  In a recent article [M. Merolle et al., Proc. Natl. Acad. Sci. USA 102, 10837
(2005)] it was argued that dynamic heterogeneity in $d$-dimensional glass
formers is a manifestation of an order-disorder phenomenon in the $d+1$
dimensions of spacetime. By considering a dynamical analogue of the free
energy, evidence was found for phase coexistence between active and inactive
regions of spacetime, and it was suggested that this phenomenon underlies the
glass transition. Here we develop these ideas further by investigating in
detail the one-dimensional Fredrickson-Andersen (FA) model in which the active
and inactive phases originate in the reducibility of the dynamics. We
illustrate the phase coexistence by considering the distributions of mesoscopic
spacetime observables. We show how the analogy with phase coexistence can be
strengthened by breaking microscopic reversibility in the FA model, leading to
a non-equilibrium theory in the directed percolation universality class.
