Two-point anisotropies in WMAP and the cosmic quadrupole

Large-scale modes in the temperature anisotropy power spectrum C_l measured by the Wilkinson Microwave Anisotropy Probe (WMAP) seem to have lower amplitudes (C₂, C₃ and C₄) than that expected in the so-called concordance Λ-cold dark matter (ΛCDM) model. In particular, the quadrupole C₂ is reported to have a smaller value than allowed by cosmic variance. This has been interpreted as a possible indication of new physics. In this paper, we re-analyse the WMAP data using the two-point angular correlation and its higher-order moments. This method, which requires a full covariance analysis, is more direct and provides better sampling of the largest modes than the standard harmonic decomposition. We show that the WMAP data are in good agreement (~=30 per cent probability) with a ΛCDM model when the WMAP data are considered as a particular realization drawn from a set of realistic ΛCDM simulations with the corresponding covariance. This is also true for the higher-order moments, shown here up to sixth order, which are consistent with the Gaussian hypothesis. The sky mask plays a major role in assessing the significance of these agreements. We recover the best-fitting model for the low-order multipoles based on the two-point correlation with different assumptions for the covariance. Assuming that the observations are a fair sample of the true model, we find C₂= 123 +/- 233, C₃= 217 +/- 241 and C₄= 212 +/- 162 (in μK²). The errors increase by about a factor of 5 if we assume the ΛCDM model. If we exclude the Galactic plane |b| < 30 from our analysis, we recover very similar values within the errors (i.e. C₂= 172, C₃= 89, C₄= 129). This indicates that the Galactic plane is not responsible for the lack of large-scale power in the WMAP data.