Testing the isotropy of the Universe with the Cosmic Microwave Background

• Main Author: Saadeh, D.
• Other Authors: Pontzen, A. ; Peiris, H. V.
• Published: University College London (University of London) 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.746451

A fundamental assumption in the standard model of cosmology is that the Universe is isotropic on large scales. Breaking this assumption leads to a set of solutions to Einstein’s field equations known as Bianchi cosmologies, of which only the subset linked to universal rotation have ever been tested against data. For the first time, we consider all the degrees of freedom in these solutions to conduct a general test of isotropy using cosmic microwave background data. We develop a new analysis framework for this study. We first analyse WMAP temperature data to test our method against previous studies searching for universal rotation. We include the effect of Bianchi power at the intermediate and small scales (i.e. up to l = 1000), and show that failure to do so results in inaccurate constraints on a significant fraction of the parameter space. We carefully assess the effects of prior choices and show that evidence for global rotation found in previous studies relies on specific a priori assumptions on some parameters. To carry out the first test of the fully anisotropy freedom, we analyse recent data from the Planck mission including, for the first time, the cosmic microwave background (CMB) polarization in the likelihood in addition to the temperature. For the vector mode (associated with vorticity) we obtain a limit on the anisotropic expansion of (σV/H)0 < 4.7 × 10^−11 (95% CI), which is an order of magnitude tighter than previous Planck results that used CMB temperature only. We also place upper limits on other modes of anisotropic expansion, with the weakest limit arising from the regular tensor mode, (σT,reg/H)0 < 1.0 × 10^−6 (95% CI). Including all degrees of freedom simultaneously for the first time, anisotropic expansion of the Universe is strongly disfavoured, with odds of 121,000:1 against.