Anisotropy of the Universe via the Pantheon supernovae sample revisited (1903.12401v2)

Abstract: 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 the maximum anisotropy level in the full Pantheon sample is $\mathrm{AL}{max}=0.361\pm0.070$ and corresponding direction $(l,b)=({123.05^{\circ}}^{+11.25^{\circ}}{-4.22^{\circ}}, {4.78^{\circ}}^{+1.80^{\circ}}_{-8.36^{\circ}})$. A robust check shows the statistical significance of maximum anisotropy level is about $2.1\sigma$. 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 rely on the inhomogeneous distribution of SNe Ia in the sky. For the DF method, we find the dipole anisotropy in the Pantheon sample is very weak. The dipole magnitude is constrained to be less than $1.16\times10^{-3}$ at $95\%$ confidence level. However, the dipole direction is well inferred by MCMC method and it points towards $(l,b)=({306.00^{\circ}}^{+82.95^{\circ}}_{-125.01^{\circ}}, {-34.20^{\circ}}^{+16.82^{\circ}}_{-54.93^{\circ}})$. 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 rely 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.

• The paper demonstrates that re-analyzing the Pantheon sample with HC and DF methods reveals potential anisotropic signals in cosmic expansion.
• It finds that the HC method shows an anisotropy level of 0.361±0.070 in certain subsamples, highlighting the impact of sample inhomogeneities.
• The study emphasizes the need for more homogeneous SNe Ia data distributions to robustly validate cosmic anisotropy and challenge the cosmological principle.

Anisotropy of the Universe via the Pantheon Supernovae Sample Revisited

The paper explores the potential cosmic anisotropy within the Universe using the extensive Pantheon sample of type Ia supernovae (SNe Ia). The cosmological principle, foundational to many models, posits a homogeneous and isotropic Universe at large scales—an assertion supported by considerable cosmological observations such as the cosmic microwave background (CMB) radiation. However, recent high-precision observations have increasingly challenged this principle, suggesting potential deviations that imply a preferred cosmic direction and, consequently, anisotropy.

The authors use the Pantheon dataset, which includes 1048 spectroscopically confirmed SNe Ia, incorporating data from various surveys such as Pan-STARRS1, SDSS, SNLS, HST, and a compilation of Low-$z$ surveys. This dataset is notably more extensive and systematically cross-calibrated compared to previous datasets like Union2, Union2.1, and JLA, thereby reducing systematic uncertainties. This makes the Pantheon sample a robust dataset for testing the potential anisotropies in cosmic expansion.

Two methodologies were employed: the Hemisphere Comparison (HC) method and the Dipole Fitting (DF) method. The HC method found an anisotropy level of $\mathrm{AL}_{max}=0.361 \pm 0.070$ in the full Pantheon sample, with a directional maximum at galactic coordinates $$(l,b)=({123.05^{\circ}}^{+11.25^{\circ}}_{-4.22^{\circ}}, {4.78^{\circ}}^{+1.80^{\circ}}_{-8.36^{\circ}})$$, having marginal statistical significance at the $2.1\sigma$ level. Crucially, the analysis revealed the strongest anisotropy was linked predominantly to the Low-$z$ and SNLS subsamples, suggesting sample inhomogeneities affect anisotropy results significantly.

In contrast, the DF method indicated a weak dipole anisotropy in the Pantheon data, with the dipole magnitude constrained to be less than $1.16 \times 10^{-3}$ at a 95% confidence level, while still able to infer a dipole direction of $$(l,b)=({306.00^{\circ}}^{+82.95^{\circ}}_{-125.01^{\circ}}, {-34.20^{\circ}}^{+16.82^{\circ}}_{-54.93^{\circ}})$$. This directional finding is notably close to the axis of the SDSS subsample's plane, inferring SDSS might be a decisive contributor to perceived dipole anisotropy within the full dataset.

Such results, highlighting sample-dependent anisotropies, call for caution in interpreting cosmic anisotropies. In the HC method, the concentrated locations of SNe Ia in particular skies—such as their clustering in the galactic south-east—pose potential biases. Meanwhile, despite the DF method finding weak anisotropy, the possible aligning influence by the SDSS plane posits an intrinsic distribution influence rather than an ascertained cosmic directional anisotropy. Practically, the paper underlines the need for more homogeneous SNe Ia data distributions before substantiating cosmic anisotropy claims.

Theoretically, these findings maintain interest in investigating potential cosmic directionalities but accentuate the influence of uneven sample distributions. In future examinations with possibly improved sample distribution homogeneity, a reevaluation of methodologies like HC and DF coupled with larger datasets or alternate astronomical phenomena might further elucidate on cosmic isotropy or lack thereof. This would be integral in either reaffirming or challenging foundational principles such as the cosmological principle that underpin our understanding of the Universe.

Overall, this investigation contributes to the discourse on universal isotropy. It reinforces the necessity of critical scrutiny on data sampling when asserting the presence of large-scale cosmic anisotropies, a detail pivotal in fortifying future astrophysical studies and models.