How isotropic is the Universe? (1605.07178v2)

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.

• The paper introduces a comprehensive test of cosmic isotropy by analyzing CMB temperature and polarization data under Bianchi models.
• It employs the ANICOSMO₂ package with the PolyChord sampler to efficiently explore scalar, vector, and tensor shear modes.
• The study constrains anisotropic expansion, notably improving vector mode limits and reinforcing the isotropic assumption of the standard model.

Anisotropy in the Cosmic Microwave Background: Analyzing Bianchi Cosmologies

In this paper, "How isotropic is the Universe?" by Saadeh et al., the authors undertake a comprehensive analysis to test the isotropy assumption of the universe within the framework of modern cosmology. The assumption that the universe is isotropic, meaning it looks the same in all directions on large scales, is a cornerstone of the $\Lambda$CDM model, which relies on the Copernican principle. However, deviations from this isotropy hypothesis, known within the context of Bianchi cosmologies, may introduce cosmic anisotropies that imprint distinct properties on the cosmic microwave background (CMB) radiation.

The paper distinguishes itself by considering all degrees of freedom for the first time using the Bianchi metrics as the theoretical foundation, which relax the isotropy assumption but maintain homogeneity. The authors implement this comprehensive analysis using the Planck satellite's CMB temperature and polarization data, thereby significantly enhancing the constraints on potential anisotropies compared to previous studies that relied solely on CMB temperature data.

The methodological approach involves employing the ANICOSMO$_2$ package, enhanced to incorporate the PolyChord sampler for exploring a high-dimensional parameter space efficiently. This allows the simultaneous examination of scalar, vector, and two tensor shear modes arising from the Bianchi VII$_h$ model, each contributing differently to the anisotropy in CMB observations. The datasets include both temperature and polarization maps, which are particularly potent for the detection of specific shear effects due to its sensitivity to polarization patterns.

The results yield stringent constraints on anisotropic expansion, disfavoring any deviation from isotropy with considerable statistical confidence. Notably, the vector mode constraint on anisotropic expansion is improved by an order of magnitude compared to previous Planck results; the anisotropic expansion is constrained to be less than $4.7 \times 10^{-11}$ at a 95% confidence interval for the vector (vorticity) mode. The probabilistic odds against anisotropic expansion in the universe, when considering all modes simultaneously, stand overwhelmingly at 121,000:1, reaffirming isotropy's compatibility with the latest observational data.

The implications of these findings are significant for both theoretical and observational cosmology. By substantially tightening the constraints on cosmic anisotropies, the research solidifies the isotropic assumption of the standard model and restricts the parameter space for potential new physics that might suggest anisotropy. Furthermore, the integration of polarization data in the analysis offers a new paradigm in assessing CMB anisotropies, paving the way for future utilizations of polarization observations to refine cosmological models further.

Speculatively, as more sensitive observations become accessible with future missions, this framework can be employed to explore deeper into the subtle nuances of cosmic isotropy. Additionally, it might provide pivotal checks against emerging theories that predict anisotropy as a symptom of new cosmological phenomena or deeper structural frameworks beyond the scope of current models.

Overall, Saadeh et al.'s work represents a robust interrogation of one of cosmology’s fundamental assumptions—a characteristic that makes it an essential reference for developing cosmological theories and models.