Collapse of a Superhorizon-sized Void in the Early Universe
  In this paper, we study the collapse of a superhorizon-sized void in the early, radiation-dominated universe using an improved general relativistic code. We find that in general a relativistic or nonrelativistic void collapses via a shock at the speed of light. This is true even if the outward velocity of the void wall is enormous. As the wall thickness decreases, the shock strength increases and the collapse time decreases up to a limit set by the shock tube solution. In addition, as the outward velocity of the wall increases, the collapse time increases somewhat. When the collapse occurs in much less than a Hubble time outside the void, gravitational forces contribute negligibly to the solution at the collapse time; non-gravitational forces (caused by pressure and velocity gradients) shape the solution almost entirely. This is true even if the wall velocity is large enough that fluid in the peak shocks outward substantially during this time. Gravitational forces are expected to be dominant only {\it after} a void has collapsed. 
Sharon L. Vadas