Detection of isotropic cosmic birefringence and its implications for
  axion-like particles including dark energy
  We investigate the possibility that axion-like particles (ALPs) with various
potentials account for the isotropic birefringence recently reported by
analyzing the Planck 2018 polarization data. For the quadratic and cosine
potentials, we obtain lower bounds on the mass, coupling constant to photon
$g$, abundance and equation of state of the ALP to produce the observed
birefringence. Especially when the ALP is responsible for dark energy, it is
possible to probe the tiny deviation of dark energy equation of state from $-1$
through the cosmic birefringence. We also explore ALPs working as early dark
energy (EDE), which alleviates the Hubble tension problem. Since the other
parameters are limited by the EDE requirements, we narrow down the ALP-photon
coupling to $10^{-19}\, {\rm GeV}^{-1}\lesssim g\lesssim 10^{-16}\, {\rm
GeV}^{-1}$ for the decay constant $f=M_\mathrm{pl}$. Therefore, the Hubble
tension and the isotropic birefringence imply that $g$ is typically the order
of $f^{-1}$, which is a non-trivial coincidence.
