The axis of evil

Abstract

We examine previous claims for a preferred axis at $(b,l)\approx (60,-100)$ in the cosmic radiation anisotropy, by generalizing the concept of multipole planarity to any shape preference (a concept we define mathematically). Contrary to earlier claims, we find that the amount of power concentrated in planar modes for $\ell=2,3$ is not inconsistent with isotropy and Gaussianity. The multipoles' alignment, however, is indeed anomalous, and extends up to $\ell=5$ rejecting statistical isotropy with a probability in excess of 99.9%. There is also an uncanny correlation of azimuthal phases between $\ell=3$ and $\ell=5$. We are unable to blame these effects on foreground contamination or large-scale systematic errors. We show how this reappraisal may be crucial in identifying the theoretical model behind the anomaly.

Figures (3)

• Figure 1: The $\ell=5$ multipole for a cleaned map in galactic coordinates (top) and aligned with $(b,l)\approx (50,-91)$ (middle.) We have superposed the multipole vectors, and the chain linking them. For comparison we plotted the $\ell =3$ multipole in its preferred frame.
• Figure 2: A plot of $m$-preference results up to $\ell=20$. We seek the direction ${\mathbf n}_\ell$ (galactic coordinates in the bottom two plots) where a given $m$ (second plot from the top) receives the highest proportion of the power (with ratio $r_{\ell}$ plotted in the top panel). We have also plotted the Monte Carlo inferred variance for $r_\ell$. We plotted results for cleaned (solid), Wiener filtered (dash), and internal linear combination (dotted) maps as described in text.
• Figure 3: $m$-preference results up to $\ell=20$ for the V-band of galactic templates. We have included free-free (solid) syncroton (dot-dash) and dust (long-dash) maps; for reference we have reproduced results for the cleaned maps (dotted line).