Viable dark energy models using pseudo-Nambu-Goldstone bosons

A canonical pseudo-Nambu-Goldstone boson (pNGB) can play the role of a dark energy field responsible for present cosmic acceleration. Confronted with the recent cosmological data, we find that the pNGB field requires a spontaneous symmetry breaking scale f close to M-P and that the initial field value be fine-tuned. It is difficult to achieve a large f in a theoretically consistent setup. A possible resolution can be achieved by increasing the Hubble friction in well-motivated particle physics models in the general setup of modified gravity theories. We show two phenomenological examples of this setup where the standard pNGB action has been modified by introducing terms motivated by Galileon cosmology. We confront those examples with the recent supernovae, PLANCK and baryon acoustic oscillation data. We find that moderate values of the dimensionless constants that increase the friction make f << M-P and the generic initial conditions also favorable by the data. We also comment on how the fifth-force constraints arising in these modified theories can be evaded.