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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.