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The POLARBEAR experiment has set new upper limits on parity violating processes or the existence of an equivalent primordial magnetic field. These would induce anisotropic cosmic birefringence, which convert parity-even (E-modes) to parity-odd (B-modes) of the polarized component of the cosmic microwave background anisotropy. The new limits are important for further constraining models of the early Universe.
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Ade et al. (2015) studied this question.
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