Can the QCD axion feed a dark energy component?

A pseudo Nambu-Goldstone boson (PNGB) coupled to a confining gauge group via an anomalous term is characterised, during the confining phase transition, by a temperature dependent mass m^2(T) ∝ T^-n. For n>2, a non-relativistic population of such particles dominating the cosmological energy density would act as dark energy (DE), accelerating the expansion. We study the possibility that a PNGB φ_b coupled to a hidden gauge group that is presently undergoing confinement could realise this scenario. To obtain the observed amount of DE, the number density of φ_b must be boosted by some mechanism. Assuming that the QCD axion φ_a constitutes the dark matter (DM), a non-adiabatic level crossing between φ_a and φ_b shortly before matter-DE equality can convert a small fraction of DM into DE, providing such mechanism and explaining the coincidence puzzle.