Space-time thermodynamics of the glass transition

We consider the probability distribution for fuctuations in dynamical action and similar quantities related to dynamic heterogeneity. We argue that the so-called glass transition is a manifestation of low action tails in these distributions where the entropy of trajectory space is subextensive in time. These low action tails are a consequence of dynamic heterogeneity and an indication of phase coexistence in trajectory space. The glass transition, where the system falls out of equilibrium, is then an order-disorder phenomenon in space-time occurring at a temperature T-g, which is a weak function of measurement time. We illustrate our perspective ideas with facilitated lattice models and note how these ideas apply more generally.