We present the results of a Chandra X-ray survey of the eight most massive galaxy clusters at z > 1.2 in the South Pole Telescope 2500 deg2 survey. We combine this sample with previously published Chandra observations of 49 massive X-ray-selected clusters at 0 survey. We combine this sample with previously published Chandra observations of 49 massive X-ray-selected clusters at 0 survey. We combine this sample with previously published Chandra observations of 49 massive X-ray-selected clusters at 0 survey. We combine this sample with previously published Chandra observations of 49 massive X-ray-selected clusters at 0 0.2{R}500 scaling like E{(z)}2. In the centers of clusters (r≲ 0.01{R}500), we find significant deviations from self-similarity ({n}e\propto E{(z)}0.2 /- 0.5), consistent with no redshift dependence. When we isolate clusters with overdense cores (i.e., cool cores), we find that the average overdensity profile has not evolved with redshift—that is, cool cores have not changed in size, density, or total mass over the past ˜9-10 Gyr. We show that the evolving “cuspiness” of clusters in the X-ray, reported by several previous studies, can be understood in the context of a cool core with fixed properties embedded in a self-similarly evolving cluster. We find no measurable evolution in the X-ray morphology of massive clusters, seemingly in tension with the rapidly rising (with redshift) rate of major mergers predicted by cosmological simulations. We show that these two results can be brought into agreement if we assume that the relaxation time after a merger is proportional to the crossing time, since the latter is proportional to H{(z)}-1.
