Given a (second countable locally compact Hausdorff) twisted étale groupoid C*-algebra with a canonical trace, the GNS-bicommutant is a twisted measured groupoid von Neumann algebra. In particular, the 'topological twist' is completely captured by a Borel 2-cocycle on the measured groupoid. We observe several consequences of this principle: 1) We show that a construction of Donvil and Vaes gives a simple separable exact C*-algebra with no twisted groupoid models, 2) We show that reduced free products and certain amalgamated free products of C*-algebras have no Cartan subalgebras. This is ongoing joint work with Ali Miller and Helena Perovic.
