In Cuffaro and Hartmann's (2021) Open Systems View, the authors suggest, "The problem is that if the physical universe is not represented as a unitarily evolving state vector, then it becomes difficult to conceive of it in terms of a multiverse of effectively independently evolving worlds."

Why is this the case? Can we not get branching if the universe is in a mixed state, and evolves non-unitarily?

In Cuffaro and Hartmann's (2021) Open Systems View, the authors suggest, "The problem is that if the physical universe is not represented as a unitarily evolving state vector, then it becomes difficult to conceive of it in terms of a multiverse of effectively independently evolving worlds."

Why is this the case? Can we not get branching if the universe is in a non-unitarily evolving mixed state represented by a density operator?

For those sceptical of non-unitary evolution in EQM, both Wallace (Emergent Multiverse 2012 §10.5) and Carroll (Reality as a Vector in Hilbert Space 2021) consider this to be a possibility.

In Everettian quantum mechanics, if the universal state evolves non-unitarily, how does this affect the structure of branching?

In Cuffaro and Hartmann's (2021) Open Systems View, the authors suggest, "The problem is that if the physical universe is not represented as a unitarily evolving state vector, then it becomes difficult to conceive of it in terms of a multiverse of effectively independently evolving worlds."

Why is this the case? Can we not get branching if the universe is in a mixed state, and evolves non-unitarily?