Ballardini, MarioGruppuso, AlessandroParadiso, Simone2026-01-122026-01-122025Ballardini, M., Gruppuso, A., Paradiso, S., Sirletti, S.S. and Natoli, P., 2025. Planck constraints on the scale dependence of isotropic cosmic birefringence. Journal of Cosmology and Astroparticle Physics, 2025(09), p.075.https://doi.org/10.1088/1475-7516/2025/09/075https://hdl.handle.net/10566/21655The rotation of the linear polarisation plane of photons during propagation, also known as cosmic birefringence, is a powerful probe of parity-violating extensions of standard electromagnetism. Using Planck legacy data, we confirm previous estimates of the isotropic birefringence angle, finding β ≃ 0.30±0.05 [deg] at 68% CL, not including the systematic error from the instrumental polarisation angle. If this is a genuine signal, it could be explained by theories of Chern-Simons-type coupled to electromagnetism, which could lead to a harmonic scale-dependent birefringence signal, if the hypothesis of an ultra-light (pseudo) scalar field does not hold. To investigate these models, we pursue two complementary approaches: first, we fit the birefringence angle estimated at different multipoles, βℓ, with a power-law model and second, we perform a non-parametric Bayesian reconstruction of it. Both methods yield results consistent with a non-vanishing constant birefringence angle. The first method shows no significant dependence on the harmonic scale (up to 1.8σ CL), while the second method demonstrates that a constant model is favored by Bayesian evidence. This conclusion is robust across all four published Planck CMB solutions. Finally, we forecast that upcoming CMB observations by Simons Observatory, LiteBIRD and a wishful CMB-Stage 4 experiment could reduce current uncertainties by a factor of approximately 7enCMBR experimentsCMBR polarisationCMBR theoryCosmic birefringenceIsotropic birefringence angleArticle