  Liquids near the glass transition exhibit dynamical heterogeneity, i.e. local
relaxation rates fluctuate strongly over space and time. Here we introduce a
simple continuum model that allows for quantitative predictions for the
correlators describing these fluctuations. We find remarkable agreement of the
model predictions for the dynamic susceptibility $\chi_4(t)$ with numerical
results for a binary hard-sphere liquid and for a Kob-Andersen Lennard-Jones
mixture. Under this model, the lifetime $\tau_{\rm ex}$ of the heterogeneities
has little effect on the position $t = t_4 \sim \tau_{\alpha}$ of the peak of
$\chi_4(t)$, but it controls the decay of $\chi_4(t)$ after the peak, and we
show how to estimate it from this decay.
