Direct observations of gravitational waves (GWs) are expected to reveal new aspects of the universe. Since GWs are emitted by the coherent bulk motion of matter, and are hardly absorbed or scattered, observations with GWs will provide complementary information to that with electromagnetic waves. Moreover, they provide completely new information that cannot be obtained by other observations, on the central part of high-energy astrophysical phenomena and on the early universe. In order to open a new field of gravitational-wave astronomy, we are contributing to and leading large-scale GW telescope projects: ground-based KAGRA telescope for astrophysical observation, and space-borne DECIGO telescope for future cosmological observation.
DECIGO is a space gravitational wave detector consisting of three drag-freespacecraft which are separated by 1,000 km each. DECIGO is aiming at the detection of gravitational waves in the frequency band mainly between 0.1Hz and 10Hz. By observing gravitational waves with this band, DECIGO will see the Universe shortly after the Big-Bang, which could not be observed by electromagnetic waves. By analyzing the waveform of gravitational waves from coalescing neutron stars we can determine the acceleration of the expansion of the Universe precisely. This will lead to better understanding of the dark energy. DECIGO will also observe the gravitational waves from the merging supermassive black holes. We expect DECIGO will be launched in 2027. Since the mission is a long-range task we are developing the technologies necessary for the final DECIGO step by step. In January 2009 we have launched a small space-demonstration module SWIMμν. With its successful operation for 1.5-year, we set an upper limit on stochastic background GWs. We are also working hard on the development of DPF (DECIGO pathfinder), which is the first milestone mission to test the key technologies with one spacecraft. DPF is one of the candidates of the small satellite mission promoted by JAXA/ISAS.