Modeling nonlinear scales with COLA: preparing for LSST-Y1

Year 1 results of the Legacy Survey of Space and Time (LSST) will provide tighter constraints on small-scale cosmology, beyond the validity of linear perturbation theory. This heightens the demand for a computationally affordable prescription that can accurately capture nonlinearities in beyond-ΛCDM models. The COmoving Lagrangian Acceleration (COLA) method, a cost-effective N-body technique, has been proposed as a viable alternative to high-resolution N-body simulations for training emulators of the nonlinear matter power spectrum. In this study, we evaluate this approach by employing COLA emulators to conduct a cosmic shear analysis with LSST-Y1 simulated data across three different nonlinear scale cuts. We use the wCDM model, for which the EuclidEmulator2 (ee2) exists as a benchmark, having been trained with high-resolution N-body simulations. We primarily utilize COLA simulations with mass resolution M_ part≈ 8 × 10^10 h^-1 M_⊙ and force resolution ℓ_ force=0.5  h^-1Mpc, though we also test refined settings with M_ part≈ 1 × 10^10 h^-1M_⊙ and force resolution ℓ_ force=0.17  h^-1Mpc. We find the performance of the COLA emulators is sensitive to the placement of high-resolution N-body reference samples inside the prior, which only ensure agreement in their local vicinity. However, the COLA emulators pass stringent criteria in goodness-of-fit and parameter bias throughout the prior, when ΛCDM predictions of ee2 are computed alongside every COLA emulator prediction, suggesting a promising approach for extended models.