A gravitational wave is the distortion (wave) of spacetime that propagates at the speed of light. Its existence had been predicted long ago by general relativity, but it was extremely challenging to observe directly. For a long time, direct detection remained elusive. In 2015, gravitational-wave detectors in US finally made the first detection of gravitational waves generated by the merger of binary black holes. Gravitational waves are believed to be generated not only by celestial objects such as binary black holes and neutron star binaries but also by high-energy and violent phenomena in the early universe, such as inflation, phase transitions, and reheating. In our group, we study various aspects, including the early universe, the evolution of the universe thereafter, and the exploration of fundamental particles beyond the standard model through gravitational wave observations.

Tests of gravity theories until now had been mostly limited to experiments and observations in the weak regime of gravity such as on the Earth and within the solar system. However, with the first observation of gravitational waves in 2015, it became possible to investigate spacetimes with extremely strong gravity, such as those around black holes, and the dynamic properties of spacetime involved in the generation and propagation of gravitational waves. The nature of gravity in such extreme environments is expected to provide insights into a quantum gravity theory and the origins of cosmic accelerated expansion. In our group, we analyze gravitational-wave observational data to precisely test general relativity in extreme environments while also to investigate the properties of unknown aspects of gravity.