Interpreting cosmic birefringence and DESI data with evolving axion in ΛCDM
Shota Nakagawa, Yuichiro Nakai, Yu-Cheng Qiu, Masaki Yamada
Abstract
Recent cosmological observations have revealed growing tensions with the standard ΛCDM model, including indications of isotropic cosmic birefringence and deviations from w = − 1 in the dark energy equation of state, as suggested by DESI and supernova measurements. In this paper, we point out that such deviations can arise even from a subdominant energy density component. We then propose a unified framework based on a dynamical axion field that simultaneously accounts for both anomalies, providing a simple and natural extension of the standard ΛCDM model. In our scenario, the axion field with 2 H 0 ≲ m ≲ 7 H 0 , where H 0 is the current Hubble constant, induces a nonzero rotation of the CMB polarization plane and modifies the present-day dark energy equation of state. This framework accommodates recent observational data with natural parameter choices, even for a string axion with a decay constant of order 10 17 GeV.