A framework for testing isotropy with the cosmic microwave background

• Published in: Monthly notices of the Royal Astronomical Society Vol. 462; no. 2; pp. 1802 - 1811
• Main Authors: Saadeh, Daniela, Feeney, Stephen M., Pontzen, Andrew, Peiris, Hiranya V., McEwen, Jason D.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 21-10-2016
• Subjects: Anisotropy; Confidence intervals; Cosmic background radiation; Cosmic microwave background; Cosmology; Isotropy; Mathematical analysis; Mathematical models; Shear; Statistical analysis; Tensors; Universe; cosmology: miscellaneous; gravitation; cosmic background radiation; early Universe
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stw1731

We present a new framework for testing the isotropy of the Universe using cosmic microwave background data, building on the nested-sampling anicosmo code. Uniquely, we are able to constrain the scalar, vector and tensor degrees of freedom alike; previous studies only considered the vector mode (linked to vorticity). We employ Bianchi type VII
h
cosmologies to model the anisotropic Universe, from which other types may be obtained by taking suitable limits. In a separate development, we improve the statistical analysis by including the effect of Bianchi power in the high-ℓ, as well as the low-ℓ, likelihood. To understand the effect of all these changes, we apply our new techniques to Wilkinson Microwave Anisotropy Probe data. We find no evidence for anisotropy, constraining shear in the vector mode to (σ
V
/H)0 < 1.7 × 10−10 (95 per cent confidence level). For the first time, we place limits on the tensor mode; unlike other modes, the tensor shear can grow from a near-isotropic early Universe. The limit on this type of shear is (σ
T, reg/H)0 < 2.4 × 10− 7 (95 per cent confidence level).