Double white dwarf mergers and type Ia supernovae

We introduce our recent studies on the double-degenerate (DD) scenario, especially the Dynamically-Driven Double-Degenerate Double-Detonation (D^6) model. We have performed hydrodynamics simulations of the D^6 model. At the first step, we have reproduced the D^6 explosion, and found that its supernova ejecta has a non-spherical shape. At the second step, we have followed the supernova remnant evolution of the supernova ejecta, and found that the non-spherical shape survives during a few 10^3 years, and it will be detected by detail observations. We have also assessed if multi-messenger observations can identify the relation between double white dwarfs and some transients. We have found that Japanese space-borne gravitational-wave observatory DECIGO should play a dominant role fot that.