Demonstrating Discreteness and Collision Error in Cosmological [ITAL]N[/ITAL]-Body Simulations of Dark Matter Gravitational Clustering

Two-body scattering and other discreteness effects are unimportant in cosmological gravitational clustering in most scenarios since the dark matter has a small particle mass. The collective field should determine evolution; two-body scattering in simulations violates the Poisson-Vlasov equations. We test this result in PM, P 3 M, Tree, and NGPM codes, noting that a collisionless code will preserve the one-dimensional character of plane-wave collapse. We find collisionality vanishing as the softening parameter approaches the mean interparticle separation. Solutions for the problem are suggested, involving greater computer power, PM-based nested grid codes, and a more conservative approach to resolution claims.