Projective representation theory for compact quantum groups and the quantum Baum–Connes assembly map

We study the theory of measurable projective representations for a compact quantum group G, i.e., actions of G on B(H) for some Hilbert space H. We show that any such measurable projective representation is inner, and is hence induced by an Ω-twisted representation for some measurable 2-cocycle Ω on G. We show that a projective representation is continuous, i.e., restricts to an action on the compact operators K(H), if and only if the associated 2-cocycle is regular, and that this condition is automatically satisfied if G is of Kac type. This allows in particular to characterise the torsion of projective type of G in terms of the projective representation theory of G. For a given regular 2-cocycle Ω, we then study Ω-twisted actions on C∗-algebras. We define deformed crossed products with respect to Ω, obtaining a twisted version of the Baaj–Skandalis duality and the Green–Julg isomorphism, and a quantum version of the Packer–Raeburn’s trick.