  The standard picture of heavy-ion collisions includes a collective expansion.
If the initial energy density in the collisions is lumpy, then a collective
expansion can convert that spatial lumpiness into correlations between
final-state particles. Correlations measurements in heavy-ion collisions show
several prominent features not present in proton-proton collisions. I argue
that many features of these correlations are related to the transference of
over-densities from the initial overlap region into momentum-space during the
quark gluon plasma phase of the expansion. I show results from a toy
Monte-Carlo to illustrate the consequences of lumpy initial conditions and a
collective expansion.
