Can a primordial magnetic field originate large-scale anomalies inWMAPdata?

Abstract

Several accurate analyses of the cosmic microwave background (CMB) temperature maps from the Wilkinson Microwave Anisotropy Probe (WMAP) have revealed a set of anomalous results, at large angular scales, that appears inconsistent with the statistical isotropy expected in the concordance cosmological model Λcold dark matter. Because these anomalies seem to indicate a preferred direction in the space, here we investigate the signatures that a primordial magnetic field, possibly present in the photon–baryon fluid during the decoupling era, could have produced in the large-angle modes of the observed CMB temperature fluctuations maps. To study these imprints, we simulate Monte Carlo CMB maps, which are statistically anisotropic due to the correlations between CMB multipoles induced by the magnetic field. Our analyses reveal the presence of the north–south angular correlations asymmetry phenomenon in these Monte Carlo maps, and we use these information to establish the statistical significance of such phenomenon observed in WMAP maps. Moreover, because a magnetic field produces planarity in the low-order CMB multipoles, where the planes are perpendicular to the preferred direction defined by the magnetic field, we investigate the possibility that two CMB anomalous phenomena, namely the north–south asymmetry and the quadrupole–octopole planes alignment, could have a common origin. Our results, for large angles, show that the correlations between low-order CMB multipoles introduced by a sufficiently intense magnetic field, can reproduce some of the large-angle anisotropic features mapped in WMAP data. We also reconfirm, at more than 95 per cent CL, the existence of a north–south power asymmetry in the WMAP 5-yr data.