Impacts of pre-initial conditions on anisotropic separate universe simulations: a boosted tidal response in the epoch of reionization

To generate initial conditions for cosmological N-body simulations, one needs to prepare a uniform distribution of simulation particles, the so-called pre-initial condition (pre-IC). The standard method to construct the pre-IC is to place the particles on the lattice grids evenly spaced in the three-dimensional spatial coordinates. However, even after the initial displacement of each particle according to cosmological perturbations, the particle distribution remains to display an artificial anisotropy. Such an artefact causes systematic effects in simulations at later time until the evolved particle distribution sufficiently erases the initial anisotropy. In this paper, we study the impacts of the pre-IC on the anisotropic separate universe simulation, where the effect of large-scale tidal field on structure formation is taken into account using the anisotropic expansion in a local background (simulation volume). To quantify the impacts, we compare the simulations employing the standard grid pre-IC and the glass one, where the latter is supposed to suppress the initial anisotropy. We show that the artificial features in the grid pre-IC simulations are seen until z similar to 9, while the glass pre-IC simulations appear to be stable and accurate over the range of scales we study. From these results we find that a coupling of the large-scale tidal field with matter clustering is enhanced compared to the leading-order prediction of perturbation theory in the quasi-non-linear regime in the redshift range 5 less than or similar to z less than or similar to 15, indicating the importance of tidal field on structure formation at such high redshifts, e.g. during the epoch of reionization.

Automated summary

This paper investigates the effects of pre-initial conditions on cosmological N-body simulations, revealing artificial features in grid pre-IC simulations while glass pre-IC simulations are more stable and accurate. The study shows that in the high redshift range, the coupling between large-scale tidal field and matter clustering is enhanced, highlighting the significance of tidal field on structure formation, especially during the epoch of reionization.