  We provide a theoretical description of dynamical heterogeneities in
glass-forming liquids, based on the premise that relaxation occurs via local
rearrangements coupled by elasticity. In our framework, the growth of the
dynamical correlation length $\xi$ and of the correlation volume $\chi_4$ are
controlled by a zero-temperature fixed point. We connect this critical behavior
to the properties of the distribution of local energy barriers at zero
temperature. Our description makes a direct connection between dynamical
heterogeneities and avalanche-type relaxation associated to dynamic
facilitation, allowing us to relate the size distribution of heterogeneities to
their time evolution. Within an avalanche, a local region relaxes multiple
times, the more the larger is the avalanche. This property, related to the
nature of the zero-temperature fixed point, directly leads to decoupling of
particle diffusion and relaxation time (the so-called Stokes-Einstein
violation). Our most salient predictions are tested and confirmed by numerical
simulations of scalar and tensorial thermal elasto-plastic models.
