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

TL;DR

This paper investigates how small-scale non-linear cosmic structures influence larger scales in a bcdm universe through non-linear mode coupling, revealing scale-dependent growth and potential signatures of modified gravity or dark energy.

Contribution

It applies the effective fluid method to quantify non-linear UV-IR mode coupling effects on large-scale structure growth in bcdm cosmology.

Findings

Scale-dependent correction to the bcdm growth function.

Effective anisotropic stress causes c and c potentials to differ.

Implications for interpreting dark energy and modified gravity signals.

Abstract

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.