Constraining Lorentz Violation using 21cm and CMB Cross Correlations

TL;DR

This paper explores how cross-correlating 21cm and CMB signals can constrain Lorentz violation through cosmic birefringence, providing forecasts for upcoming experiments and highlighting the potential to improve current bounds.

Contribution

It introduces a novel method using 21cm-CMB cross-correlations to constrain Lorentz violation parameters, with detailed forecasts for future experiments.

Findings

Best constraints of approximately 4.4° for CMB birefringence angle.

Constraints around 100° for 21cm birefringence angle.

Forecasts for SKA, HIRAX, PUMA, and Planck-like experiments.

Abstract

Lorentz symmetry is a fundamental pillar of modern Physics, yet high-energy theories often predict its violation. One potential signature of such a violation is cosmic birefringence - rotation of the polarization plane of photons due to Chern-Simons coupling in Maxwell's electrodynamics. This rotation angle, aka birefringence angle, depends upon the distance travelled by the photon and is thus different for CMB and 21cm photons. While the rotation angle in CMB, i.e., $\alpha_\mathrm{CMB}$, has been tightly constrained by CMB experiments, the potential of the 21cm cosmological signal to constrain this parameter, as well as constrain $\alpha_\mathrm{21cm}$, remains largely unexplored. In this work, we provide constraints on both these angles by cross-correlating 21cm and CMB signals. Using the Fisher matrix formalism, we give our forecasts for 21cm experiments, including SKA, HIRAX, and PUMA, and Planck like CMB experiment. We find that best constraints $\sigma_{\alpha_\mathrm{CMB}} \sim 4.4^\circ$ and $\sigma_{\alpha_\mathrm{21cm}} \sim 100^\circ$ are found using $C_\ell^{T_{21} B_\mathrm{CMB}}$ and $C_\ell^{T_{21} B_{21}}$ respectively. Since birefringence hasn't yet been detected in 21cm, we choose the fiducial value $\alpha_\mathrm{21cm}^\mathrm{fid}=0$ assuming the null hypothesis.