  As liquids approach the glass transition temperature, dynamical heterogeneity
emerges as a crucial universal feature of their behavior. Dynamic facilitation,
where local motion triggers further motion nearby, plays a major role in this
phenomenon. Here we show that long-range, elastically-mediated facilitation
appears below the mode-coupling temperature, adding to the short-range
component present at all temperatures. Our results suggest deep connections
between the supercooled liquid and glass states, and pave the way for a deeper
understanding of dynamical heterogeneity in glassy systems.
