$n\pi$ phase ambiguity of cosmic birefringence

TL;DR

The paper discusses a phase ambiguity in the measurement of cosmic birefringence rotation angle, which affects the interpretation of its origin, and explores ways to partially resolve this ambiguity using anisotropic signals and power spectrum shapes.

Contribution

It identifies a $n\pi$ phase ambiguity in cosmic birefringence measurements and analyzes how anisotropic birefringence and power spectrum shapes can help resolve it.

Findings

Phase ambiguity significantly impacts interpretation of cosmic birefringence.

Anisotropic birefringence can partly break the phase ambiguity.

Existing experimental constraints on the birefringence angle are discussed.

Abstract

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.