Isotropic cosmic birefringence from early dark energy

Understand

A tantalizing hint of isotropic cosmic birefringence has been found in the $E B$ cross-power spectrum of the cosmic microwave background (CMB) polarization data with a statistical significance of $3\sigma$.

• A pseudoscalar field coupled to the CMB photons via the Chern-Simons term can explain this observation.
• The same field may also be responsible for early dark energy (EDE), which alleviates the so-called Hubble tension.
• Since the EDE field evolves significantly during the recombination epoch, the conventional formula that relates $E B$ to the difference between the $E$- and $B$-mode auto-power spectra is no longer valid.