Scale-dependent CMB power asymmetry from primordial speed of sound and a generalized $\delta$N formalism

TL;DR

This paper proposes a mechanism where spatial variations in the primordial sound speed during inflation lead to observable hemispherical power asymmetry in the CMB, using a generalized $ $ formalism that accounts for super-Hubble perturbations.

Contribution

It extends the $ $ formalism to include spatially varying sound speed, providing a novel explanation for CMB power asymmetry with scale-dependent features.

Findings

Asymmetry amplitudes are scale-dependent and consistent with observations.

The model constrains primordial non-Gaussianities.

The generalized formalism links super-Hubble perturbations to observable asymmetries.

Abstract

We explore a plausible mechanism that the hemispherical power asymmetry in the CMB is produced by the spatial variation of the primordial sound speed parameter. We suggest that in a generalized approach of the $\delta N$ formalism the local e-folding number may depend on some other primordial parameters besides the initial values of inflaton. Here the $\delta N$ formalism is extended by considering the effects of a spatially varying sound speed parameter caused by a super-Hubble perturbation of a light field. Using this generalized $\delta N$ formalism, we systematically calculate the asymmetric primordial spectrum in the model of multi-speed inflation by taking into account the constraints of primordial non-Gaussianities. We further discuss specific model constraints, and the corresponding asymmetry amplitudes are found to be scale-dependent, which can accommodate current observations of the power asymmetry at different length scales.