A viable explanation of the CMB dipolar statistical anisotropy

TL;DR

This paper examines the challenge of explaining the observed dipolar anisotropy in the CMB without conflicting with bounds on non-Gaussianity and quadrupole moments, and proposes an alternative model consistent with observations.

Contribution

It demonstrates the difficulty of models linking dipolar anisotropy with non-Gaussianity and introduces a new scenario that aligns with observational constraints.

Findings

Most existing models are nearly excluded by data.

Non-linear effects of dipolar anisotropy produce quadrupole and non-Gaussianity.

A simple alternative scenario can explain the anisotropy within observational bounds.

Abstract

The presence of a dipolar statistical anisotropy in the spectrum of cosmic microwave background (CMB) fluctuations was reported by the Wilkinson Microwave Anisotropy Probe (WMAP), and has recently been confirmed in the Planck 2013 analysis of the temperature anisotropies. At the same time, the Planck 2013 results report a stringent bound on the amplitude of the local-type non-Gaussianity. We show that the non-linear effect of the dipolar anisotropy generates not only a quadrupole moment in the CMB but also a local-type non-Gaussianity. Consequently, it is not easy to build models having a large dipolar modulation and at the same time a sufficiently small quadrupole and level of local bispectral anisotropy to agree with the present data. In particular, most models proposed so far are almost excluded, or are at best marginally consistent with observational data. We present a simple alternative scenario that may explain the dipolar statistical anisotropy while satisfying the observational bounds on both the quadrupole moment and local-type non-Gaussianity.