### Initial conditions for cosmological N-body simulations of the scalar sector of theories of Newtonian, Relativistic and Modified Gravity *[Cross-Listing]*

(0 votes over all institutions)

We provide a prescription for setting initial conditions for cosmological N-body simulations, which simultaneously employ Lagrangian meshes (`particles’) and Eulerian grids (`fields’). Our description is based on coordinate systems in arbitrary geometry, and can therefore be used in any metric theory of gravity. We apply our prescription to a choice of Effective Field Theory of Modified Gravity, and show how already in the linear regime, particle trajectories are curved. For some viable models of modified gravity, the Dark Matter trajectories are affected at the level of 5% at Mpc scales. Moreover, we show initial conditions for a simulation where a scalar modification of gravity is modelled in a Lagrangian particle-like description.