Redefining Big Bang Cosmology with a Fresh Perspective

Demonstrating the homogeneity and isotropy of the universe at massive sizes is a challenging task. This chapter aims to explore the impact of matter distribution in homogeneity up to the last scattering surface time within the framework of spatially homogeneous yet anisotropic models. These models give rise to anisotropies on large angular scales (larger than
> 2
) that closely resemble those found in the conventional FriedmannLemaitre-Robertson-Walker (FLRW) geometries.

Regarding these anisotropic models, the imprint left on the cosmic microwave background radiation due to primordial density fluctuations, manifested as fractional temperature variations, follows the same expression as in FLRW models. Precisely, under adiabatic initial conditions, we observe the retrieval of the classical Sachs-Wolfe effect, as the anisotropy in the overall cosmic expansion remains minimal at the time of the last scattering surface.

This finding aligns with prior research on these anisotropic models, indicating their gradual "isotropization" to a degree where observational distinctions from FLRW models become imperceptible, provided that the Hubble parameters along orthogonal directions are approximately equal to their current values. In conclusion, the observation of Sachs-Wolfe effect plateau does not permit us to distinguish between FLRW models and the anisotropic Kantowski-Sachs and Bianchi type-III models. This work also established upper bounds on the anisotropic parameters' present-day values based on COBE observations.