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

Abstract  From the time of CMB decoupling onwards we investigate cosmological evolution subject to a strongly interacting SU(2) gauge theory of Yang–Mills scale, Λ ∼ 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 ultralight (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 for Δtmγ=0, the duration of the epoch stretching from the present to the point where the photon starts to be Meissner massive, is obtained: Δtmγ=0∼2.2 billion years.