Engaged in the most advanced research that has gained the attention of the world. We asked Professor Yutaka Fujita, "what is so exciting about working in Theoretical Astrophysics"?

Professor Yutaka Fujita, at Tokyo Metropolitan University, Faculty of Science and Graduate School of Science, carries out research to understand the formation mechanism of the Fermi bubbles, giant gamma-ray structures, observed in the center of the Milky Way Galaxy. This research has gained the attention of the world. We asked Professor Fujita to tell us more about his research and what is important at his laboratory.

The whole world is taking notice of Professor Fujita's research on his theory regarding the Milky Way Galaxy

⸺ First, what is your specialty?

My specialty is astrophysics. Within this, I work in the field called "theoretical astrophysics." My work involves various equations in physics and seeks to unravel the mysteries of the universe.

⸺ So how is space research carried out?

Broadly speaking, there are two methods in astrophysics, "observation" and "theory." Many Japanese researchers specialize in one research method, either observation or theory. Observation specialists use high-performance telescopes located around the world and use the obtained data as the basis for ideas on the origin of various space phenomena. On the other hand, researchers like me specialize in theories, and try to describe phenomena occurring in the universe and explain how they work using equations of physics, which are based on observational data and various research papers.

⸺ I heard the research paper "Evidence for powerful winds and the associated reverse shock as the origin of the Fermi bubbles" published in December 2022 attracted a great deal of attention worldwide.

I'm pleased that it was well received in more than 30 media outlets, mostly from overseas. I was also surprised to learn that there are videos on YouTube by people from overseas explaining the contents of my research papers in an easy-to-understand manner.

⸺ Can you tell us about the research described in that paper?

Simply stated, my research seeks to theoretically understand the formation mechanism of Fermi bubbles, giant gamma-ray structures observed in the center of the Milky Way Galaxy.

As a detailed explanation, there is a black hole with a mass approx. 4 million times that of the Sun. It swallows everything around it, but it actually releases some energy just before doing so. As a result, Fermi bubbles may be created by the enormous energy discharged from the black hole at the center of the Galaxy and the long-term eruption of high-speed winds from the Galactic center. The basic points of my paper are the theoretical presentation of this idea.

Fermi bubbles have attracted the attention of researchers around the world, and many theories have been proposed to explain the reasons for such phenomenon. In my paper, I use the latest observational data to formulate a theory that is more convincing than any previous theory. I think that is why my paper has been introduced by the media overseas.

⸺ Especially for massive entities such as galaxies, accuracy of a theory cannot be verified by conducting experiments. So how can the "certainty" of a theory be determined in space research?

It is difficult to explain, but when we look at a theory on paper, if it can be explained naturally, without being unreasonable, it will be supported and become an established theory. Since there is so much we do not know about the universe, we often make "assumptions" when theorizing. However, such assumptions are often subjective, and an explanation should use them as little as possible. If there are fewer assumptions, and if the explanation makes sense, especially when viewed by multiple researchers, the theory is more likely to be supported as "certain."

⸺ In space research, there are multiple theories for a single phenomenon, and eliminating some of these theories is what keeps research moving one step forward, right?

Yes. In space research, a discovery of the century, such as theory that reveals how everything works from the very beginning, is extremely rare. In most cases, a theory is established as various ideas are presented, and over the decades, the most certain ideas survive.

For example, consider the theory that the universe began with a Big Bang and that it is expanding. This is now inserted in school textbooks and known by many. However, there used to be many different theories. In the past, many people, even professionals in astronomy and astrophysics, rejected the theory that the universe had a beginning and that it was expanding. However, as observations advanced, data supporting the Big Bang appeared one by one. As a result, over the decades, the most certain theory regarding how the universe exists was established and that "the universe had a beginning and that it is expanding."

⸺ I understand that in space research, it takes a long time for the true significance of research results to become apparent. What motivates you to take on this research?

In one word, "curiosity." I have loved space since I was a child. It started when my grandfather bought me a telescope for my birthday. When I was in the third grade, I was called the "astronomy boy" and spent many nights in astronomical observation. As I kept watching the stars, I started to have questions about them and the universe, such as "Why is it like that?" My search for the answers to questions raised by my curiosity has led me to the academic field called theoretical astrophysics, and my current research.

Participating in the "XRISM Satellite Project" along with names such as JAXA and NASA

⸺ Will you tell us about some features of your laboratory?

The greatest feature of my laboratory is that we are trying to discover the mysteries of the universe from both theoretical and observational perspectives in collaboration with research teams in Japan and overseas. As I mentioned when discussing my method about space research, there are not many laboratories in Japan that handle both theory and observation.

Our laboratory also participates in the "XRISM Satellite Project" along with participation by institutions such as the Experimental Astrophysics Laboratory of Tokyo Metropolitan University, JAXA (Japan Aerospace Exploration Agency), NASA (National Aeronautics and Space Administration), and the University of Tokyo. The XRISM satellite was successfully launched on September 7, 2023, and the equipment is currently being adjusted for various observations. Our laboratory also seeks to gather the latest data from the instruments board on the XRISM manufactured by the Experimental Astrophysics Laboratory and other laboratories in order to advance our theoretical research.

⸺ Specifically, what phenomena would you like to observe using the XRISM satellite?

Regarding phenomena, by capturing the wind blowing in "galaxy clusters," the largest astronomical objects in the universe, we are trying to observe the movement of gas inside the cluster and the movement of the cluster itself.

Nevertheless, galaxy clusters are extremely large and distant, exceeding 10 million light years across. We cannot understand their movements just by ordinary observation of visible shapes, even if we spend our entire lifetime. So we use the "Doppler effect" phenomenon for this purpose. Many know the Doppler effect from when an ambulance approaches and the siren has a high pitch. But as it moves away, the pitch is lower. Sound is a wave generated by the vibration of an object, and light is also a wave. Consequently, by observing the light in the X-ray range, the speed of the gas can be determined by the Doppler effect.


The XRISM satellite is equipped with an instrument that can precisely measure the energy of X-ray light. The advantage of X-rays is that they can even be used to observe radiation from gas as hot as 100 million degrees that is emitted by clusters of galaxies. By analyzing this observation data, we hope to conduct theoretical research on the gas movement in galaxy clusters and black holes in the future.

I want students to develop "physical intuition"

⸺ How many students are affiliated to your laboratory now?

As of November 2023, there are six students- three undergraduate students and three graduate students. There are also three affiliated project researchers and visiting researchers, of which two are foreigners.

⸺ What is important when training students and researchers?

Developing physical intuition. When students see an equation, I want them to be able to visualize the phenomenon taking place and understand its meaning, rather than just recognizing the equation simply as variables.

Theoretical astrophysicists like myself can see the world represented by equations just by looking at them. I can imagine what is happening from the formulas! In my laboratory, I want students to reach that level. If mathematical expressions and phenomena can be connected, various phenomena can be represented with mathematical expressions. It also makes research activities enjoyable.

This means that persons who enjoy physics and like physical ideas do well at my laboratory. They don't need to know the details of constellations and celestial bodies. Rather, if they have loved physics and mathematics since middle or high school, they are more likely to enjoy research. I would welcome anyone who likes to tinker with mathematical formulas and think this way to our laboratory.


The universe is "everything" The real charm of theoretical astrophysics is that you can test all kinds of physics

⸺ What is so exciting about studying and working in the field of theoretical astrophysics?

There are many different fields of physics, but when dealing with the universe, we can test all those different types of physics. The universe is "everything." It is filled with all the phenomena we consider in physics. Consequently a vast amount of knowledge is required. On the other hand, as you accumulate knowledge, you can present the beginning of the universe and astronomical phenomena with mathematical formulas and understand them accurately. There is a strange feeling when you approach the realm of God, as if you are great in some way. This is perhaps the best thing about pursuing this research.

⸺ Finally, do you have a message for students who would like to enter Tokyo metropolitan university?

To master space research, a truly broad range of knowledge is required. Therefore, show interest in various things, and study with all your curiosity while in high school.

The environment of the university will be very comfortable. You can work on the latest research including the XRISM satellite, and the National Astronomical Observatory of Japan and JAXA's Sagamihara Campus are also located nearby. In addition, Minami Osawa Campus has much greenery, and the area around the station is a very nice place to live.

I personally want students to be ambitious as to challenge difficult research themes while still young. In the long run, I intend to conduct theoretical research to solve the mysteries of the universe as a whole, using various observational data available throughout the world. Let's work together to unravel the mysteries of the universe.