Anisotropic Generalization of the {\Lambda}CDM Universe Model with Application to the Hubble Tension

TL;DR

This paper derives an exact anisotropic Bianchi type I solution extending the LambdaCDM model to explore if cosmic anisotropy can resolve the Hubble tension, concluding it is too small to do so.

Contribution

It presents the first exact analytic anisotropic generalization of the LambdaCDM universe model suitable for analyzing the Hubble tension.

Findings

Anisotropic expansion is too small to resolve the Hubble tension.

Exact solutions show anisotropy cannot significantly affect early universe nucleosynthesis.

Previous suggestions of anisotropy solving the Hubble tension are not supported by the exact model.

Abstract

I deduce an exact and analytic Bianchi type I solution of Einsteins field equations, which generalizes the isotropic LambdaCDM universe model to a corresponding model with anisotropic expansion. The main point of the article is to present the anisotropic generalization of the LambdaCDM universe model in a way suitable for investigating how anisotropic expansion modifies observable properties of the LambdaCDM universe model. Although such generalizations of the isotropic LambdaCDM universe model have been considered earlier, they have never been presented in this form before. Several physical properties of the model are pointed out and compared with properties of special cases, such as the isotropic LambdaCDM universe model. The solution is then used to investigate the Hubble tension. It has recently been suggested that the cosmic largescale anisotropy may solve the Hubble tension. I consider those earlier suggestions and find that the formulae of these papers lead to the result that the anisotropy of the cosmic expansion is too small to solve the Hubble tension. Then, I investigate the problem in a new way, using the exact solution of the field equations. This gives the result that the cosmic expansion anisotropy is still too small to solve the Hubble tension in the general Bianchi type I universe with dust and LIVE (Lorentz Invariant Vacuum Energy with a constant energy density, which is represented by the cosmological constant) and anisotropic expansion in all three directions even if one neglects the constraints coming from the requirement that the anisotropy should be sufficiently small so that it does not have any significant effect upon the results coming from the calculations of the comic nucleosynthesis during the first ten minutes of the universe. If this constraint is taken into account, the cosmic expansion anisotropy is much too small to solve the Hubble tension.