Cosmic Birefringence as a Probe of Dark-Energy Field Excursions and UV Geometry

We show that any rolling dark-energy scalar field inducing isotropic cosmic birefringence mustsatisfy a universal relation between its late-time field excursion and its coupling to electromagnetism.Using a controlled phenomenological reconstruction of the scalar dynamics, we derive a robust sub-Planckian estimate of the field excursion, ∆ϕeff ∼ 10−2MPl. Combining this with current hints ofcosmic birefringence at the level β ∼ 10−3 rad, we obtain a model-independent requirement on theaxion-photon coupling, gϕγ ∼ 10−19 GeV−1. In string-motivated compactifications, this translatesinto a direct constraint on microscopic parameters, linking late-time cosmology to compactifica-tion data through cγ Vp ∼ O(102). This establishes cosmic birefringence as a direct observationalconstraint on the microscopic origin of dark energy.