Axion-like particles are dark matter candidates motivated by the Peccei-Quinn mechanism and also occur in effective field theories where their masses and photon couplings are independent. We estimate the dispersion of circularly polarized photons in a background of oscillating axion-like particles (ALPs) with the standard gaγaFμνF~μν/4 coupling to photons. This leads to birefringence or rotation of linear polarization by ALP dark matter. Cosmic microwave background (CMB) birefringence limits Δα≲(1.0)∘ enable us to constrain the axion-photon coupling gaγ≲10−17−10−12GeV−1, for ultra-light ALP masses ma∼10−27−10−24 eV. This improves upon previous axion-photon coupling limits by up to four orders of magnitude. Future CMB observations could tighten limits by another one to two orders.
@article{arxiv.1811.07873,
title = {Axion-like Dark Matter Constraints from CMB Birefringence},
author = {Günter Sigl and Pranjal Trivedi},
journal= {arXiv preprint arXiv:1811.07873},
year = {2019}
}