A Planck-scale axion and SU(2) Yang-Mills dynamics: Present acceleration and the fate of the photon

From the time of CMB decoupling onwards we investigate cosmological evolution subject to a strongly interacting SU(2) gauge theory of Yang-Mills scale $\Lambda\sim 10^{-4}$ eV (masquerading as the $U(1)_{Y}$ factor of the SM at present). This theory is coupled to a spatially homogeneous and ultra-light (Planck-scale) axion field. As first pointed out by Frieman et al. such an axion is a viable candidate for quintessence, i.e. dynamical dark energy, being associated with today's cosmological acceleration. An upper limit $\Delta t_{m_\gamma=0}$ for the duration of the epoch stretching from the present to the point where the photon starts to be Meissner massive is predicted: $\Delta t_{m_\gamma=0}\sim 2$ billion years