Is the cold spot responsible for the CMB North-South asymmetry?

Several intriguing phenomena, unlikely within the standard inflationary cosmology, were reported in the cosmic microwave background (CMB) data from the Wilkinson Microwave Anisotropy Probe and appear to be uncorrelated. Two of these phenomena, termed CMB anomalies, are representative of their disparate nature: the North-South asymmetry in the CMB angular-correlation strength, inconsistent with an isotropic universe, and the cold spot, producing a significant deviation from Gaussianity. We find a correlation between them, at medium angular scales ($\ell =11–20$): we show that a successive diminution of the cold spot (absolute value) temperature implies a monotonic decrease of the North-South asymmetry power, and moreover we find that the cold spot supplies 60% of such power.

Figure 1

Top: This is the V map with multipole components $\ell =11–20$, obtained using the KQ75 mask. Bottom: This plot shows the temperature distribution of this V map, considering data outside the KQ75 region, where (i) the straight (blue) line is the expected Gaussian distribution with variance ${\sigma }_{\mathrm{G}}^2=4.65\times {10}^{-4}{\mathrm{mK}}^2$, (ii) the histogram (red) curve corresponds to the negative pixel’s temperatures, (iii) the dashed (violet) line corresponds to the positive pixel’s temperatures, and (iv) the triangles represent the distribution of the negative pixels’ temperatures corresponding to a modified V map where the cold-spot temperature is 70% of its original value.

Figure 2

Top: This is the sigma-map WMAP-V obtained using ${\gamma }_0=45°$, $N_{\mathrm{bins}}=45$, and $N_{\mathrm{caps}}=3072$. The NS-asymmetry phenomenon is clearly seen in the uneven hemispherical distribution of the angular correlation’s strengths, depicted as red and blue pixels (large and small ${\sigma }_J$, respectively). Bottom: Angular power spectra of the sigma map shown above (100%-case) and sigma maps from several cases: $X=90$, 70, 50, 30, and 0, where the cold-spot temperature in the V map is a fraction, $X\%$, of its original value. The solid (dashed) line corresponds to the mean (95.4% C.L.) spectra of 1000 sigma-maps MC. The statistical significance of the NS-asymmetry is revealed at 97.6% C.L. in the 100%-case, while for the 70%-case it is 91.5% C.L. The dipole component of the above sigma map, displayed in the inset, points toward $(l,b)\simeq (225°,-45°)$.