Testing the isotropy of the Universe with type Ia supernovae in a model-independent way

TL;DR

This study tests the isotropy of the universe using type Ia supernovae in a model-independent way, fitting parameters simultaneously to constrain anisotropy levels and find results consistent with isotropy but allowing moderate anisotropy.

Contribution

It introduces a model-independent method to constrain universe anisotropy using SNe Ia data by fitting light-curve and cosmological parameters simultaneously.

Findings

Constraints on anisotropy parameters are consistent with isotropy.

No evidence found for a preferred direction in the universe.

Moderate anisotropy levels are still permissible.

Abstract

In this paper, we study an anisotropic universe model with Bianchi-I metric using Joint Light-curve Analysis (JLA) sample of type Ia supernovae (SNe Ia). Because light-curve parameters of SNe Ia vary with different cosmological models and SNe Ia samples, we fit the SNe Ia light-curve parameters and cosmological parameters simultaneously employing Markov Chain Monte Carlo method. Therefore, the results on the amount of deviation from isotropy of the dark energy equation of state ($\delta$), and the level of anisotropy of the large-scale geometry ($\Sigma_0$) at present, are totally model-independent. The constraints on the skewness and cosmic shear are $-0.101<\delta<0.071$ and $-0.007<\Sigma_0<0.008$. This result is consistent with a standard isotropic universe ($\delta=\Sigma_0=0$). However, a moderate level of anisotropy in the geometry of the Universe and the equation of state of dark energy, is allowed. Besides, there is no obvious evidence for a preferred direction of anisotropic axis in this model.