A field-level emulator for extra fundamental forces

Abstract

We present a field-level framework to emulate the effects of extra fundamental forces on the cosmic web. This approach is designed to enable field-level inference with data from Stage IV cosmological surveys. Building on the reaction method, which models the nonlinear matter power spectrum in modified gravity as corrections to a "pseudo'' $\Lambda$CDM cosmology, we extend the method to full field-level predictions by applying it to the output of N-body simulations, including both positions and velocities. Our neural network predicts the field-level correction ("reaction'') to a pseudo$\Lambda$CDM simulation whose linear clustering matches that of the target. The emulator achieves sub-percent accuracy across a broad range of summary statistics, including 0.4% agreement in the matter power spectrum at scales k < 1 Mpc/h, and 2% accuracy in redshift-space distortion multipoles at k < 0.3 Mpc/h. We also validate the emulator against N-body simulations with increased force resolution and time steps, confirming the robustness of its performance. These results demonstrate that our framework is a practical and reliable tool for incorporating screened modified gravity models into field-level cosmological inference, enabling stringent tests of extra fundamental forces at cosmological scales.