Metrics on Visual Boundaries of CAT(0) Spaces

Abstract: A famous open problem asks whether the asymptotic dimension of a CAT(0) group is necessarily finite. For hyperbolic groups, it is known that asymptotic dimension of the group is bounded above by the dimension of the boundary plus one, which is known to be finite. For CAT(0) groups, the latter quantity is also known to be finite, so one approach is to try proving a similar inequality. So far those efforts have failed. Motivated by these questions we work toward understanding the relationship between large scale dimension of CAT(0) groups and small scale dimension of the group's boundary by shifting attention to the linearly controlled dimension of the boundary. To do that, one must choose appropriate metrics for the boundaries. In this paper, we suggest two candidates and develop some basic properties. Under one choice, we show that linearly controlled dimension of the boundary remains finite; under another choice, we prove that macroscopic dimension of the group is bounded above by twice the linearly controlled dimension of the boundary plus one. Other useful results are established, some basic examples are analyzed, and a variety of open questions are posed.