A field-level reaction for screened modified gravity

TL;DR

This paper introduces a neural network-based framework to emulate nonlinear effects of screened modified gravity on the cosmic web at the field level, enabling more accurate cosmological inference.

Contribution

It extends the reaction method to full field-level predictions, providing a fast, accurate emulator for modified gravity effects in cosmological simulations.

Findings

Achieves sub-percent accuracy in matter power spectrum predictions.

Provides 2% accuracy in redshift-space distortion multipoles.

Validated robustness against high-resolution N-body simulations.

Abstract

We present a field-level reaction framework to emulate the nonlinear 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 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. We focus on modifications to gravity that are scale-independent at the linear level, allowing us to isolate and emulate nonlinear 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-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.