Sketch of a cooperative rearranging region in a glass and the part of the energy landscape related to it. The curve shows schematically the potential energy V as a function of R⃗1, R⃗2,..., which are generalized coordinates indicating molecular positions. Blue indicates immobile molecules and red the most mobile ones. The arrows are in the direction of motion. The figure shows in a pictorial way that since more molecules have to move cooperatively approaching the glass transition, the associated effective energy barrier grows and the range of dynamic correlations increases. Bauer et al.  have measured these correlations as a function of temperature and have related them to the effective energy barriers, confirming this cooperative behavior for several different glass formers.