Numerical evolution of general relativistic voids

In this paper, we study the evolution of a superhorizon-sized void embedded in a radiation-dominated Friedmann-Robertson-Walker universe. We numerically solve the spherically symmetric general relativistic equations in comoving, synchronous coordinates. Initially, the fluid inside the void is taken to be homogeneous and nonexpanding. When the fluid inside the void is relativistic, we find that radiation diffuses into the void at approximately the speed of light as a strong shock---the void collapses in general. We also find the surprising result that the cosmic collapse time (the first-crossing time) for a relativistic or nonrelativistic void is much smaller than previously thought, because it depends not only on the initial void radius, but also on the ratio of the temperature inside the void (or the particle mass if the void is nonrelativistic) to the temperature outside. Under certain conditions (e.g., if the void is empty enough), the collapse occurs in less than the Hubble time outside the void. This quick collapse time revises the current picture of superhorizon-sized void evolution after first-order inflation. In addition, it introduces the possibility that superhorizon-sized voids may thermalize and homogenize relatively quickly.