Frontier Research Institute for Interdisciplinary Sciences
Tohoku University

After almost 100 years passed from the foundation of the general relativity by A.Einstein, it was finally reported in February 2016 that we have detected gravitational waves whose existences were first predicted by the general relativity. Gravitational waves are a sort of ripples in spacetime which is one of physical consequences in the general relativity where spacetime itself is dynamical. The observation of gravitational waves strongly support the existence of dynamics of spacetime and then the theory of gravity and related topics must be central focuses in the coming decades.

On the other hand, many cosmological observations such as cosmic microwave background and type-Ia supernova indicate that the universe is acceleratedly expanding. However suppose that gravitational force is always attractive, the expansion speed of the universe will slow down since the gravitational force will attract the spacetime. Therefore the fact that the universe has experienced a stage of accelerated expansion would imply that gravitational force could be repulsive at least on cosmologically large scales, which in turn might be an indication that the theory of gravity is modified from that of the general relativity.

To reveal the fundamental law of gravity in our universe and test it by means of various observations, we are developing non-linear and/or non-perturbative analytical methods beyond linear perturbation theory trying to find unknown theoretical predictions from theories of gravity by invoking those methods.