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Session 86 - Cosmology: Theory.
Display session, Friday, January 09
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