Anisotropic Universes in Light of Background Cosmological Observations

TL;DR

This paper investigates the possibility of small anisotropic expansions in the universe by constraining the shear parameter using recent cosmological data, challenging the assumption of isotropy in standard cosmology.

Contribution

It introduces anisotropic models within the Bianchi I framework, constrains the shear parameter with observational data, and explores their implications for cosmological assumptions.

Findings

The shear parameter $oldsymbol{ ext{Sigma}_0}$ can be non-zero, with some models excluding $ ext{Sigma}_0=0$ at 2$oldsymbol{\sigma}$ confidence.

Anisotropic models are compatible with current data but generally less favored than $oldsymbol{ ext{Lambda CDM}}$ due to additional parameters.

A specific scalar field coupled to a 2-form field can produce a shear around $oldsymbol{10^{-4}}$, challenging the isotropy assumption.

Abstract

The cosmological principle is a cornerstone of the standard cosmological model. However, recent observations suggest potential deviations from this assumption, hinting at a small anisotropic expansion. Such an expansion can arise from sources that break rotational invariance. A minimal realization of this scenario is described by a Bianchi I geometry, where the degree of anisotropy is quantified by the shear parameter $\Sigma$. In this work, we constrain the present-day value of the shear, $\Sigma_0$, by confronting theoretical predictions with recent cosmological data. We implement various anisotropic models within the Boltzmann code \texttt{CLASS} and explore their parameter space using the sampler \texttt{MontePython}. Although our results show that $\Sigma_0$ is model-dependent, notably, in one specific scenario considering a homogeneous scalar field coupled to a 2-form field, $\Sigma_0 = 0$ is excluded at the $2\sigma$ confidence level, with mean value around $|\Sigma_0| \sim 10^{-4}$ while remaining consistent with observations. These findings challenge the conventional assumption that cosmic shear is negligible in the present universe. Moreover, the anisotropic expansion in this model is driven by a steep scalar field potential, a feature often found in supergravity-inspired scenarios. While anisotropic models offer interesting alternatives and could help explain some cosmological anomalies, they generally introduce additional parameters, making the standard $\Lambda$CDM model statistically favored in most cases. Still, they remain compatible with current observations and provide new perspectives on features not fully explained within the standard framework. These results highlight the importance of further exploring anisotropic cosmologies to better understand their implications.