Gravitational Waves (GWs) are predicted from the theory of general relativity in 1916 by A.Einstein. However, GWs have not been detected directly as changes of the space-time. GW is the last question from A.Einstein to us.

Emitting lights and spreading gravitational potential of "mass" is the fundamental method to send informations about the entity. We have already obtained a lot of knowledge about the Universe by the electro-magnetic waves such as visible lights, x-ray and so on. On the other hand, we have no method to see the Universe by gravitational waves which are generated from masses. So, GWs also can be a new window to see the Universe in addition to the electro-magnetic waves.

Within 40 thousand years after the Big Bang, electro-magnetic waves (EMWs) were scattered a lot because of the extreme density of the Universe. Consequently, we cannot see those ages with EMWs. On the other hand, nutrinos and GWs are never scattered just after the Big Bang. So, We can see the beginning of the Universe with GWs and neutrinos.

GWs are the first information that are emitted from " big deals" in the Universe such as the generation of black halls, neutron stars coalescences and so on.

The property of gravity has not been verified precisely compared with other kinds of forces such as the strong force, the weak force and electro-magnetic force because of its extreme smallness. However, GWs can be a good probes to research the property of gravity because GWs are emitted from the space-time where the gravity is as strong as the other forces, such as black holes and neutron stars.