Faraday scaling and the Bicep2 observations

Understand

As repeatedly speculated in the past, the linear polarization of the Cosmic Microwave Background can be rotated via the Faraday effect.

• An economic explanation of the recent Bicep2 observations, not relying on long-wavelength tensor modes of the geometry, would stipulate that the detected B mode comes exclusively from a Faraday rotated E mode polarization.
• We show hereunder that this interpretation is ruled out by the existing upper limits on the B mode polarization obtained by independent experiments at observational frequencies much lower than the operating frequency of the Bicep2 experiment.
• We then derive the fraction of the observed B mode polarization ascribable to the Faraday effect and suggest a dedicated experimental strategy for its detection.