Testing the cosmic anisotropy with supernovae data: Hemisphere comparison and dipole fitting

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.PACS numbers: 98.80.Es, 95.36.+x *

Anisotropy of the Universe via the Pantheon supernovae sample revisited

“…However, the maximum anisotropy level in Sun & Wang (2018b) is ALmax = 0.136 +0.009 −0.005 , much less than 0.361 and the corresponding direction (l, b) = (37 • ± 40 • , 33 • ± 16 • ) totally deviates from the preferred direction in this paper, which could mean the 500 axes used in their work are not enough. The maximum anisotropy direction in the JLA sample (Deng & Wei 2018b) is not consistent with that in this paper, but the submaximum anisotropy direction (l, b) = (119.47 •+46.13 • −23.39 • , −28.39 •+17.01 • −6.52 • ) is about 33.35 • away from the preferred direction in this paper. Interestingly, in the Union2 sample (Antoniou & Perivolaropoulos 2010), the maximum anisotropy level is ALmax = 0.43 ± 0.06 and the corresponding direction (l, b) = (129 •+23 • −3 • , −18 •+10 • −11 • ) is 23.52 • away from the preferred direction in the Pantheon sample.…”

“…A similar cosmic anisotropy has been found in the Union2.1 sample (Yang et al 2014;Javanmardi et al 2015;Bengaly et al 2015;Li et al 2015;Lin et al 2016b). Different with the Union2 or Union2.1 sample, the JLA sample doesn't show any convincing evidence for the existence of the cosmic anisotropy (Bengaly et al 2015;Lin et al 2016a;Chang et al 2018a;Wang & Wang 2018;Deng & Wei 2018b;Sun & Wang 2018a). Constraining the anisotropic amplitude and direction in ΛCDM, ωCDM and CPL models with the JLA sample gave a zero result (Lin et al 2016a).…”

“…Constrained by the JLA sample, an anisotropic universe model with Bianchi-I metric was found to be consistent with the isotropic universe (Wang & Wang 2018). No significant dipole anisotropy (Deng & Wei 2018b;Sun & Wang 2018a) was found in JLA sample by using the CosmoMC (Lewis & Bridle 2002).…”

“…As standard candles, the supernovae of type Ia (SNe Ia) have been used to investigate the accelerating expansion of the Universe (Riess et al 1998;Perlmutter et al 1999), and the possible deviations from the isotropic universe ( Heneka et al 2014;Bengaly et al 2015;Li et al 2015;Javanmardi et al 2015;Lin et al 2016a,b;Salehi & Aftabi 2016;Salehi & Setare 2017;Li & Lin 2017;Ghodsi et al 2017;Wang & Wang 2018;Andrade et al 2018b;Chang et al 2018a;Deng & Wei 2018b), with the datasets given by the Union2 sample (Amanullah et al 2010), the Union2.1 sample (Suzuki et al 2012) and the "Joint Lightcurve Analysis" (JLA) sample (Betoule et al 2014). Antoniou & Perivolaropoulos (2010) first used the hemisphere comparison (HC) method to the Union2 sample and they found a certain cosmological preferred direction with maximum accelerating expansion rate.…”

Bianchi I and V cosmologies with Hu-Sawicki f(R) gravity in Palatini formalism

“…In addition, some recent probes and experiments, for example, Wilkinson Microwave Anisotropy Probe (WMAP) [32][33][34][35] and Planks results [36,37] also have confirmed the presence of small anisotropies. In a recent paper [38], the authors have used two methods, namely the hemisphere comparison (HC) method and dipole fitting (DF) method in probing the possible preferred direction in the distribution of type Ia supernovae (SNIa). They used these methods in order to test cosmic anisotropy with supernovae data.…”