Abstract
Recently, BICEP2 measurements of the cosmic microwave background (CMB) B-mode polarization has indicated the presence of primordial gravitational waves at degree angular scales, inferring the tensor-to-scalar ratio of r=0.2 and a running scalar spectral index, provided that dust contamination is low. In this Letter, we show that the existence of the fluctuations of cosmological birefringence can give rise to CMB B-mode polarization that fits BICEP2 data with r<0.11 and no running of the scalar spectral index. When dust contribution is taken into account, we derive an upper limit on the cosmological birefringence, Aβ2<0.0075, where A is the amplitude of birefringence fluctuations that couple to electromagnetism with a coupling strength β.
Highlights
BICEP2 measurements of the cosmic microwave background (CMB) B-mode polarization has indicated the presence of primordial gravitational waves at degree angular scales, inferring the tensor-to-scalar ratio of r = 0.2 and a running scalar spectral index, provided that dust contamination is low
WMAP+SPT CMB data has placed an upper limit on the contribution of tensor modes to the CMB anisotropy, in terms of the tensor-to-scalar ratio, which is r < 0.18 at 95% confidence level, tightening to r < 0.11 when including measurements of the Hubble constant and baryon acoustic oscillations (BAO) [7]
Planck Collaboration XVI has quoted r < 0.11 using a combination of Planck, SPT, and ACT anisotropy data, plus WMAP polarization; the constraint relaxes to r < 0.26 (95% confidence) when running of the scalar spectral index is allowed with dns/d ln k = −0.022±0.010 (68%) [8]
Summary
Horizonsized tensor perturbation induces large-scale temperature anisotropy of the cosmic microwave background (CMB) via the Sachs-Wolfe effect [5]. WMAP+SPT CMB data has placed an upper limit on the contribution of tensor modes to the CMB anisotropy, in terms of the tensor-to-scalar ratio, which is r < 0.18 at 95% confidence level, tightening to r < 0.11 when including measurements of the Hubble constant and baryon acoustic oscillations (BAO) [7]. BICEP2 CMB experiment has found an excess of B-mode power at degree angular scales, indicating the presence of tensor modes with r = 0.20+−00..0075 and dns/d ln k = −0.028 ± 0.009 [9].
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