  The Lichnerowicz formula yields an index theoretic obstruction to positive
scalar curvature metrics on closed spin manifolds. The most general form of
this obstruction is due to Rosenberg and takes values in the $K$-theory of the
group $C^*$-algebra of the fundamental group of the underlying manifold. We
give an overview of recent results clarifying the relation of the Rosenberg
index to notions from large scale geometry like enlargeability and
essentialness. One central topic is the concept of $K$-homology classes of
infinite $K$-area. This notion, which in its original form is due to Gromov, is
put in a general context and systematically used as a link between
geometrically defined large scale properties and index theoretic
considerations. In particular, we prove essentialness and the non-vanishing of
the Rosenberg index for manifolds of infinite $K$-area.
