Confronting pNGB quintessence with data
  We analyze the observational constraints on the model where a pseudo-Nambu-Goldstone boson (pNGB) plays the role of dark energy. The constraints are derived by using the latest Gold set of 182 type Ia supernovae and the CMB shift parameter. We allow for both the initial value of the scalar field and the present value of the energy density in the pNGB to vary. We find that - compared to previous analyses - the allowed portion of parameter space has shrunk around the region where the pNGB does not evolve significantly. 
Koushik Dutta, Lorenzo Sorbo