How isotropic is the Universe?

TL;DR

This paper conducts a comprehensive test of the Universe's isotropy using Planck CMB data, placing tight constraints on anisotropic expansion modes and strongly disfavoring anisotropy.

Contribution

It performs the first all-degrees-of-freedom test of isotropy with CMB data, providing the most stringent limits on anisotropic expansion modes to date.

Findings

Vector mode anisotropic expansion limit: $(\sigma_V/H)_0 < 4.7 imes 10^{-11}$ (95% CI)

Tensor mode limit: $(\sigma_{T, m reg}/H)_0<1.0 imes 10^{-6}$ (95% CI)

Strong statistical evidence against Universe anisotropy, odds of 121,000:1.

Abstract

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, only a subset of which have ever been tested against data. For the first time, we consider all degrees of freedom in these solutions to conduct a general test of isotropy using cosmic microwave background temperature and polarization data from Planck. For the vector mode (associated with vorticity), we obtain a limit on the anisotropic expansion of $(\sigma_V/H)_0 < 4.7 \times 10^{-11}$ (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, $(\sigma_{T,\rm reg}/H)_0<1.0 \times 10^{-6}$ (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.