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). The viability of this postulate is discussed in view of cosmological and (astro)particle physics bounds. The gauge 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. A prediction of 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 obtained: $\Delta t_{m_\gamma=0}\sim 2.2$ billion years. 
Francesco Giacosa and Ralf Hofmann