Cross-correlation between 21-cm radiation and CMB B modes from the
  cosmic birefringence in the presence of a light scalar field
  We study the cross-correlation between the 21cm and CMB B mode fluctuations
which are induced by the cosmic birefringence when the non-constant scalar
field couples to the electromagnetic field strength. Such multi-wavelength
signals can potentially probe the reionization history of the Universe and also
explore the nature of fundamental physics such as the parity violation and the
scalar field dynamics in the early Universe. We illustrate the feasibility to
detect such 21cm-B mode cross-correlations through commonly discussed scalar
field models, the quintessence-like scalar field which is responsible for the
current dark energy and the axion-like scalar field which is responsible for
the current dark matter density.
