Cosmic Birefringence Test of the Hubble Tension

An early dark energy component consisting of a cosmic pseudo Nambu-Goldstone boson has been recently proposed to resolve the Hubble tension -- the four-sigma discrepancy between precision measurements of the expansion rate of the universe. Here we point out that such an axion-like component may be expected to couple to electromagnetism by a Chern-Simons term, and will thereby induce an anisotropic cosmic birefringence signal in the polarization of the cosmic microwave background (CMB). We show that observations of the rotation-angle power spectrum and cross-correlation with CMB temperature anisotropy can confirm the presence of this early dark energy component. Future CMB data as expected from the CMB-S4 experiment will improve sensitivity to this effect by two orders of magnitude and help in discriminating between different Hubble tension scenarios.