Quantum Mechanics - Is There One Wave Function for the Whole Universe?

In quantum mechanics, the world is not made up of little things. You really need to take seriously the fact that there is just one wave function for the whole system. If you define it in the conventional way, with little pieces making up big things, no quanta mechanical system counts as being emergent,. I think that's just a mistake, because people are still embedded in a classical way of thinking. The universe appears roughly homogeneous and isotropic reflecting translational and rotational symmetries of physics. But i don't think it's likely that that's true - we just wanted to figure out how many times this could happen. People were throwing around laurence variants during inflation