Quantum state of fields in $SU(\infty)$ Quantum Gravity ($SU(\infty)$-QGR)

Our Universe is ruled by quantum mechanics and should be treated as a quantum system. $SU(\infty)$-QGR is a recently proposed quantum model for the Universe, in which gravity is associated to $SU(\infty)$ symmetry of its Hilbert space. Fragmentation of its infinite dimensional state due to random quantum fluctuations divides the Universe to approximately isolated subsystems. In addition to parameters of their {\it internal} finite rank symmetries, states and dynamics of subsystems are characterized by 4 continuous parameters and the perceived classical spacetime is their effective representation, reflecting quantum states of subsystems and their relative evolution. At lowest order the effective Lagrangian of $SU(\infty)$-QGR~has the form of Yang-Mills gauge theories for both $SU(\infty)$ - gravity - and internal symmetries defined on the aforementioned 4D parameter space. In the present work we study more thoroughly some of the fundamental aspects of $SU(\infty)$-QGR. Specifically, we clarify the impact of the degeneracies of $\mathcal{SU}(\infty)$ algebra on the construction of the model; describe mixed states of subsystems and their purification; calculate measures of their entanglement to the rest of the Universe; and discuss their role in the emergence of local gauge symmetries. We also describe the relationship between what is called {\it internal space} of $SU(\infty)$ Yang-Mills with the 4D parameter space, and analytically demonstrate irrelevance of the geometry of parameter space for physical observables. Along with these topics, we demonstrate the equivalence of two sets of criteria for compositeness of a quantum system, and show uniqueness of the limit of various algebras leading to $\mathcal{SU}(\infty)$.