Comparison between hemisphere comparison method and dipole-fitting method in tracing the anisotropic expansion of the Universe use the Union2 dataset

TL;DR

This study compares hemisphere comparison and dipole-fitting methods in analyzing the anisotropic expansion of the Universe using the Union2 supernova dataset, revealing significant differences influenced by data distribution.

Contribution

It provides a comprehensive comparison of HC and DF methods for detecting cosmic anisotropy, highlighting their sensitivities and limitations with real data.

Findings

HC and DF methods produce approximately opposite preferred directions.

Results depend heavily on the sky distribution of supernovae.

Nonuniform data distribution may cause conflicting anisotropy signals.

Abstract

Type-Ia supernovae (SNe Ia) are often used as the standard candles to probe the anisotropic expansion of the Universe. In this paper, we make a comprehensive comparison between the hemisphere comparison (HC) method and dipole-fitting (DF) method in searching for the cosmological preferred direction using the Union2 dataset, a compilation of 557 well-calibrated SNe Ia. We find that the directions of the faintest SNe Ia derived from these two methods are approximately opposite. Monte Carlo simulations show that the results of the HC method strongly depend on the distribution of the data points in the sky. The coincidence that the HC method and DF method give two completely opposite directions may be due to the extremely nonuniform distribution of the Union2 dataset.