Anisotropy of the Universe via the Pantheon supernovae sample revisited

We employ the hemisphere comparison (HC) method and the dipole fitting (DF) method to investigate the cosmic anisotropy in the recently released Pantheon sample of type Ia supernovae (SNe Ia) and five combinations among Pantheon. For the HC method, we find that the maximum anisotropy level in the full Pantheon sample is ALmax = 0. 361 ± 0. 070 and corresponding direction | (l, b) = (123. 05^{ }^+11. 25^{ }-₄. ₂₂^, 4. 78^{ }^+1. 80^{ }-₈. ₃₆^) |⁠. A robust check shows that the statistical significance of maximum anisotropy level is about 2. 1σ. We also find that the low-z and SNLS subsamples have decisive impact on the overall anisotropy while other three subsamples have little impact. Moreover, the anisotropy level map significantly relies on the inhomogeneous distribution of SNe Ia in the sky. For the DF method, we find that the dipole anisotropy in the Pantheon sample is very weak. The dipole magnitude is constrained to be less than 1. 16 × 10−3 at |95{\ per\ cent}| confidence level. However, the dipole direction is well inferred by Markov chain Monte Carlo method and it points towards | (l, b) = (306. 00^{ }^+82. 95^{ }-₁₂₅. ₀₁^, -34. 20^{ }^+16. 82^{ }-₅₄. ₉₃^) |⁠. This direction is very close to the axial direction to the plane of SDSS subsample. It may imply that SDSS subsample is the decisive part to the dipole anisotropy in the full Pantheon sample. All these facts imply that the cosmic anisotropy found in Pantheon sample significantly relies on the inhomogeneous distribution of SNe Ia in the sky. More homogeneous distribution of SNe Ia is necessary to search for a more convincing cosmic anisotropy.