Boundaries of cocompact proper CAT(0) spaces

A proper CAT(0) metric space XX is cocompact   if it has a compact generating domain with respect to its full isometry group. Any proper CAT(0) space, cocompact or not, has a compact metrizable boundary at infinity ∂∞X∂∞X; indeed, up to homeomorphism, this boundary is arbitrary. However, cocompactness imposes restrictions on what the boundary can be. Swenson showed that the boundary of a cocompact XX has to be finite-dimensional. Here we show more: the dimension of ∂∞X∂∞X has to be equal to the global Čech cohomological dimension of ∂∞X∂∞X. For example: a compact manifold with non-empty boundary cannot be ∂∞X∂∞X with XX cocompact. We include two consequences of this topological/geometric fact: (1) The dimension of the boundary is a quasi-isometry invariant of CAT(0) groups. (2) Geodesic segments in a cocompact XX can “almost” be extended to geodesic rays, i.e. XX is almost geodesically complete.