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**Session 86 - Cosmology: Theory.**

*Display session, Friday, January 09*

*Exhibit Hall, *

## [86.03] Fundamental Limitations on Cosmological Clustering Simulations due to ``N-Bodies''

*R. J. Splinter (Hewlett-Packard Co), A. L. Melott, S. F. Shandarin (U. Kansas), Y. Suto (U. Tokyo)*
Fundamental physical considerations and past tests suggest that
there may be a problem with discreteness error in N--body methods
widely used in cosmological clustering studies. This could cause
problems with accuracy when coupled to hydrodynamics codes.

We therefore investigate some of the effects that discreteness and
two--body scattering may have on N-body simulations with
``realistic'' cosmological initial conditions. For the initial
power spectrum of the density fluctuations we choose a pure
power-law with the form P(k) \propto k^-1. Such a spectrum
approximates many candidate cosmological models on scales of order
10--100 h^-1 Mpc while keeping the simplicity of self-similar
evolution.

We use an identical subset of particles from the initial conditions
for a 128^3 Particle--Mesh (PM) calculation as the initial
conditions for a variety of Particle--Particle--Particle Mesh
(P^3M) and Tree code runs. The force softening length and particle
number in the P^3M and Tree code runs are varied and results are
compared with those of the PM run. In particular, we investigate
the effect of mass resolution, since most ``high resolution'' codes
only have high resolution in gravitational force, not in mass.

Since the codes never agree well on scales below the mean comoving
interparticle separation, we find little justification to use
results on these scales to make quantitative predictions in
cosmology. The range of values found for some quantities spans
50%, but others, such as the amount of mass in high density
regions, can be off by a factor of three or more.

**Program
listing for Friday**