Constraints on Scalar-Field Dark Energy from Galaxy Cluster Gas Mass   Fraction versus Redshift
  We use the Allen et al. (2004) Chandra measurements of x-ray gas mass fraction of 26 rich clusters to place constraints on the scalar-field dark energy model with inverse power law potential energy density. The constraints are consistent with, and typically more constraining than, those from other cosmological tests, and mildly favor the Einstein cosmological constant limit of the dark energy model. 
Gang Chen and Bharat Ratra