Studying the Nature of Dark Energy with Galaxy Clusters
  We report on the status of our effort to constrain the nature of dark energy through the evolution of the cluster mass function. Chandra temperature profiles for 31 clusters from a local cluster sample are shown. The X-ray appearance of the proto supermassive binary black hole at the center of the cluster Abell 400 is described. Preliminary weak lensing results obtained with Megacam@MMT for a redshift z=0.5 cluster from a distant cluster sample are given. 
Thomas H. Reiprich (1), Daniel S. Hudson (1), Thomas Erben (1), Craig   L. Sarazin (2) ((1) AIfA, (2) UVa)