Simulating Voids

We present a novel method for simulation of the interior of large cosmic voids, suitable for study of the formation and evolution of objects lying within such regions. Following Birkhoff's theorem, void regions dynamically evolve as universes with cosmological parameters that depend on the underdensity of the void. We derive the values of $Omega_M$, $Omega_{Lambda}$, and $H_0$ that describe this evolution. We examine how the growth rate of structure and scale factor in a void differ from the background universe. Together with a prescription for the power spectrum of fluctuations, these equations provide the initial conditions for running specialized void simulations. The increased efficiency of such simulations, in comparison with general-purpose simulations, allows an improvement of upwards of twenty in the mass resolution. As a sanity check, we run a moderate resolution simulation ($N=128^3$ particles) and confirm that the resulting mass function of void halos is consistent with other theoretical and numerical models.Comment: Accepted to ApJ., Dec. 11, 2003. 9 pages, 3 postscript figures. Changes made to respond to referee's comment