Anisotropic cosmic birefringence estimates for forthcoming CMB experiments

Cosmic Birefringence (CB) is a unique probe of physics beyond the Standard Model, potentially sourced by an axion-like field coupled to photons via a Chern-Simons interaction. Recent Planck analyses have strengthened evidence for a non-zero isotropic rotation angle, yet its anisotropic component remains undetected, despite being the key to distinguishing a homogeneous quintessence-like field from spatially varying axion-like dark matter. We forecast the sensitivity of forthcoming ground- and space-based CMB experiments to anisotropic CB, applying a patch-based localization technique to simulated CMB-plus-noise maps with white noise levels representative of Simons Observatory SAT- and LiteBIRD-like sensitivities. Local rotation angles are reconstructed on independent sky patches by minimizing the $χ^2_{EB}$ statistic built from the $EB$ D-estimator. Under our idealized simulations, the forecast angular power spectrum improves current Planck-based constraints on the CB anisotropy statistical uncertainty by a factor of 26-40 for a satellite-like mission and 11-15 for a realistic ground-based footprint. Translating these into constraints on the scale-invariant amplitude, fitting the reconstructed bandpowers with a scale-invariant amplitude model, we forecast with a 95 \% C.L. $A_{\rm CB} < 3.5\times10^{-4}\,{\rm deg}^2$ for the space-like mission and $A_{\rm CB} < 7.1\times10^{-4}\,{\rm deg}^2$ for the ground-based footprint, improving on the current full-sky Planck constraint by a factor of about 20 and 10, respectively. These values represent upper bounds on the achievable gain, and indicate that forthcoming CMB experiments will be able to further shed light on anisotropic CB.

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Authors

Matteo Longo Minnolo; Matteo Billi; Giuseppe Puglisi

Published

Oct 2, 2026

Paper type

preprint

Source

Arxiv