Revisiting cosmic anisotropy with the Pantheon+ compilation

Abstract

We investigate cosmic anisotropy within the updated Pantheon+ sample using both the dipole fitting (DF) and hemisphere comparison (HC) methods. With the DF method, the dipole signal within the full sample is statistically weak. However, the low-$z$ subsample yields a dipole signal of $A_{\mathrm{D}}=0.952_{-0.403}^{+0.454}\times 10^{-3}$ at $\sim 2\ddot{I}ƒ$ significance, pointing towards $(l,b)=(149.77^{\circ },-12.20^{\circ })$. This signal is predominantly driven by a combined subset of surveys 5, 56, 63, and 150, which is characterized by an amplitude of $A_{\mathrm{D}}=1.730_{-0.715}^{+0.554}\times 10^{-3}$ towards $(l,b)=(153.05^{\circ },-1.25^{\circ })$. For the HC method, the full sample yields a maximum anisotropy level of ${\mathrm{AL}}_{\max }=0.289\pm 0.052$ oriented towards $(l,b)=(127.97^{\circ },17.90^{\circ })$ with a $1.56\ddot{I}ƒ$ significance. This preferred direction is primarily determined by the highly inhomogeneous SNLS subsample, whereas the low-$z$ and high-$z$ subsamples act to suppress the anisotropy level along this axis. These subsample-dependent results suggest that the apparent anisotropy arises from local structures or the inhomogeneous distribution of the datasets rather than an intrinsic cosmic anisotropy.

16 pages, 11 figures, 3 tables