A Hemispherical Power Asymmetry from Inflation

TL;DR

This paper explains the observed hemispherical power asymmetry in the CMB using a multi-field inflation model, specifically the curvaton scenario, which introduces a superhorizon perturbation to account for the asymmetry without violating homogeneity constraints.

Contribution

It demonstrates that single-field inflation cannot produce the asymmetry, but a multi-field curvaton model with a superhorizon perturbation can, providing testable predictions for future CMB observations.

Findings

Single-field inflation cannot generate the asymmetry.

Curvaton model can produce the power asymmetry.

Predictions include non-Gaussianity and modified inflationary relations.

Abstract

Measurements of CMB temperature fluctuations by the Wilkinson Microwave Anisotropy Probe (WMAP) indicate that the fluctuation amplitude in one half of the sky differs from the amplitude in the other half. We show that such an asymmetry cannot be generated during single-field slow-roll inflation without violating constraints to the homogeneity of the Universe. In contrast, a multi-field inflationary theory, the curvaton model, can produce this power asymmetry without violating the homogeneity constraint. The mechanism requires the introduction of a large-amplitude superhorizon perturbation to the curvaton field, possibly a pre-inflationary remnant or a superhorizon curvaton-web structure. The model makes several predictions, including non-Gaussianity and modifications to the inflationary consistency relation, that will be tested with forthcoming CMB experiments.