A violation of parity symmetry in electromagnetism would rotate the polarisation of the cosmic microwave background (CMB) in an effect known as cosmic birefringence. In the past, attempts to measure isotropic cosmic birefringence have been limited by the uncertainty in the calibration of the instrument’s polarisation angle. In this talk I will present the novel methodology that allowed us to bypass that limitation by using Galactic foreground emission as our calibrator. Its application to WMAP and Planck data yields a birefringence angle of β≈0.3º, with a statistical significance of 3σ. This measurement could be explained by a Chern-Simons coupling between photons and a pseudoscalar field like those predicted by ultra-light axionlike particles or Early Dark Energy. High-precision measurements of the CMB polarisation will allow us to distinguish between these two effects, potentially shedding more light on the Hubble tension.