Transients from Initial Conditions in Cosmological Simulations

We study the impact of setting initial conditions in numerical simulations using the standard procedure based on the Zel’dovich approximation (ZA). As it is well known from perturbation theory, ZA initial conditions have incorrect second and higher-order growth and therefore excite long-lived transients in the evolution of the statistical properties of density and velocity fields. We also study the improvement brought by using more accurate initial conditions based on second-order Lagrangian perturbation theory (2LPT). We show that 2LPT initial conditions reduce transients significantly and thus are much more appropriate for numerical simulations devoted to precision cosmology. Using controlled numerical experiments with ZA and 2LPT initial conditions we show that simulations started at redshift zi = 49 using the ZA underestimate the power spectrum in the nonlinear regime by about 2, 4, 8% at z = 0, 1, 3 respectively, whereas the mass function of dark matter halos is underestimated by 5% at m = 1015M⊙/h (z = 0) and 10% at m = 2 × 10 14M⊙/h (z = 1). The clustering of halos is also affected to the few percent level at z = 0. These systematics effects are typically larger than statistical uncertainties in recent mass function and power spectrum fitting formulae extracted from numerical simulations. At large scales, the measured transients in higher-order correlations can be understood from first principle calculations based on perturbation theory.