Non-linear mode coupling and the growth of perturbations in ΛCDM

Abstract

Cosmic structures at small non-linear scales k > L ∼ 0.2h Mpc−1 have an impact on the longer (quasi-)linear wavelengths with k < L via non-linear UV-IR mode coupling. We evaluate this effect for a ΛCDM universe applying the effective fluid method of Baumann, Nicolis, Senatore and Zaldarriaga. For k < L the ΛCDM growth function for the density contrast is found to receive a scale dependent correction and an effective anisotropic stress sources a shift between the two gravitational potentials, setting ϕ−ψ≠0. Since such a situation is generically considered as a signature of modified gravity and/or dark energy, these effects should be taken into account before any conclusions on the dark sector are drawn from the interpretation of future observations.