Initial conditions for cosmological N-body simulations of the scalar sector of theories of Newtonian, Relativistic and Modified Gravity

TL;DR

This paper develops a method to set initial conditions for cosmological N-body simulations in various gravity theories, extending the Zel'dovich Approximation to account for scale-dependent effects and scalar modifications.

Contribution

It introduces a generalized approach for initial conditions in modified gravity simulations, including scale dependence and scalar field modeling, improving accuracy over standard methods.

Findings

Initial conditions can differ by up to 5% at Mpc scales for modified gravity.

Extended Zel'dovich Approximation accounts for curved particle paths in nontrivial theories.

Method enables more accurate simulations for testing gravity beyond the standard model.

Abstract

We present a description for setting initial particle displacements and field values for simulations of arbitrary metric theories of gravity, for perfect and imperfect fluids with arbitrary characteristics. We extend the Zel'dovich Approximation to nontrivial theories of gravity, and show how scale dependence implies curved particle paths, even in the entirely linear regime of perturbations. For a viable choice of Effective Field Theory of Modified Gravity, initial conditions set at high redshifts are affected at the level of up to 5% at Mpc scales, which exemplifies the importance of going beyond {\Lambda}-Cold Dark Matter initial conditions for modifications of gravity outside of the quasi-static approximation. In addition, we show initial conditions for a simulation where a scalar modification of gravity is modelled in a Lagrangian particle-like description. Our description paves the way for simulations and mock galaxy catalogs under theories of gravity beyond the standard model, crucial for progress towards precision tests of gravity and cosmology.