A New Solution for the Observed Isotropic Cosmic Birefringence Angle and its Implications for the Anisotropic Counterpart through a Boltzmann Approach

Understand

Cosmic Birefringence (CB) is a phenomenon in which the polarization of the Cosmic Microwave Background (CMB) radiation is rotated as it travels through space due to the coupling between photons and an axion-like field.

• We look for a solution able to explain the result obtained from the \textit{Planck} Public Release 4 (PR4), which has provided a hint of detection of the CB angle, $\alpha=(0.30\pm0.11)^{\circ}$.
• In addition to the solutions, already present in the literature, which need a non-negligible evolution in time of the axion-like field during recombination, we find a new region of the parameter space which allows for a nearly constant time evolution of such a field in the same epoch.
• The latter reinforces the possibility to employ the commonly used relations connecting the observed CMB spectra with the unrotated ones, through trigonometric functions of the CB angle.