  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 $\Lambda$CDM universe
applying the effective fluid method of Baumann, Nicolis, Senatore and
Zaldarriaga. For $k<L$ the $\Lambda$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 $\phi$ - $\psi \neq 0$. 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.
