest in the world. It is recorded that Yusuke Hagihara, then Director of the Tokyo Astronomical Observatory, emphasized its significance to the Emperor Showa during the annual Imperial Lecture (Kōshō Hajime). Because its construction took five years, it is said that, for the first time, a five-year national government obligation authorization was applied to secure its budget.
After entering graduate school, I conducted theoretical research on the mechanisms responsible for the formation of spiral structure in galaxies. In parallel with this theoretical work, I also carried out observations of spiral galaxies using this telescope. Observations at that time required manually adjusting the position of a glass photographic plate attached to the top of the telescope every minute during an hour-long exposure. When the exposure was finished and the plate was developed in a darkroom, the spiral galaxy would gradually emerge. I was deeply moved by the experience of being able to use such a large telescope. However, as dark skies disappeared within Japan, domestic observations could no longer compete with those conducted at leading observatories overseas.
Consequently, beginning in 1984, we began to develop an ambitious plan to construct the world’s largest telescope, with an 8-meter diameter, on the summit of Mauna Kea on the island of Hawai‘i, at an altitude of 4,200 meters. Although this became a major project with a construction cost of approximately 40 billion yen, it received formal endorsement from the Science Council of Japan and other bodies. In 1988, the Tokyo Astronomical Observatory became independent of the University of Tokyo and was reorganized as the National Astronomical Observatory of Japan, a national inter-university research institute. Construction of the telescope began in 1991. Because the Hawai‘i Observatory would be the first national research facility established overseas, it was also necessary to establish its legal and institutional framework. The telescope was named the Subaru Telescope, inspired by the famous phrase “Hoshi wa Subaru” (“Pleiades, most pronounced stars”) from one of the oldest Japanese literature written by a female about 1000 years ago (Makura no Sōshi).
I was responsible for the technical aspects of this project and considered how to achieve the world’s highest performance. The idea was to make the 8-meter mirror as thin and flexible as possible and support it with 261 robotic actuators, adjusting the force distribution every second to maintain the mirror in its ideal shape at all times. Although this was an unprecedented approach, the Subaru Telescope—constructed by bringing together the advanced technological capabilities of Japanese industry—was completed in 1999. For the inauguration ceremony, Her Imperial Highness Princess Sayako graciously visited the summit. Full-scale scientific observations with the Subaru Telescope began in 2000, and it has since produced a remarkably wide range of outstanding scientific results.
Probing Cosmic History by Observing the Most Distant Galaxies
In astronomy, we define the enormous distance that light travels in one year as one light-year. In 2006, using the Subaru Telescope, we discovered galaxies at distances of about 13 billion light-years—far beyond the 2-billion-light-year universe observed by Hubble—and held the world record for five years. Light from a galaxy 10 billion light-years away takes 10 billion years to reach the Earth. Just as archaeologists reconstruct Earth’s history from fossils preserved in ancient strata, astronomers explore the past history of the universe by looking ever farther into space. Although our record was surpassed in 2011, Japanese researchers have continued to update world records one after another since then.
In astronomy, it is now customary to solicit outstanding observing proposals from researchers around the world. In recent years, Japanese astronomers have achieved remarkable results in studies of cosmic history by building upon discoveries made with the Subaru Telescope, using facilities such as the ALMA radio telescope—jointly constructed by Japan, the United States, and Europe on the Atacama Plateau in the Andes—as well as a space telescope launched by NASA in 2021. These efforts have led to striking advances, particularly in our understanding of the early universe.
The most distant galaxy confirmed to date lies at a distance of about 13.4 billion light-years. This means that we are observing the universe as it was roughly 400 million years after its birth. Our understanding of the formation of the first stars and galaxies—the so-called “first light” of the universe—has advanced considerably.
Dark Matter and Dark Energy
A cosmological model that consistently explains both the observations of the cosmic microwave background radiation from about 380,000 years after the Big Bang and the observed distribution of galaxies roughly 10 billion years after the Big Bang, as studied with the Subaru Telescope and other facilities, has been widely accepted by researchers as the standard cosmological model. However, this standard model suggests that cosmic expansion began to accelerate against gravity about 7 billion years ago. If so, approximately two-thirds of the total energy density of the universe must consist of an unknown form of dark energy that drives this accelerated expansion. Furthermore, about one-quarter of the universe is composed of dark matter, which exerts gravitational influence but neither emits nor reflects light and is therefore invisible. As a result, the ordinary matter familiar to us accounts for only about 5 percent of the total content of the universe.
The greatest challenge in twenty-first-century cosmology is to elucidate the nature of dark matter and dark energy. Although their true identities remain entirely unknown, research is actively underway to investigate the distribution of dark matter through its gravitational effects, making full use of the exceptional capabilities of the Subaru Telescope.
Planets Beyond the Solar System
Alongside progress in studies of the universe as a whole, another major highlight of recent research is the exploration of planets. With regard to observations of Saturn, Jupiter, and their moons within our solar system, Japan’s space agency, JAXA, has launched numerous spacecraft. Missions such as Hayabusa2, which successfully returned grains of sand from an asteroid, are steadily advancing, and our detailed understanding of the solar system is deepening day by day. Many of you have probably seen images from these missions in newspapers and on television.
What I would like to focus on here, however, is the study of planets beyond our solar system. Since the first confirmation of a planet outside the solar system in 1995, more than 6,000 such exoplanets have been identified in just 30 years. In a universe this vast, few astronomers believe that life emerged and civilization developed only on Earth. There must be many planets in the universe that harbor life.
To enable concrete discoveries, we developed what might be called “smart glasses” for the Subaru Telescope. The Subaru Telescope itself already has a “smart mirror” that adjusts its shape every second to achieve excellent image quality. However, atmospheric turbulence above the telescope blurs the images, making them about ten times less sharp than the theoretical resolution would allow. By attaching an additional set of compact, rapidly adjustable “smart glasses” and correcting the distortions at a rate of 1,000 times per second, it becomes possible to suppress the twinkling of starlight and achieve a tenfold improvement in image sharpness. With the practical implementation of this technology, the Subaru Telescope has succeeded in directly imaging several planets outside our solar system. By dispersing the faint light from such planets into a spectrum using a diffraction grating, we should be able to determine whether their atmospheres contain molecules such as oxygen or methane—potential indicators of life.
However, dividing such extremely faint light into a detailed spectrum pushes even the Subaru Telescope beyond its limits. For this reason, Japan, the United States, Canada, and India are jointly pursuing plans to construct the Thirty Meter Telescope (TMT), an extremely large telescope with a 30-meter-diameter mirror, adjacent to the Subaru Telescope. Although this lies more than a decade in the future, once next-generation telescopes such as the TMT come into operation, they should enable the discovery of planets that host life and even allow direct tests of the accelerated expansion of the universe. Some of the profound mysteries discussed today may then finally find their answers.
Human Civilization
Finally, I would like to reflect on our view of the universe and its relationship to human civilization.
According to Big Bang cosmology, hydrogen and helium were formed during the first three minutes of the universe. In the rapidly cooling early universe, however, there was no time to create heavier elements. All atoms heavier than hydrogen—such as carbon, oxygen, and iron—are products of nuclear fusion reactions that took place later inside countless stars.
If we could interview a carbon atom in the protein of one of our hairs and ask, “Where did you come from?”, it might answer: “In fact, I was born inside a nearby star about eight billion years ago. When that star reached the end of its life and broke apart, by a fortunate chain of events I now find myself serving as part of your hair.” In other words, without stars, we would not exist. In this sense, we are truly “children of the stars,” or even “citizens of the universe.”
Human civilization has advanced at a remarkable pace over the past century. The invention of the airplane in 1902, the first radio broadcast in 1920, and the discovery of the expansion of the universe in 1929 all occurred about 100 years ago. In Japan, average life expectancy has doubled over this same period. Although the future cannot be predicted, we must ask whether humanity, living in an era marked by intensifying ethnic conflicts and struggles for dominance, can overcome the risks of nuclear war and environmental destruction and mature into a sustainable civilization.
Given the momentum of progress in observational astronomy, it is likely that planets showing signs of life will be discovered in the near future. It is even possible that we may find planets that exhibit signals of civilization that cannot be explained by natural phenomena. We are on the threshold of such an era.
Even if there were, hypothetically, 100,000 civilizations more advanced than Earth’s within our Milky Way galaxy, the average distance to the nearest one would be about 1,000 light-years. That is so far that even light would take 1,000 years to travel. If the stable lifetime of modern human civilization is less than 1,000 years, then even if we were to detect a neighboring civilization, humanity might destroy itself before a reply to a simple greeting could ever return. The universe is that vast and immense.
From the perspective of a 13.8-billion-year cosmic history, a 4.6-billion-year history of Earth, a 10,000-year history of human civilization, and the mere 100 years of modern civilization that has acquired the power to damage the global environment, I believe it is essential for all of us to adopt a broad, long-term perspective. By overcoming division and conflict, and by thinking in terms of 100-year and 1,000-year timescales, we can strive together toward a future in which human civilization is sustained and endures.