Q. Tell us about your specialty and what you are finding with your research?
My specialty is particle physics. The aim is to study fundamental laws - what the elementary subatomic constituents of matter are, and how they work - by looking closely at matter. Thanks to great advancements in particle physics in the 20th century, the basic structure of matter is pretty well understood: matter is made up of atoms, and in the center of an atom is an atomic nucleus with electrons orbiting around it. An atomic nucleus consists of protons and neutrons, and they are made of even smaller particles, called quarks. To give you an idea of how small a quark is, an atom is one hundred millionth of a centimeter, and a quark is at least a billion times smaller than that. So we know the matter around us at an ultramicroscopic level.
When it comes to the universe, however, there is still so much to uncover. I came to realize that research on elementary particles has a lot to do with the universe in many different ways. For instance, if you look back at the history of the universe, there was a time when there were no stars or galaxies but only matter - dark matter and atoms. And if you look further back, nuclei and electrons were moving around separately. As you go back even further, the universe becomes the world of smaller particles. So I’m basically studying these particles in order to understand the universe.
Q. What is the research you are working on now?
Distribution of matter (mainly dark matter) in the region called the COSMOS field. (courtesy: NASA, ESA, P. Simon (University of Bonn) and T. Schrabback (Leiden Observatory))
What I’m most interested in now is dark matter and dark energy. In my career as a physicist, I have often conducted theoretical research, so I have my own theory to explain what dark matter could be. To test this theory, I work with others to design experiments, I talk with mathematicians about calculations that may help, and I consult astronomers about what and how to observe. Working this way, I would like to confirm the validity of my theory.
Dark energy is very mysterious. It just keeps being generated as the universe expands. From the point of view of the law of gravity, there is one longstanding theory that the universe would stop expanding when it got to a certain point, and then it would shrink. It is just like when you throw a ball up in the air: because of the pull of the Earth’s gravity, the speed of the ball’s ascent slows, it stops in the air, and then it falls back down. But recent observation has shown that the expansion of the universe, which had been once slowing down, is now accelerating. There is an increase in some sort of energy that is slowing down and speeding up the universe’s expansion. And, this is called dark energy. So I want to study the history of the expansion of the universe in detail in order to address the mystery of dark energy and investigate the future of the universe. I think we’ll be able to predict the future of the universe - whether it will continue to grow eternally or whether there will be an end - by looking into the history of its expansion. We are now in the planning stage of a project to study the history of the expansion of the universe in much greater detail than ever before, using telescopes.
Q. Dark matter is invisible, so how was it detected? And how much do we know about it now?
The Virgo Cluster. In the 1930s, the Swiss-born American astronomer Fritz Zwicky studied the motion of the galaxies in the Virgo Cluster, and suggested the existence of dark matter. (courtesy: Günter Kerschhuber (Gahberg Observatory))
The existence of dark matter became known through observations of a galaxy cluster in the 1930s. A cluster of galaxies comprises many galaxies, each of them moving very fast. Yet somehow they all stay together without spinning out of the group. For the cluster to stay together, the galaxies need to pull at each other strongly enough with their own gravity. However, the amount of gravity that would exist in a galaxy based on the visible stars is not sufficient to maintain the cluster. Therefore, scientists made the assumption that there is also another type of matter, which is invisible and heavy.
Further evidence of the existence of dark matter came through observations of spiral galaxies. Based on the laws of gravity, scientists had predicted that the rotation velocity of the center of a spiral galaxy would be greater, and that the outer edges would be rotating more slowly. However, investigation has revealed that the rotation velocity actually remains about the same. In order to maintain such a galactic rotation, there must be matter that encircles the outer part of the galaxy and exerts gravity on it. And this is dark matter, which does not emit light but has great mass.
The total mass of dark matter in the universe is about five times that of currently observable conventional matter, and it is becoming evident that dark matter is the source of the structure in the current universe. Dark matter was created in the huge explosion called the Big Bang. Then dark matter started to cluster because of its gravitational force and pulled in regular atoms to form stars, which later developed into galaxies. Stars are the source of life. Had it not been for dark matter causing the formation of stars, we would not exist today. Also, dark matter allows our solar system to stay in our galaxy despite the fact that the solar system is moving at 220 km per second. The gravity of dark matter anchors the solar system firmly in its current position, and prevents it from spinning away beyond the galaxy. It is also thought that a great quantity of dark matter exists around us, even running through our bodies.
Q. How is the beginning of the universe perceived today?
All-sky map of light emitted from the universe 380,000 years after its birth. Red indicates high temperature and blue indicates low temperature. (courtesy: NASA / WMAP Science Team)
Today, the universe is 13.7 billion years old and flat. But when it began with the Big Bang, it is thought to have been very small and wrinkled - this is when the universe had just been created, at the age of a trillionth of a hundred trillionth of a second. The theory called inflation explains that this wrinkly universe was then stretched flat, as if it had been ironed. And then it started expanding. In the early stages, all parts of the universe were even - there was no gathering of anything anywhere in the universe. And what happened later is as I explained earlier: dark matter gradually began to cluster, its gravity initiating star formation.
We also know that the universe used to be hot, not cold as it is today. This was supported by radio telescope observations that measured light emitted from the universe when it was only 380,000 years old. It takes time for light to reach us, and therefore, the farther away the light comes from, the younger the universe we are looking at. Based on this principle, by analyzing the light of the universe at the age of 380,000 years, scientists discovered that at that time the universe was hot. Light from the universe prior to the age of 380,000 years is undetectable as light or radio waves, so we are not certain about the condition of the universe before that. If we can understand dark matter, it would be possible to learn about the universe at an even younger age.
Q. What made you interested in particle physics?
The sky looks blue because of the interaction of sunlight with atoms in the atmosphere.
Particles are the smallest and the most fundamental of all matter. I’ve had a desire to know and understand things at a fundamental level since I was a child, and this is how I ended up with particles. You ask questions about many things as a child, and all these wonders can be explained with the study of particles.
For example, why is the sky blue? It has something to do with the interaction between sunlight and atoms, such as atoms of oxygen, in the atmosphere. Sunlight contains a mixture of light of various colors, from blue light to red light, and the color of light differs depending on the wavelength. Blue light has a short wavelength and red light has a long wavelength. Blue light, with a short wavelength, has a higher chance of bouncing off atoms in the atmosphere and scatters in all directions. The reason the sky looks blue is that blue light bounces and scatters in the atmosphere. So when you pursue a question from your childhood, such as "Why is the sky blue?", you face the fundamental questions about what things are made of and how they work. And when one question is solved, you say, "Then, what about this?" And you keep looking deeper and deeper. And the next thing I knew, I was deeply involved in particles.
Q. What is the beauty of particle physics to you?
It’s that you study not for applications to something but out of pure curiosity. Research into the universe may not directly help our life, yet understanding the universe relates to the fundamental matters of humankind. The question of how the universe began is linked with the question of where we humans came from. Stars were able to form thanks to dark matter, and thanks to the formation of stars various elements were born, which became the source of our life. Thinking that our bodies are made of fragments of stars leaves me in wonderment. To learn about the universe is to learn about your own existence and to trace our history. It feels essential to me, and I find it fascinating.
