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, so-called the
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
artifact 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 $z\sim 9$,
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 $5\lesssim z\lesssim 15$, indicating the importance of
tidal field on structure formation at such high redshifts, e.g. during the
epoch of reionization.
