  A coarse-grained model of dense hard sphere colloids building on simple
notions of particle mobility and spatial coherence is presented and shown to
reproduce results of experiments and simulations for key quantities such as the
intermediate scattering function, the particle mean-square displacement and the
$\chi_{4}$ mobility correlation function. All results are explained by two
emerging and interrelated dynamical properties: i) a rate of intermittent
events, quakes, which decreases as the inverse of the system age t; ii) a
length scale characterizing correlated domains, which increases linearly in log
t. This leads to simple and accurate scaling forms expressed in terms of a
single scaling variable Finally, we propose a method to experimentally extract
the growing length scale of an aging colloid and suggest that a suitable
scaling of the probability density function of particle displacement can
experimentally reveal the rate of quakes.
