Precision cosmology in muddy waters: Cosmological constraints and N-body
  codes
  Future large-scale structure surveys of the Universe will aim to constrain
the cosmological model and the true nature of dark energy with unprecedented
accuracy. In order for these surveys to achieve their designed goals, they will
require predictions for the nonlinear matter power spectrum to sub-percent
accuracy. Through the use of a large ensemble of cosmological N-body
simulations, we demonstrate that if we do not understand the uncertainties
associated with simulating structure formation, i.e. knowledge of the `true'
simulation parameters, and simply seek to marginalize over them, then the
constraining power of such future surveys can be significantly reduced.
However, for the parameters {n_s, h, Om_b, Om_m}, this effect can be largely
mitigated by adding the information from a CMB experiment, like Planck. In
contrast, for the amplitude of fluctuations sigma8 and the time-evolving
equation of state of dark energy {w_0, w_a}, the mitigation is mild. On
marginalizing over the simulation parameters, we find that the dark-energy
figure of merit can be degraded by ~2. This is likely an optimistic assessment,
since we do not take into account other important simulation parameters. A
caveat is our assumption that the Hessian of the likelihood function does not
vary significantly when moving from our adopted to the 'true' simulation
parameter set. This paper therefore provides strong motivation for rigorous
convergence testing of N-body codes to meet the future challenges of precision
cosmology.
