Fast and accurate predictions of the nonlinear matter power spectrum for general models of Dark Energy and Modified Gravity

TL;DR

This paper introduces a model-independent framework for predicting the nonlinear matter power spectrum in various dark energy and modified gravity models, achieving high accuracy with minimal additional parameters.

Contribution

It develops a flexible, physically motivated approach embedded in the halo model reaction framework, enabling accurate nonlinear predictions for general cosmologies with few extra parameters.

Findings

Sub-percent agreement with exact solutions and emulators at z≤1 and k≤5 h/Mpc.

Only 3 additional free constants needed for 2% accuracy in nonlinear regime.

Implementation of the approach into ReACT v2.0 code.

Abstract

We embed linear and nonlinear parametrisations of beyond standard cosmological physics in the halo model reaction framework, providing a model-independent prescription for the nonlinear matter power spectrum. As an application, we focus on Horndeski theories, using the Effective Field Theory of Dark Energy (EFTofDE) to parameterise linear and quasi-nonlinear perturbations. In the nonlinear regime we investigate both a nonlinear parameterised-post Friedmannian (nPPF) approach as well as a physically motivated and approximate phenomenological model based on the error function (Erf). We compare the parameterised approaches' predictions of the nonlinear matter power spectrum to the exact solutions, as well as state-of-the-art emulators, in an evolving dark energy scenario and two well studied modified gravity models, finding sub-percent agreement in the reaction using the Erf model at $z\leq1$ and $k\leq 5~h/{\rm Mpc}$. This suggests only an additional 3 free constants, above the background and linear theory parameters, are sufficient to model nonlinear, non-standard cosmology in the matter power spectrum at scales down to $k \leq 3h~/{\rm Mpc}$ within $2\%$ accuracy. We implement the parametrisations into ver.2.0 of the ReACT code.