Cosmic microwave background bispectrum on small angular scales

TL;DR

This paper analyzes the non-linear evolution of cosmological perturbations on small scales, showing it causes deviations from Gaussianity in the CMB bispectrum, with implications for primordial non-Gaussianity measurements.

Contribution

It provides an analytical estimate of the CMB bispectrum induced by non-linear dark matter evolution on small angular scales, a novel contribution to understanding non-Gaussianity.

Findings

Non-linear evolution contributes a bispectrum equivalent to fNL~25.

Deviation from Gaussianity is significant on scales l~1000 to l~3000.

Dark matter non-linearities dominate modes coupling on small scales.

Abstract

This article investigates the non-linear evolution of cosmological perturbations on sub-Hubble scales in order to evaluate the unavoidable deviations from Gaussianity that arise from the non-linear dynamics. It shows that the dominant contribution to modes coupling in the cosmic microwave background temperature anisotropies on small angular scales is driven by the sub-Hubble non-linear evolution of the dark matter component. The perturbation equations, involving in particular the first moments of the Boltzmann equation for the photons, are integrated up to second order in perturbations. An analytical analysis of the solutions gives a physical understanding of the result as well as an estimation of its order of magnitude. This allows to quantify the expected deviation from Gaussianity of the cosmic microwave background temperature anisotropy and, in particular, to compute its bispectrum on small angular scales. Restricting to equilateral configurations, we show that the non-linear evolution accounts for a contribution that would be equivalent to a constant primordial non-Gaussianity of order fNL~25 on scales ranging approximately from l~1000 to l~3000.