Macroscopic decoherence is about the interaction of a quantum system with its environment, resulting in a mixture state for the subsystem of interest. But if yo

Macroscopic decoherence is about the interaction of a quantum system with its environment, resulting in a mixture state for the subsystem of interest. But if you describe the whole universe as a quantum system there is no environment, no decoherence, no many worlds, the universe will remain in a pure state if it was originally in a pure state.