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A new key to solving the mysteries of the universe
What exactly happened at the beginning of the universe, that is, at the very moment when the universe was born? That is one of the great mysteries that science has yet to solve. But how can we find out without a time machine?
“We can “watch” it using gravitational waves.”
So says Professor Masaki Ando, whose research focuses on gravitational wave observation.
“The earliest image of the universe humans can see today is the cosmic microwave background radiation. This is electromagnetic radiation arriving at Earth from all directions, dating back to 13.8 billion years ago, or approximately 380,000 years after the birth of our universe. It is, in principle, impossible to observe electromagnetic waves from an earlier time. This is because the early universe was a high-energy fire ball, so light could not pass through straightly without getting scattered, similarly to how the Sun becomes "invisible" on a cloudy day as the light get scattered. The same happens when we observe the early universe: we cannot see electromagnetic waves older than 380,000 years after its birth. However, gravitational waves, which have completely different properties from electromagnetic waves, can pass through such conditions. Therefore, we can directly observe signals that were emitted when the universe was born.”
According to Ando, it is possible to observe the universe even as early as 10-24 seconds after its birth.
“According to the cosmological inflation theory, the universe expanded rapidly right after its birth. However, there are more than one hundred different theoretical models proposed. We still do not know which one is correct. We may be able to find out by observing gravitational waves. It is also believed that the Big Bang was a result of the inflation's energy converting into heat after the inflation ended. However, we do not yet know precisely how the inflation ended, or how the Big Bang began. By observing gravitational waves, we may be able to find out this as well.”
But the beginning of the universe is not the only mystery gravitational waves might help solve.
“Until recently, astronomers have been observing the universe mainly by using electromagnetic waves. Galileo, for example, used visible light for his telescope. As time went on, observational capabilities expanded to various wavelengths, such as, X-rays and gamma rays for short wavelengths and radio waves for long wavelengths. However, gravitational waves are completely different from electromagnetic waves and can travel through anything. Therefore, they can be a means of directly observing what is happening at the center of high-energy phenomena such as the center of a black hole, a supernova explosion, or the merger of a binary star (two stars orbiting around a common center of gravity). In other words, they could be the new key to solving the mysteries of the universe.”
These are exciting prospects indeed.
So, how can we observe these gravitational waves? Is there such a thing as a gravitational wave telescope? But first, we have to clarify what gravitational waves are.
Gravitational waves predicted by Einstein
Newton's “discovery” of gravity helped us understand not only how the planets were orbiting the Sun but also why things on Earth seemed to be stuck to it. The true nature of gravity, however, remained unknown for a long time until Einstein found the answer to this mystery, proposing that gravity is the curvature of space-time created by a massive object. Ando continues.
“There is an often-used analogy of a trampoline and a bowling ball. If a bowling ball is placed on a trampoline, it will distort the fabric in the middle. If you place a small marble near that distortion, the marble will roll toward the bowling ball. That is what we call gravity. The distortion in the trampoline’s fabric represents the curvature of space-time. Let us imagine that the bowling ball is the Sun, and the marble is Earth. The Sun “distorts” the surrounding space-time, with Earth in motion around the Sun following the curvature created. This is gravity as depicted by Einstein's theory of general relativity.”
What are gravitational waves then?
“Gravitational waves, also predicted by Einstein's general theory of relativity, are the wave-like transmissions of these “distortions” caused by the motion of an object in space-time. When the bowling ball on the trampoline moves, you can see the way the oscillations in its wake get transmitted throughout the trampoline. When a massive event occurs somewhere in the universe, such as the merger of two black holes, space-time is greatly distorted, and the distortion is transmitted to Earth as gravitational waves. This is what we observe with gravitational wave detectors. But detecting gravitational waves is incredibly difficult.”
This is because gravitational waves are the result of massive events but are astronomically small: the distance between the Sun and Earth changes by only one hydrogen atom. The difference is so small you can only imagine it with great difficulty. How can we detect such a slight change?
“By using lasers and mirrors,” says Ando.
KAGRA, Japan's gravitational wave detector
Researchers worldwide have been hard at work to detect gravitational waves for a long time, and various detectors have been proposed. The first gravitational wave detector in Japan was called TAMA300 and it began operating in 1999 at the National Astronomical Observatory of Japan in Mitaka, Tokyo. Unfortunately, this first-generation detector has not been successful in detecting gravitational waves.
Currently, three institutions worldwide have been operating second generation of gravitational wave detectors. LIGO, undergoing a major upgrade in 2015, has been in operation at two locations in the U.S. Advanced Virgo in Italy began operating in 2016. Finally, KAGRA, run by the Institute for Cosmic Ray Research at the University of Tokyo with domestic and international collaborators, was completed in 2019. KAGRA is located approximately 200 meters underground at/in the Kamioka Mine in Hida City, Gifu Prefecture. Ando has been closely involved in the construction since the project's inception and was a key member to its detector design.
Let us take KAGRA as an example to see how a gravitational wave detector works.
Although KAGRA is considered a “telescope,” it is different from its optical counterparts, which operate using lenses. Instead, it operates with a device called a laser interferometer-type detector, which is an L-shaped intersection of two three-kilometer-long vacuum tunnels (arms) through which laser light travels.
“Where the arms intersect, there is a beamsplitter, a device that allows half of the light to pass through and half to be reflected. The emitted laser beam is thus split into two halves, each of which travels to and gets reflected from a sapphire mirror (end mirror), set up at an equal distance from the beamsplitter. Then, a photodetector converts the laser into an electrical signal. If the two separate laser beams travel the same distance back and forth, when they merge at the photodetector, the peaks and troughs of the light waves cancel each other out, resulting in no laser power at the photodetector. If there is a difference in distance, the peaks and troughs of the light waves will not cancel each other perfectly, resulting in a power leakage interpreted as a signal. This is the principle used to detect gravitational waves.”
To review: if a gravitational wave reaches where KAGRA is located, it will distort the space-time in its vicinity. That means that one arm will contract, and the other arm will extend. Then the distance traveled by the laser light, which is split in two directions, will be different. As a result, the two beams of light received by the photodetector interfere with each other, and a gravitational wave is detected. This sounds simple, but the device itself is highly sophisticated and extremely complex.
“My job was to create the basic design of the laser interferometer (the entire system, including the arms). I had to choose specifications of mirrors to use and their positions for optimal sensitivity. I also had to work on the design of the anti-vibration system which helps to avoid the effects of heat and vibration. I was still in my early thirties when I started working on this project.”
What makes KAGRA different from LIGO and Virgo is that the detector is installed underground.
“It is installed underground to reduce the influence of vibrations in the grounds. For the same reason, the mirrors are suspended from above in a pendulum-like contraption to prevent it from swinging. Another source of “noise,” or unwanted signal, is heat. Thermal vibration, arbitrary vibration at the molecular level, causes errors. To prevent this, KAGRA is equipped with a device that cools the mirrors to approximately -253 Celsius."
KAGRA started operating in 2020 but has not yet captured gravitational waves.
Planning to launch a gravitational wave detector… in space
“The world's first gravitational wave detection was achieved by the American LIGO project in 2015. It was a gravitational wave coming from the merger of two black holes in a binary star system.
“Currently, KAGRA, LIGO, and Virgo share observational data and cooperate. So, if LIGO or Virgo detects a gravitational wave signal, it will be easier for KAGRA to find that signal even if it is buried in noise. Gaining such statistical insight will help us find the clues of a true positive gravitational wave signal. So, perhaps during the observational period this (2025) summer or fall, we may be able to witness the historic moment of KAGRA’s first gravitational wave detection.”
Thus, while Ando looks forward to KAGRA’s first successful gravitational wave detection, a project he has poured his heart and soul into, he has also started working on an even bigger project: the launch of a gravitational wave detector… in space.
“I think KAGRA is not powerful enough to observe the universe at 10-24 seconds after its birth. On Earth, there is a lot of noise, such as ground vibrations, which limits our ability to observe in the low-frequency range. There is no ground vibration in space. Thus, the arm length can be significantly longer than on the ground, which dramatically increases sensitivity. The arms of DECIGO, a new project we are proposing, would be 1,000 km long.”
1,000 km is equivalent to the beeline between Tokyo and Kumamoto in Kyushu Prefecture. How can such long arms be built? Actual tunnels like those of KAGRA are not needed in the vacuum and zero gravity of space. Three satellites equipped with mirrors would be placed in precise orbits, and laser beams would be fired at the mirrors to make observations.
“We are now aiming to launch a gravitational wave detector with an arm length of 100 km, one step ahead of DECIGO. Our goal is sometime in the 2030s, with DECIGO following in the 2040s or 2050s. Other countries are also planning to launch gravitational wave detectors in space. However, currently, DECIGO is almost the only mission with the sensitivity to observe the birth of the universe. By that time, I will probably be too old to be involved in the mission but I hope to be part of the first steps. Launching this satellite in a collective dream, which I am sure will one day become reality.”
Young researchers, be bold!
Ando is also working on the development of a gravitational wave detector based on a different concept from laser interferometry. The device is called TOBA, or a “torsion-bar antenna.”
“There are two bars that detect gravitational waves called test masses. They are sensitive to the twisting motion caused by the tidal force caused by gravitational waves. The advantage is that it can be relatively compact and still provide good sensitivity, but it requires a very precise laser measurement of the angle between the bars.”
The current prototype has a height of 2 meters and an area of about 3.5 square meters. Ando is considering using TOBA for detecting phenomena other than gravitational waves, for example, earthquakes.
“At the moment of an earthquake, extreme movements are occurring along fault lines. While these movements do not emit gravitational waves, they do change the gravitational field, and those changes get transmitted at the speed of light. If we could capture these signals with TOBA, we would be able to send out alerts earlier than we can now. Ten extra seconds would allow us to stop a car, a surgery, or an elevator. Ten seconds could make a difference.”
In addition to being a world-class researcher in gravitational wave detection, Ando is also a pioneer in quantum optics, a field in which he has been involved since his undergraduate days doing laser research.
“When we use lasers to measure things with ever more precision, we run into a limit posed by quantum fluctuations, which we can overcome by clever design. One of the possible methods is to use quantum optomechanics. Controlling quantum noise in quantum computers is a very active research area. These are the current frontiers of optics. To me, optics itself is a fascinating research subject along with the actual detection of gravitational waves.”
Ando decided to pursue physics when he was a high school senior. He says he loved and was influenced by science fiction novels.
“It was a book titled “Contact” written by Carl Sagan, which was later made into a movie starring Jodie Foster. In the novel's finale, a mysterious radio signal from outer space is finally decoded and turns out to be the pi. Perhaps this led me to start thinking about the fundamental laws that govern the universe even as a high school student (laughs). I think that is how I got started,” Ando said nostalgically and happily.
It is very similar to the image of gravitational waves coming from the edge of the universe, is it not?
His answer to a question about the joys of being a researcher is reflective of him as a person as well.
“Participating in the expansion of humanity’s boundaries. What I am doing may be very small, but I am contributing to the progress of human knowledge, and it is very exciting to see such a connection. Even if not many people read my paper, the fact that I wrote it will remain, and our knowledge will be advanced by the accumulation of such research, however small. I think that is the joy of doing research in the natural sciences.”
Ando hopes that young people will live a life full of such joys.
“I hope that instead of going down a set route, young people will discover the right path for themselves. I think young people should strive to be as bold as they can be.”
※Year of interview:2025
Interview/Text: OTA Minoru
Photography: KAIZUKA Junichi
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