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. Confronting with the
recent cosmological data, we find that the pNGB field requires spontaneous
symmetry breaking scale $f$ close to $M_P$ and the initial field value
fine-tuned. It is difficult to achieve a large $f$ in a theoretically
consistent set-up. A possible resolution can be achieved by increasing the
Hubble friction in well motivated particle physics models in the general set-up
of modified gravity theories. We show two phenomenological examples of this
set-up where the standard pNGB action have been modified by introducing terms
motivated from galileon cosmology. We confront those examples with the recent
supernovae, PLANCK and BAO data. We find that moderate values of the
dimensionless constants that increase the friction, make $f << M_{P}$ and the
generic initial conditions also favourable by the data. We also comment how the
fifth force constraints arising in these modified theories can be evaded.
