Anisotropy of Cosmic Acceleration

Abstract: In this paper, we study the anisotropy of cosmic acceleration by dividing the Union2 Type Ia supernova dataset into 12 subsets according to their positions in Galactic coordinate system. In each region, we derive the deceleration parameter $q_0$ as the diagnostic to quantify the anisotropy level in the corresponding direction, and construct $q_0$ anisotropic maps by combining these $q_0$ values. In addition to the monopole component, we find the significant dipole effect in the $q_0$-maps with the amplitude $A_1=0.466^{+0.255}_{-0.205}$, which deviates from zero at more than 2-$\sigma$ level. The direction of the best-fit dipole is ($\theta=108.8^{\circ}$, $\phi=187.0^{\circ}$) in Galactic system. Interesting enough, we find the direction of this dipole is nearly perpendicular to the CMB kinematic dipole, and the angle between them is $95.7^{\circ}$. The perpendicular relation is anomalous at the 1-in-10 level.