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Observations of the cosmic microwave background radiation (CMB) have greatly advanced our understanding of the early universe. Among the four Stokes parameters in conventional CMB measurements, the T-mode (temperature) and the E- and B-modes (linear polarization) have already been detected, whereas the V-mode, corresponding to circular polarization, remains unobserved. Upcoming experiments such as the Simons Observatory and LiteBIRD satellite are expected to improve sensitivity to V-mode signals by two orders of magnitude compared to current capabilities. In standard cosmology, the V-mode amplitude is expected to be negligibly small, implying that its detection would point to new physics beyond the standard model of cosmology. In this talk, I present a novel and concrete mechanism in which hypothetical dark photons generate a measurable V-mode polarization signal. |
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