Twisted matrix factorizations and loop groups

The data of a compact Lie group $G$ and a degree 4 cohomology class on its classifying space leads to invariants in low-dimensional topology as well as important representations of the infinite dimensional group of loops in $G$. Previous work with Mike Hopkins and Constantin Teleman brought the twisted equivariant topological K-theory of $G$ into the game, but only on the level of equivalence classes. After reviewing these ideas I will describe ongoing work with Teleman which gives a geometric construction of the representation categories.