Kilobyte Cosmic Birefringence from ALP Domain Walls
  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. $N$-bit, of information with $N = {\cal O}(10^{3-4})$ being equal to the
number of domains at the last scattering surface, and thus the name, $kilobyte~
cosmic~ birefringence$. The magnitude of the isotropic birefringence is
consistent with the recently reported value, while the anisotropic one is
determined by the structure of domains at the last scattering surface. The
predicted cosmic birefringence is universal over a wide range of the ALP mass
and coupling to photons. The detection of both signals will be a smoking-gun
evidence for the ALP domain walls without strings.
