Homotopy of posets, net-cohomology and superselection sectors in globally hyperbolic spacetimes

TL;DR

This paper extends the analysis of superselection sectors and their charge structures from Minkowski space to arbitrary globally hyperbolic spacetimes, using poset net-cohomology to encode topological properties.

Contribution

It demonstrates that the category of DHR-type sectors in curved spacetimes forms a C*-category with tensor and conjugation structures, generalizing Minkowski space results.

Findings

The sector category is equivalent to 1-cocycles of the poset.

The fundamental group of the poset matches that of the spacetime.

Net-cohomology is invariant under changes of the index set.

Abstract

We study sharply localized sectors, known as sectors of DHR-type, of a net of local observables, in arbitrary globally hyperbolic spacetimes with dimension $\geq 3$. We show that these sectors define, has it happens in Minkowski space, a $\mathrm{C}^*-$category in which the charge structure manifests itself by the existence of a tensor product, a permutation symmetry and a conjugation. The mathematical framework is that of the net-cohomology of posets according to J.E. Roberts. The net of local observables is indexed by a poset formed by a basis for the topology of the spacetime ordered under inclusion. The category of sectors, is equivalent to the category of 1-cocycles of the poset with values in the net. We succeed to analyze the structure of this category because we show how topological properties of the spacetime are encoded in the poset used as index set: the first homotopy group of a poset is introduced and it is shown that the fundamental group of the poset and the one of the underlying spacetime are isomorphic; any 1-cocycle defines a unitary representation of these fundamental groups. Another important result is the invariance of the net-cohomology under a suitable change of index set of the net.