Cosmic microwave background constraints on cosmological models with large-scale isotropy breaking

TL;DR

This paper investigates models that break large-scale isotropy in the cosmic microwave background, assessing their ability to explain observed anomalies and their impact on cosmological parameter estimation.

Contribution

It evaluates the evidence for isotropy-breaking models using Bayesian analysis and examines their effects on cosmological parameters, especially the power spectrum normalization.

Findings

Weak Bayesian evidence supports isotropy-breaking models.

Only the power spectrum normalization is significantly affected.

Models with dipole/quadrupole terms do not strongly explain anomalies.

Abstract

Several anomalies appear to be present in the large-angle cosmic microwave background (CMB) anisotropy maps of WMAP, including the alignment of large-scale multipoles. Models in which isotropy is spontaneously broken (e.g., by a scalar field) have been proposed as explanations for these anomalies, as have models in which a preferred direction is imposed during inflation. We examine models inspired by these, in which isotropy is broken by a multiplicative factor with dipole and/or quadrupole terms. We evaluate the evidence provided by the multipole alignment using a Bayesian framework, finding that the evidence in favor of the model is generally weak. We also compute approximate changes in estimated cosmological parameters in the broken-isotropy models. Only the overall normalization of the power spectrum is modified significantly.