CMB Power Asymmetry from Non-Gaussian Modulation

TL;DR

This paper explores how non-Gaussianity in inflationary perturbations can cause large-scale modulations in the CMB power spectrum, potentially explaining observed asymmetries without breaking isotropy.

Contribution

It demonstrates that infrared-divergent bispectra can produce significant CMB power asymmetry consistent with current non-Gaussianity constraints.

Findings

Non-Gaussianity can induce observable CMB power asymmetry.

Infrared divergence in the bispectrum is crucial for the effect.

Constraints on inflation duration arise from asymmetry limits.

Abstract

Non-Gaussianity in the inflationary perturbations can couple observable scales to modes of much longer wavelength (even superhorizon), leaving as a signature a large-angle modulation of the observed cosmic microwave background (CMB) power spectrum. This provides an alternative origin for a power asymmetry which is otherwise often ascribed to a breaking of statistical isotropy. The non-Gaussian modulation effect can be significant even for typical ~10^{-5} perturbations, while respecting current constraints on non-Gaussianity, if the squeezed limit of the bispectrum is sufficiently infrared divergent. Just such a strongly infrared-divergent bispectrum has been claimed for inflation models with a non-Bunch-Davies initial state, for instance. Upper limits on the observed CMB power asymmetry place stringent constraints on the duration of inflation in such models.