The Schrödinger equation inherently contains a multiverse. The disagreement is about whether the wave function described by the equation collapses to a single universe upon measurement (i.e. whether the equation stops holding upon measurement), or whether the different branches continue to exist (i.e. the equation continues to hold at all times), each with a different measurement outcome. Regardless, between measurements the different branches exist in parallel. It’s what allows quantum computation to be a thing.
> The Schrödinger equation inherently contains a multiverse.
A simple counterexample is superdeterminism, in which the different measurement outcomes are an illusion and instead there is always a single pre-determined measurement outcome. Note that this does not violate Bell's inequality for hidden variable theories of quantum mechanics, as Bell's inequality only applies to hidden variables uncorrelated to the choice of measurement: in superdeterminism, both are predetermined so perfectly correlated.
When taking the entire universe as a quantum system governed by the Schrödinger equation, then ψ is the universal wavefunction, and its state vector can be decomposed into pointer states that represent the “branches” of MW.
Non of that honkey ponkey is needed if you just give up locality and a hard deterministic explanation like De-Broglie-Bohm gives all the same correct measurements and conclusions like Copenhagen interpretation without multiverses and "wave function collapses".
Copenhagen interpretation is just "easier" (like oops all our calculations about the univers don't seemt to fit, lets invent "dark matter") when the correct explanations makes any real world calculation practically impossible (thus ending most of physics further study) as any atom depends on every other atom at any time.
De Broglie–Bohm doesn’t remove anything from the wave function, and thus all the pointer states therein contained are still present. The theory basically claims that one of them is special and really exists, whereas the others only mathematically exist, but philosophically it’s not clear what the difference would be. The Broglie–Bohm ontology is bigger than MW rather than smaller.
