Testing the Cosmic Anisotropy with Supernovae Data: Hemisphere Comparison and Dipole Fitting

TL;DR

This paper investigates cosmic anisotropy using supernova data by comparing hemisphere and dipole fitting methods, revealing their differing effectiveness in detecting multiple preferred directions.

Contribution

It systematically compares hemisphere and dipole fitting methods on supernova datasets, highlighting their strengths and limitations in identifying cosmic anisotropy.

Findings

Both methods generally agree when a single preferred direction exists.

Dipole fitting often fails with multiple preferred directions.

Hemisphere comparison remains effective in complex anisotropy cases.

Abstract

The cosmological principle is one of the cornerstones in modern cosmology. It assumes that the universe is homogeneous and isotropic on cosmic scales. Both the homogeneity and the isotropy of the universe should be tested carefully. In the present work, we are interested in probing the possible preferred direction in the distribution of type Ia supernovae (SNIa). To our best knowledge, two main methods have been used in almost all of the relevant works in the literature, namely the hemisphere comparison (HC) method and the dipole fitting (DF) method. However, the results from these two methods are not always approximately coincident with each other. In this work, we test the cosmic anisotropy by using these two methods with the Joint Light-Curve Analysis (JLA) and simulated SNIa datasets. In many cases, both methods work well, and their results are consistent with each other. However, in the cases with two (or even more) preferred directions, the DF method fails while the HC method still works well. This might shed new light on our understanding of these two methods.