Observational Constraints on Axions as Quintessence in String Theory
  We study the observational constraints for the axion models in string theory
which can successfully act as quintessence. The evolution of the universe in
this model is sensitive to the initial value of the axion field. This initial
value of the axion field controls deviation of the cosmic evolution from the
\Lambda CDM behaviour. We use the recent Union2 Supernova Type Ia dataset as
well as the data from BAO measurements, the WMAP measurement of the shift
parameter and the $H(z)$ measurements. Using these data, the reconstructed
equation of state of the axion field has extremely small deviation from the
cosmological constant \omega = -1 even at 2\sigma confidence level. One
interesting outcome of this analysis is that one can put bound on the SUSY
breaking scale using cosmological measurements assuming the values of different
string related parameters and vice versa.
