Some people advocate for relational quantum mechanics, which is an interpretation (or at least Wikipedia calls it an interpretation) of QM where quantum states are thought of as representing a relationship between "subject" and "object" systems, rather than as objectively-real states of the object system.
In particular, RQM claims that there is no quantum state for the whole universe; quantum states only arise when we divide the universe into two pieces and describe the relationship between the two. It's been described by its proponents as "one-world" (or even "zero-world") as opposed to the many-worlds that seem to inevitably result if you apply standard QM to the whole universe.
That sounds like it would have to produce a difference in physical predictions between RQM and standard QM. They might be indistinguishable by any experiment we could realistically do locally, but on a cosmological scale the fact that RQM (somehow) forbids a whole-universe quantum state from existing seems like it should result in some different behavior from standard QM.
Is that the case? Is there a known thought experiment that shows predictions of RQM diverging from those of standard QM—in other words, is it really a different theory, not an interpretation? Or if they always give the same predictions, how can they be conceptually reconciled at the cosmological scale?
