Cosmic Birefringence from Monodromic Axion Dark Energy

TL;DR

This paper proposes a string theory motivated axion model with monodromy potential to explain observed cosmic birefringence, constraining axion parameters and exploring implications for dark energy and string theory conjectures.

Contribution

It introduces a novel axion dark energy model with monodromy potential that accounts for cosmic birefringence and addresses super-Planckian field range issues.

Findings

Upper bound on axion decay constant: $f_a \,\lesssim\, 10^{16}$ GeV

Energy scale of axion dark energy is around GUT scale

Implications for string swampland conjectures and oscillatory features in dark energy

Abstract

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. We further study the implications of cosmic birefringence for the underlying theory and its consequences for the string swampland conjectures. We finally discuss oscillatory features in the dark energy sector and the expected cosmic birefringence tomography.