Dark Matter and Modified Gravity
We review on the models of dark energy which generates the accelerating expansion on the present universe. There have been proposed so many kinds of models. The motivations why we consider such models are explained and some of the models based on the modified gravity are very briefly reviewed. It is now well-known that the different models can generate the identical expansion history of the universe. One way to select the model in the real world, we may investigate the propagation of the gravitational wave. We discuss how the propagation could depend on the models with some examples.