Accurate predictions from small boxes: variance suppression via the Zel'dovich approximation

Abstract Simulations have become an indispensable tool for accurate modelling of observables measured in galaxy surveys, but can be expensive if very large dynamic range scale is required. We describe how to combine Lagrangian perturbation theory models with N-body simulations reduce the effects finite computational volume prediction ensemble average properties within context control variates. In particular we use fact that Zel'dovich displacements, computed during initial condition generation any simulation, correlate strongly final density field. Since all correlators biased tracers arbitrary precision these pairing `simulation' realization allows hundredfold reductions sample variance power spectrum or correlation function estimation. variates accurately extend matter tracer field emulators larger scales than previously possible, as well improving measurements statistics which are inherently limited small volumes, such hydrodynamical formation and reionization.