Searching for a preferred direction with Union2.1 data

A cosmological preferred direction was reported from the Type Ia supernovae data in recent years. We use the Union2.1 data to give a simple classification of such studies for the first time. Because the maximum anisotropic direction is independent of isotropic dark energy models, we adopt two cosmological models ( CDM, wCDM) for the hemisphere comparison analysis and the cold dark matter ( CDM) model for a dipole-fitting approach. In hemisphere comparison method, the matter density and the equation of state of dark energy are adopted as the diagnostic qualities in the CDM model and the wCDM model, respectively. In the dipole-fitting approach, we fit the fluctuation of distance modulus. We find that there is a null signal for the hemisphere comparison method, while a preferred direction (b = -14. 3 10. 1, l = 307.1 16.2 ) for the dipole-fitting method. This result indicates that the dipole fit is more sensitive than the hemisphere comparison method.