A Physical Bias in Cosmological Simulations

Numerical simulation plays an important role in the study of structureformation of the universe. However, the mass resolution in current simulationsis still poor. Due to technical difficulties, it is necessary to use bothgreatly reduced number density of particles and greatly raised unit particlemass. Consequently, the particle masses used in cosmological simulations areabout $10^{70}$ times larger than the $Gev$ candidates in particle physics.This is a huge physical bias that cannot be neglected when interpreting theresults of the simulations. Here we discuss how such a bias affects Cold DarkMatter (CDM) cosmological simulations. We find that the small scale propertiesof the CDM particle system are changed in two aspects. 1) An upper limit isimposed on the spatial resolution of the simulation results. 2) Mostimportantly, an unexpected short mean free path is produced, and thecorresponding two body scattering cross section is close to the value expectedin the Self-Interaction Dark Matter(SIDM) model. Since the mean free path ofreal CDM particle systems is much longer than that in the simulations, ourresults imply 1) that there is probably no 'cusp problem' in real CDM halos,and 2) that a much longer time is needed to form new virialized halos in realCDM particle systems than in the simulations. This last result can help usunderstand the 'substructure problem'. Our discussion can also explain why themassive halos in the simulations may have smaller concentration coefficients.Comment: 4 pages,no figures. Submitted to ApJ