Forging New Scientific Frontiers in Japan

“We have already more than 15 years of history here at this Institute. We are in a stable stage and doing quite well; this is not to be demolished, even though we have a new director,” Yokoyama says with a laugh and exuding enthusiasm. “But we must do some new things, and we have amazing opportunities ahead of us.”

One of Yokoyama’s two top priorities in his vision for Kavli IPMU is to move the Institute into the forefront of multi-messenger astronomy—a rapidly growing field with the power to probe the universe as never before. The field brings together theory and observations of forms of matter and energy beyond light, the mainstay “messenger” of astronomy for centuries. These additional messengers include the ghostly particles known as neutrinos and the ripples in spacetime known as gravitational waves.

In recent years, astrophysicists have begun making detections of combos of these messengers, which each ferry critical information about extreme cosmic phenomena, such as black hole and neutron star collisions. Greater insights into those events, in turn, offer novel insights into the forces of nature that shape the universe.

The rise of multi-messenger science had seemed extremely farfetched even just 40 years ago, Yokoyama relates. In the mid-1980s, during applications for graduate school, Yokoyama faced a panel of professors who asked him what else could be used to observe the universe besides light. He offered the speculative answer of neutrinos and gravitational waves, a response which prompted laughers from the interviewers. “They thought, ‘Oh, what a crazy student he is’,” Yokoyama recalls with a laugh. A mere few years later though, in 1987, the first neutrinos from beyond the solar system were captured by early generations of neutrino observatories, namely Kamiokande in Japan and the IMB (Irvine-Michigan-Brookhaven) detector in the U.S. Then, much later, in 2015, gravitational waves at last became a detectable phenomenon, thanks to LIGO and observatories that have followed.

Yokoyama’s point is that what seems prohibitively hard to do at present in moving astrophysics and related fields forward can in fact be done. “What we are discussing now, even though it may look like a ridiculous thing to realize, can be realized in 30 years’ time,” Yokoyama says. “So I must encourage young people and young scientists for the future.”

As a case in point, Yokoyama envisions Kavli IPMU focusing more on space-based gravitational wave detectors, a challenging technology for which efforts are underway. Such a space-based array could be spread vastly farther apart than today’s ground-based detectors, an architecture necessary to detect, for instance, gravitational waves from supermassive black holes within galaxies.

But Yokoyama is thinking even bigger, toward second- or even third-generation space-based arrays that could possibly capture primordial gravitational waves emitted in the earliest moments of the universe. Doing so would offer a profoundly unprecedented way to study the Big Bang and fundamental physics.

“Directly detecting these primordial gravitational waves coming from the beginning of the universe, when the universe was rapidly expanding during what we theorize as an inflationary phase, would let us understand the universe in new depth,” says Yokoyama.

To accomplish this feat and other paradigm-shifting science, Yokoyama’s second top priority in his vision for Kavli IPMU comes into play. He intends for Kavli IPMU to become “a gateway to the world” by “promoting a new stage of cooperation,” he says, that modern science requires in order to make profound new discoveries.

“There are many interesting projects being planned in Japan, but they are too big to be realized in Japan alone,” says Yokoyama. “I want to have Kavli IPMU to serve as a gateway for collaborating with our international partners.”

Within this vision of expanded multi-messenger astronomy and frontier-forging on major scientific collaborations, Kavli IPMU is uniquely positioned to succeed, Yokoyama says, given the Institute’s dedicated researchers who are already working on a vast scope of research activities.

“We have an excellent selection of people covering diverse fields in a unique environment,” says Yokoyama. “Everyone is keen to understand what is going on in the universe, how the universe was created, and why we are here.”

The scope of science at Kavli IPMU is reflected by the Institute’s twenty-odd research program areas, which cover the gamut from fundamental to theoretical to experimental and observational. A mere sampling of these research program areas includes mathematics, quantum field theory, string theory, astroparticle physics, observational cosmology, inflation and the early universe, dark matter, supernovae, and much more.

On the experimental and observational side, Kavli IPMU members are involved in a rich array of projects, including Japan-based particle and astroparticle physics experiments such as KamLAND, BelleII, and Super-Kamiokande, including its next-generation upgrade, Hyper-Kamiokande.

Outside of Japan, Kavli IPMU leads international efforts behind the Hyper Suprime-Cam (HSC) and Prime Focus Spectrograph (PFS), both installed on the 8.2-meter Subaru Telescope at the National Astronomical Observatory of Japan in Hawaii. These two instruments in particular will be playing a significant role in multi-messenger astronomy, following up on gravitational wave detections to locate the waves’ sources in the sky.

To this research portfolio, Yokoyama seeks to add not just multi-messenger astronomy, but also deeper studies of essence of the newest messengers—the fundamental force of gravity.

“Kavli IPMU has been more focused on the material, matter part of the universe and studying energy and momentum,” says Yokoyama. “I would like us to understand the identity and character gravity and the geometrical structure of the universe.”

In this vein, Yokoyama is bridging his own background as a leading international researcher in theoretical cosmology and gravitational waves with Kavli IPMU. In many other ways as well, Yokoyama’s accepting of the director role at Kavli IPMU follows naturally from his career as a central figure in the Japanese research community.

Fittingly, Yokoyama earned his PhD at the University of Tokyo, the host university for Kavli IPMU. In 2008, shortly after the original founding of IPMU—and even before its Kavli endowment in 2012—Yokoyama joined as a Visiting Senior Scientist, meaning he has been with the Institute since the very beginning.

Over his scientific career, Yokoyama has also held positions at Kyoto University and Osaka University in Japan, and as a spokesperson for KAGRA, Japan’s gravitational wave detector. Yokoyama also jointly serves as director of the Research Center for the Early Universe (RESCEU), likewise at the University of Tokyo. Enhancing collaboration between RESCEU and Kavli IPMU will further the missions for both, Yokoyama says.

In addition, Yokoyama has previously worked in the United States at Stanford University, home to the Kavli Institute for Particles Astrophysics and Cosmology (KIPAC), and at the Fermi National Accelerator Laboratory, where he interacted with members of the Kavli Institute for Cosmological Physics (KICP) at the University of Chicago. Those interactions continue through today, especially with KICP member Joshua Frieman, who serves as chair of the External Advisory Committee for Kavli IPMU, a group that annually reviews the activities and scientific achievements of the Institute.

Overall, at home and abroad in his pursuits, Yokoyama has helped propel the increasingly intertwining fields of cosmology and fundamental physics. Paradoxically, cosmology operates on the largest possible scales, encompassing the billions of light-years of observable and unobservable universe, while fundamental physics operates on the smallest possible scales of particles and forces.

“Theoretically, information in the universe connects on all the different length scales,” says Yokoyama, jokingly adding, “It’s like an ouroboros, the snake eating its own tail.”

To push ever farther into these frontiers of the largest and the smallest, Yokoyama returns to his point of encouraging the researchers of today and tomorrow to think big.

Says Yokoyama: “I am much excited to be the new director at Kavli IPMU and to see how we all can make a very big contribution to this wonderful thing, science.”