Power spectrum for the small-scale Universe
  The first objects to arise in a cold dark matter universe present a daunting
challenge for models of structure formation. In the ultra small-scale limit,
CDM structures form nearly simultaneously across a wide range of scales.
Hierarchical clustering no longer provides a guiding principle for theoretical
analyses and the computation time required to carry out credible simulations
becomes prohibitively high. To gain insight into this problem, we perform
high-resolution (N=720^3 - 1584^3) simulations of an Einstein-de Sitter
cosmology where the initial power spectrum is P(k) propto k^n, with -2.5 < n <
-1. Self-similar scaling is established for n=-1 and n=-2 more convincingly
than in previous, lower-resolution simulations and for the first time,
self-similar scaling is established for an n=-2.25 simulation. However, finite
box-size effects induce departures from self-similar scaling in our n=-2.5
simulation. We compare our results with the predictions for the power spectrum
from (one-loop) perturbation theory and demonstrate that the renormalization
group approach suggested by McDonald improves perturbation theory's ability to
predict the power spectrum in the quasilinear regime. In the nonlinear regime,
our power spectra differ significantly from the widely used fitting formulae of
Peacock & Dodds and Smith et al. and a new fitting formula is presented.
Implications of our results for the stable clustering hypothesis vs. halo model
debate are discussed. Our power spectra are inconsistent with predictions of
the stable clustering hypothesis in the high-k limit and lend credence to the
halo model. Nevertheless, the fitting formula advocated in this paper is purely
empirical and not derived from a specific formulation of the halo model.
