Precision cosmology in muddy waters: Cosmological constraints and N-body codes

TL;DR

This paper examines how uncertainties in N-body simulations impact the precision of future cosmological constraints, emphasizing the need for rigorous convergence testing to achieve sub-percent accuracy in modeling the nonlinear matter power spectrum.

Contribution

It highlights the importance of understanding simulation uncertainties and demonstrates their effect on cosmological parameter constraints, especially for dark energy parameters.

Findings

Marginalizing over simulation parameters reduces survey constraining power.

Adding CMB data mitigates parameter constraint degradation.

Dark-energy figure of merit can be degraded by approximately 2 when marginalizing over simulation uncertainties.

Abstract

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.