$\text{Hi-COLA}$: Fast, approximate simulations of structure formation in Horndeski gravity

Abstract

We introduce $\text{Hi-COLA}$, a code designed to run fast, approximate $\text{N}$-body simulations of non-linear structure formation in reduced Horndeski gravity. Given an input Lagrangian, $\text{Hi-COLA}$ dynamically constructs the appropriate field equations and consistently solves for the cosmological background, linear growth, and screened fifth force of that theory. Hence $\text{Hi-COLA}$ is a general, adaptable, and useful tool that allows the mildly non-linear regime of many Horndeski theories to be investigated for the first time, at low computational cost. In this work, we first describe the screening approximations and simulation setup of $\text{Hi-COLA}$ for theories with Vainshtein screening. We validate the code against traditional $\text{N}$-body simulations for cubic Galileon gravity, finding $2.5\%$ agreement up to $k_{\max }=1.2h/\mathrm{Mpc}$. To demonstrate the flexibility of $\text{Hi-COLA}$, we additionally run the first simulations of an extended shift-symmetric gravity theory. We use the consistency and modularity of $\text{Hi-COLA}$ to dissect how the modified background, linear growth, and screened fifth force all contribute to departures from $\Lambda$CDM in the non-linear matter power spectrum. $\text{Hi-COLA}$ can be found at https://github.com/Hi-COLACode/Hi-COLA .

51 pages, 20 figures, 5 tables, comments welcome. Updated to match the version published in JCAP

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