Constraints on cosmic birefringence using $E$-mode polarisation

Understand

A birefringent universe could show itself through a rotation of the plane of polarisation of the cosmic microwave background photons.

• This is usually investigated using polarisation $B$ modes, which is degenerate with miscalibration of the orientation of the polarimeters.
• Here we point out an independent method for extracting the birefringence angle using only temperature and $E$-mode signals.
• We forecast that, with an ideal cosmic-variance-limited experiment, we could constrain a birefringence angle of $0.3^\circ$ with $3\,\sigma$ statistical significance, which is close to the current constraints using $B$ modes.