Directional Dependence of ΛCDM Cosmological Parameters

We study hemispherical power asymmetry in the Wilkinson Microwave Anisotropy Probe 9 yr data. We analyze the combined V- and W-band sky maps, after application of the KQ85 mask, and find that the asymmetry is statistically significant at the 3.4σ confidence level for l = 2-600, where the data are signal-dominated, with a preferred asymmetry direction (l, b) = (227, –27). Individual asymmetry axes estimated from six independent multipole ranges are all consistent with this direction. Subsequently, we estimate cosmological parameters on different parts of the sky and show that the parameters A_s, n_s , and Ω_b are the most sensitive to this power asymmetry. In particular, for the two opposite hemispheres aligned with the preferred asymmetry axis, we find n_s = 0.959 ± 0.022 and n_s = 0.989 ± 0.024, respectively.