Observational Constraints on Axion(s) with a Cosmological Constant
  The present work deals with a dark energy model that has an oscillating
scalar field potential along with a cosmological constant (CC). The oscillating
part of the potential represents the contribution of a light axion field in the
dark energy that has its origin in the String-Axiverse scenario. The model has
been confronted with the latest cosmological observations. The results show
that a sub-Planckian value of the axion field decay constant is consistent at
1$\sigma$ confidence level. The oscillating feature in the scalar field
evolution and in the equation of state for the dark energy can be observed at
2$\sigma$ confidence interval. It is also observed that cluster number counts
in this axion model are suppressed compared to the $\Lambda$CDM and this
suppression is enhanced for the sub-Planckian values for the axion decay
constant.
