Comparison between the Planck observations and the model. The ratios ⟨ Q ⟩/⟨ I ⟩ and ⟨ U ⟩/⟨ I ⟩ derived from the polarization data are given by the magenta point. The statistical and systematic uncertainties are shown by the solid and dashed error bars, respectively. The dashed grey lines show the solutions for φ₀ = −5°, 0°, and 5°, with θ₀ varying from 10° to 80°. The solid grey lines indicate how the ratios change with φ₀, for a given θ₀ angle. The model results are calculated here for p₀ = 4%.