# On the road to percent accuracy II: calibration of the non-linear matter power spectrum for arbitrary cosmologies

## TL;DR

This paper presents an emulator that predicts the non-linear matter power spectrum for various cosmologies beyond $\\Lambda$CDM using only standard $\\Lambda$CDM simulations, achieving high accuracy and enabling efficient testing of extended models.

## Contribution

It introduces a novel emulator approach that leverages standard simulations and analytical models to accurately predict non-linear spectra for extended cosmologies, reducing computational costs.

## Key findings

- Emulator achieves errors less than 1% in non-linear regime for tested cosmologies.
- Method effectively models $f(R)$ gravity and massive neutrinos.
- Reduces need for expensive beyond-$\\Lambda$CDM$ simulations.

## Abstract

We introduce an emulator approach to predict the non-linear matter power spectrum for broad classes of beyond-$\Lambda$CDM cosmologies, using only a suite of $\Lambda$CDM $N$-body simulations. By including a range of suitably modified initial conditions in the simulations, and rescaling the resulting emulator predictions with analytical `halo model reactions', accurate non-linear matter power spectra for general extensions to the standard $\Lambda$CDM model can be calculated. We optimise the emulator design by substituting the simulation suite with non-linear predictions from the standard {\sc halofit} tool. We review the performance of the emulator for artificially generated departures from the standard cosmology as well as for theoretically motivated models, such as $f (R)$ gravity and massive neutrinos. For the majority of cosmologies we have tested, the emulator can reproduce the matter power spectrum with errors $\lesssim 1\%$ deep into the highly non-linear regime. This work demonstrates that with a well-designed suite of $\Lambda$CDM simulations, extensions to the standard cosmological model can be tested in the non-linear regime without any reliance on expensive beyond-$\Lambda$CDM simulations.