Using time-resolved photoluminescence experiments and Monte Carlo simulatio
n, we identify two distinct temperature regimes in the energy relaxation of
excitons in a GaAs/AlGaAs double quantum structure. Below a critical tempe
rature. the system is glassy and exhibits slow, nonexponential energy relax
ation to the longest times observed. At higher temperatures, the system ach
ieves thermal equilibrium in finite time. The transition between these regi
mes is marked by a dramatic critical behavior: a dynamical broadening of th
e energy distribution at the glass temperature. Simulations reproduce this
critical behavior and allow for the first time the determination of the den
sity of localized states.