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

Cosmic structures at small non-linear scales $k>L\sim 0.2 h$ 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 $Λ$CDM universe applying the effective fluid method of Baumann, Nicolis, Senatore and Zaldarriaga. For $k<L$ the $Λ$CDM 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 $ϕ$ - $ψ\neq 0$. 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.

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