Searching for a preferred direction with Union2.1 data

Abstract

A cosmological preferred direction was reported from the type Ia supernovae (SNe Ia) 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 (ΛCDM, wCDM) for the hemisphere comparison analysis and ΛCDM model for dipole fit approach. In hemisphere comparison method, the matter density and the equation of state of dark energy are adopted as the diagnostic qualities in the ΛCDM model and wCDM model, respectively. In dipole fit 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 fit method. This result indicates that the dipole fit is more sensitive than the hemisphere comparison method.

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

“…An alternative method is to analyze the SNe Ia data in a model-independent way (Antoniou & Perivolaropoulos 2010;Mariano & Perivolaropoulos 2012;Cai & Tuo 2012;Cai et al 2013;Kalus et al 2013;Zhang et al 2013;Yang et al 2014). The advantage of this method is that it does not rely on the particular cosmological model.…”

The variation of the fine structure constant: testing the dipole model with thermonuclear supernovae

“…Also the direction of the dipole term is different in both cases, although their respective amplitudes are similar. Moreover, the direction of the dipole when the correct dependence on α is considered is at variance with the results of King et al (2012) for distant quasars and Yang et al (2014) for SNIa.…”

“…Moreover, it follows from Table 1 that the values of A, B and D are considerably different for the two sets of data studied here. In particular, the amplitude of the monopole term (A) is significantly larger when the complete Union 2.1 dataset is employed, and moreover for both datasets we obtain values that are considerably larger than that obtained by King et al (2012) employing the many multiplet method, and that of Yang et al (2014) using Type Ia supernovae, but disregarding the effects of a varying α. However, we remark that given the large uncertainties in the determination of A our results are compatible with a null result for model 2, which is obtained using the more reliable data.…”

“…The ultimate reason for this is that the magnitudes of the reported variations of α result in modest variations of the peak bolometric magnitudes of distant SNIa, and thus the differences in the positions of the SNIa of the Union 2.1 compilation are too small when compared with the leading terms intervening in the calculation of the luminosity distances of Type Ia supernova. To this regard, it is worth mentioning that Yang et al (2014) have found that the SNIa data can be better explained when a dipole model pointing towards (b = −14.3…”

“…They did so employing the Union 2 compilation of luminosity distances (Amanullah et al 2010;Suzuki et al 2012). More recently, Yang et al (2014) searched for a preferred direction using the Union 2.1 sample and found a preferred direction which can be well approximated by a dipole fit. However, none of these studies took into account the dependence of the Chandrasekhar limiting mass on the precise value of α.…”