Present‐day cluster bolometric X‐ray luminosity function. The solid curve shows the observed luminosity function (Ebeling et al. 1997). The dashed curve shows the predicted luminosity function for a closed universe (﹩\Omega _{0}=1﹩, ﹩\Omega _{\Lambda }=0﹩), while the dotted curve shows the predicted luminosity function for an open universe (﹩\Omega _{0}=0.3﹩, ﹩\Omega _{\Lambda }=0﹩). The flat universe is omitted here for clarity, since the predicted curve essentially matches the predicted curve for the open universe.