# Cross-correlation between 21-cm radiation and CMB B modes from the cosmic birefringence in the presence of a light scalar field

## TL;DR

This paper investigates the cross-correlation between 21cm radiation and CMB B modes caused by cosmic birefringence linked to a dynamic scalar field, offering a new probe into early Universe physics and reionization.

## Contribution

It demonstrates the potential to detect 21cm-CMB B mode cross-correlations induced by scalar fields, connecting dark energy, dark matter, and fundamental physics.

## Key findings

- Feasibility of detecting 21cm-B mode cross-correlations.
- Application to quintessence-like scalar fields.
- Application to axion-like scalar fields.

## Abstract

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.