$n\pi$ phase ambiguity of cosmic birefringence

Understand

We point out that the rotation angle $\beta$ of cosmic birefringence, which is a recently reported parity-violating signal in the cosmic microwave background (CMB), has a phase ambiguity of $n\pi \,(n\in\mathbb{Z})$.

• This ambiguity has a significant impact on the interpretation of the origin of cosmic birefringence.
• Assuming an axion-like particle as the origin of cosmic birefringence, this ambiguity can be partly broken by the anisotropic cosmic birefringence and the shape of the CMB angular power spectra.
• We also discuss constraints on $\beta$ from existing experimental results.