Cosmic Birefringence from Monodromic Axion Dark Energy

Understand

The recently reported non-zero isotropic birefringence angle in Planck 2018 polarization data provides a tantalizing hint for new physics of axions.

• In this paper, we explain this by a string theory motivated axion with a monodromy potential that plays the role of dark energy.
• Upon using the birefringence measurement and the constraint on the equation of state for dark energy in this scenario, we find an upper bound on the axion decay constant as $f_a \lesssim 10^{16}$ GeV.
• This naturally gives an energy scale of order GUT and can resolve the theoretical issue of super-Planckian field range of the conventional axion dark energy model.