X-rays and Cosmology
  The role of X-ray observations for cosmological studies and conclusions is briefly explored. X-rays currently yield cosmologically interesting results on the abundances, evolution and gas content of clusters of galaxies, on the clustering and evolution of active galaxies and on the X-ray Background. They are unlikely in the long term future to give the most precise values of the cosmological parameters, although in the short term the baryon fraction of clusters and the Sunyaev-Zeldovich effect will remain important determinants and checks for some parameters. X-rays will however continue to play an important role in studying the astrophysics of the formation and growth of black holes, galaxies, groups and clusters. It is possible that this role will be crucial, if winds from active galaxies are responsible for breaking the simple gravitational scaling laws for clusters. 
A. C. Fabian (Institute of Astronomy, University of Cambridge)