Kilobyte Cosmic Birefringence from ALP Domain Walls

Understand

ALP domain walls without strings may be formed in the early Universe.

• We point out that such ALP domain walls lead to both isotropic and anisotropic birefringence of cosmic microwave background (CMB) polarization, which reflects spatial configuration of the domain walls at the recombination.
• The polarization plane of the CMB photon coming from each domain is either not rotated at all or rotated by a fixed angle.
• For domain walls following the scaling solution, the cosmic birefringence of CMB is characterized by $2^{N}$, i.e.