  We show how dynamical heterogeneities in glass forming systems emerge as a
consequence of the existence of dynamical constraints, and we offer an
interpretation of the glass transition as an entropy crisis in trajectory space
(space-time) rather than in configuration space. To illustrate our general
ideas, we analyze the one dimensional (d=1) Fredrickson-Andersen and East
models. Dynamics of such dynamically constrained systems are shown to be
isomorphic to the statics of d+1 dimensional dense mixtures of polydisperse
non-interpenetrating domains. The domains coincide with arrested regions in
trajectory space.
