A field-level reaction for screened modified gravity

ABSTRACT We present a field-level reaction framework to emulate the non-linear effects of screened modified gravity 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 non-linear matter power spectrum in modified gravity as corrections to a ‘pseudo’ Lambda cold dark matter ($\Lambda$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. We focus on modifications to gravity that are scale-independent at the linear level, allowing us to isolate and emulate non-linear deviations, particularly screening effects. 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-per cent accuracy across a broad range of summary statistics, including 0.4 per cent agreement in the matter power spectrum at scales $k < 1$ Mpc h–1, and 2 per cent accuracy in redshift-space distortion multipoles at $k < 0.3$ Mpc h–1. 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.