Abstract
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.
Highlights
The Bicep2 Collaboration [1] recently measured the B-mode polarization for angular scales of the order of the degree and at a pivot frequency νp 1⁄4 150 GHz: GBl
If the whole effect is attributed to the primordial tensor modes, amplified by the pumping action of the gravitational field in a Friedmann-Robertson-Walker geometry, the present frequency of the corresponding gravitons would be Oð10−17Þ Hz
In what follows the sole assumptions will be instead that the tensor modes of the geometry are absent and that whole Bicep2 signal comes from the Faraday rotation of the polarization plane of the cosmic microwave background (CMB in what follows). If this is the case it will be shown that we are led into an interesting contradiction: direct upper limits on the B-mode polarization at frequencies ν ≪ νp are violated if the detected signal is ascribed to a purported Faraday effect
Summary
The Bicep2 Collaboration [1] recently measured the B-mode polarization for angular scales of the order of the degree and at a pivot frequency νp 1⁄4 150 GHz: GBl. The Bicep2 Collaboration [1] recently measured the B-mode polarization for angular scales of the order of the degree and at a pivot frequency νp 1⁄4 150 GHz: GBl lðl þ 2π where GlB denotes the angular power spectrum of the B-mode autocorrelation, l ≃ π=θ is the multipole moment and θ is the angular scale.
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