The cross-correlation between 21cm and CMB B modes from the cosmic birefringence in existence of 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.