Demonstrating Discreteness and Collision Error in Cosmological N-body Simulations of Dark Matter Gravitational Clustering

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  • 21 September 1996
Abstract
Two-body scattering is unimportant in cosmological gravitational clustering in most scenarios, as the dark matter has a small particle mass. The collective field should determine evolution: Two-body scattering in simulations violates the physics which is being modeled. We test for this, noting that a collisionless code will preserve the one-dimensional character of plane wave collapse. P$^3$M, the workhorse of most cosmological N-body simulations, fails miserably unless its softening parameter is so large it becomes a PM code -- which passes the test easily. This error calls into question all ``high resolution'' results in which forces are resolved below the mean interparticle separation. Since dark matter usually dominates the gravitational field in cosmological hydrodynamic simulations, these too will be affected. Some approaches to solving the problem are suggested, mostly involving greater computer power, PM-based nested grid codes, and a more conservative approach to resolution claims. We test a nested-grid code which achieves high resolution without becoming collision-dominated.

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