  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.
