Cosmic birefringence tomography and calibration-independence with
  reionization signals in the CMB
  The search for cosmic polarization rotation or birefringence in the CMB is
well-motivated because it can provide powerful constraints on parity-violating
new physics, such as axion-like particles. In this paper we point out that
since the CMB polarization is produced at two very different redshifts - it is
generated at both reionization and recombination - new parity-violating physics
can generically rotate the polarization signals from these different sources by
different amounts. We explore two implications of this. First, measurements of
CMB birefringence are challenging because the effect is degenerate with a
miscalibration of CMB polarization angles; however, by taking the difference of
the reionization and recombination birefringence angles (measured from
different CMB angular scales), we can obtain a cosmological signal that is
immune to instrumental angle miscalibration. Second, we note that the
combination with other methods for probing birefringence can give tomographic
information, constraining the redshift origin of any physics producing
birefringence. We forecast that the difference of the reionization and
recombination birefringence angles can be competitively determined to within
~0.05 degrees for future CMB satellites such as LiteBIRD. Although much further
work is needed, we argue that foreground mitigation for this measurement should
be less challenging than for inflationary B-mode searches on similar scales due
to larger signals and lower foregrounds.
