Cosmological constraints on anisotropic Thurston geometries

Much of modern cosmology relies on the Cosmological Principle, the assumption that the Universe is isotropic and homogeneous on sufficiently large scales, but it remains worthwhile to examine cosmological models that violate this principle slightly. We examine a class of such spacetimes that maintain homogeneity but break isotropy through their underlying local spatial geometries. These spacetimes are endowed with one of five anisotropic model geometries of Thurston's geometrization theorem, and their evolution is sourced with perfect fluid dust and cosmological constant. We show that the background evolution of these spacetimes induces fluctuations in the observed cosmic microwave background (CMB) temperature with amplitudes coupled to the curvature parameter $\Omega_K$. In order for these fluctuations to be compatible with the observed CMB angular power spectrum, we find $|\Omega_K| \lesssim 10^{-5}$ is required in all five geometries, with two geometries requiring $|\Omega_K| \lesssim 10^{-10}$. This strongly limits the cosmological consequences of these models.