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arXiv · 2610.09748

Cosmic Birefringence from Large Lepton Asymmetry

Kentaro Kasai·Ippei Obata
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Abstract

Cosmic birefringence, the rotation of the polarization plane of CMB photons, provides an intriguing probe of physics beyond the Standard Model. In this paper, we investigate a vector Chern-Simons coupling between the neutrino current and photons as a possible origin of isotropic cosmic birefringence. Firstly, we perform the first analysis of anisotropic cosmic birefringence induced by the neutrino current. We find that the anisotropic signal sourced by lepton-number isocurvature fluctuations can be important on large scales, whereas on small scales, the anisotropic signal induced by gravitational effects becomes dominant comparing with the axion-like particle (ALP) scenario. Secondly, we show that a large lepton asymmetry generated through the Affleck-Dine mechanism can successfully account for the observed isotropic birefringence, and discuss its possible connection to the recently reported low helium-4 abundances in Subaru EMPRESS and Atacama Cosmology Telescope (ACT).

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Kentaro Kasai, Ippei Obata. 2026-10-07. Cosmic Birefringence from Large Lepton Asymmetry. https://arxiv.org/abs/2610.09748

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