Implications of Cosmic Birefringence for Multi-Field ALP Dark Matter

Cosmic birefringence, characterized by the observed rotation of the polarization plane of the cosmic microwave background (CMB) radiation, serves as a critical probe for testing theories beyond the standard cosmological scenario. As a major component of the universe, dark matter plays a pivotal role in cosmic evolution, particularly in the formation of large-scale structures. However, its fundamental nature remains elusive. Axion-like particles (ALPs), as promising dark matter candidates, possess unique advantages in naturally explaining such phenomena. Previous studies on the implications of cosmic birefringence for these ultralight ALP fields have focused on single-field models with conventional potentials. These models face exclusion due to the washout effect - a suppression of the CMB polarization power spectrum induced by oscillatory dynamics of the scalar field within the mass range of less than $10^{-23}$ eV. To address this limitation, we develop a more general theoretical framework incorporating two ALP fields, providing analytical approximations and numerical calculations. Our findings reveal that the superposition of two ALP fields with distinct masses can relax the constraints imposed by the washout effect and reconcile with observations.