Parity in the cosmic microwave background: Space oddity

We search for a direction in the sky that exhibits parity symmetry under reflections through a plane. We use the natural estimator, which compares the power in even and odd ℓ + m multipoles, and apply minimal blind masking of outliers to the Internal Linear Combination map in order to avoid large errors in the reconstruction of multipoles. The multipoles of the cut sky are calculated both naively and by using the covariance inversion method, and we estimate the significance of our results using ΛCDM simulations. Focusing on low multipoles, 2 ≤ ℓ ≤ ℓ_max with ℓ_max = 5, 6, or even 7, we find two perpendicular directions of even and odd parity in the map. While the even parity direction does not appear significant, the odd direction is quite significant - at least a 3.6σ effect.