  We analyse the global structure of time-dependent geometries dual to
expanding plasmas, considering two examples: the boost invariant Bjorken flow,
and the conformal soliton flow. While the geometry dual to the Bjorken flow is
constructed in a perturbation expansion at late proper time, the conformal
soliton flow has an exact dual (which corresponds to a Poincare patch of
Schwarzschild-AdS). In particular, we discuss the position and area of event
and apparent horizons in the two geometries. The conformal soliton geometry
offers a sharp distinction between event and apparent horizon; whereas the area
of the event horizon diverges, that of the apparent horizon stays finite and
constant. This suggests that the entropy of the corresponding CFT state is
related to the apparent horizon rather than the event horizon.
