Implication of dressed form of relational observable on von Neumann algebra

TL;DR

This paper explores how relational observables in quantum gravity can be expressed as dressed operators, revealing algebraic structures like Type II$_ty$ and Type II$_1$ algebras in different backgrounds such as quasi-de Sitter and de Sitter space.

Contribution

It demonstrates that the algebraic structure of the background spacetime can be characterized by the dressed form of relational observables, highlighting differences between isometry-preserving and breaking backgrounds.

Findings

Quasi-de Sitter space corresponds to a Type II$_ty$ algebra with a diverging trace.

De Sitter space is described by a Type II$_1$ algebra with a finite trace.

Dressing of relational observables can be local or nonlocal depending on background symmetries.

Abstract

In quantum gravity, physically meaningful operator is required to be invariant under the diffeomorphisms. Such gauge invariant operator is typically given by the relational observable, the operator localized in relation to some background states. We point out that the relational observable can be comprehensively written in the form of the dressed operator. For the background having boundary where the diffeomorphisms are not gauged, we can use the gravitational Wilson line for dressing, then the relational observable is nonlocal. In contrast, when the background breaks some isometries, as can be found in quasi-de Sitter space, dressing can be local, which is a kind of St\"uckelberg mechanism. Since dressing resembles the outer automorphism in the von Neumann algebra, we may investigate the algebraic structure of the background by considering the dressed form of the relational observable. From this, we can understand that quasi-de Sitter space is described by the Type II$_\infty$ algebra where the trace diverges in the decoupling limit of gravity. It is different from the Type II$_1$ algebra of de Sitter space where the finite size of trace can be defined in the same limit. This shows that the isometry preserving and breaking backgrounds are quite different in the algebraic structure no matter how small the breaking effect is.