Rovelli & others, in Relational Quantum Mechanics (RQM) take the simple ontological picture of the Copenhagen Picture and relativise it. This is what I was suggesting in this question, though I was looking for a specific mathematical treatment.
In the Copenhagen Picture a distinction is made between the microscopic observed system, taken to be quantum mechanical; and the macroscopic observer which is classical.
They argue that this 'mixed' picture is 'klutzy' for three reasons.
That reality shouldn't arbitrarily divide between a quantum mechanical microscopic system and a classical macroscopic one. Physical Reality should be quantum mechanical all the way through. (I say Physical as this is the reality probed by Physics, that is susceptible to Physicalism. I do not mean by this is the only way of theorising reality).
Observers shouldn't be characterized by human beings operating classical macroscopic apparatus, but by interactions between observer & observed - and the observer need not be macroscopic, classical or animate. An electron can observe a proton, for example.
Interactions are going on all the time. A proper description should take into all interactions. That is rather than consider a system of observer & observed; one should consider a single system, which is then divided into observer & observed; and one should consider all possible divisions. These should be consistent. Also reflexive - division A can observe division B, but also division B can observe division A. Simple Transitivity doesn't hold - though if A can observe B, and B can observe C; then A ought to Observe C. Though I suspect its mendable with enough ingenuity.
(The removal of animate or conscious observers also removes one of conceptual mystifications in the interpretations of QM that has seeped into popular culture that somehow consciousness is implicated in QM. This however doesn't negate a Kantian separation of reality into phenomena & noumena since the theoretical physical description of reality is not immediate to the Sensability and is mediated instrumentally).
Rovelli insists that a measurement is always done between an observer and an observed; hence both must be mentioned. One cannot say the 'momentum of a particle is such-and-such', but that the 'momentum of the particle is such-and such for this observer'.
In a broad sense Rovelli has applied Einsteins insight about the 'relativity' of observation. In fact this goes further back to Galileo about the relativity of rest & velocity. Also Leibniz, Huygens & Descartes about the relational conception of space & time. Though interestingly Newton broke with this to develop his physics - he proscribed absolute space & time; and he did this on pragmatic grounds - it made his theory work; he understood the justice of the philosophical arguments but the mathematics of his day - including calculs - wasn't upto such a subtle view.
One can then argue that it is 'meaningless' to speak of the momentum of the universe as a whole, for who or what is there to observe it.
One could say that the universe observes itself; but this possibility is ruled out by Breuer. In fact, he shows that there is information about the whole universe that is impossible to access by any observer, even in principle.
What is the ontological status of information that is permanently inaccessible by any conceivable physical observer?