X-ray Cluster Surveys, Cosmology, and Dark Energy

I will discuss how observations of the number density of the high-redshift galaxy clusters can be used for constraining cosmological parameters An emphasys will be given to new results from the recently completed 400 square degrees survey carried out using essentially the entire archive of ROSAT PSPC pointings This survey provides a large number of serendipitously discovered high-redshift galaxy clusters Each X-ray detection is optically confirmed and spectroscopic redshifts were obtained for all objects The most massive distant clusters 43 objects at z 0 35 were followed up with Chandra This program is nearly completed These observations provide the most accurate to-date measurement of the cluster number density evolution The 400d distant cluster sample provides a new independent measurement of the dark energy equation of state