We investigate the
relationship between the metric boundary and the Gromov boundary of a hyperbolic
metric space. We show that the Gromov boundary is a quotient of the metric
boundary and the quotient map is continuous, and that therefore a word-hyperbolic
group has an amenable action on the metric boundary of its Cayley graph.
Furthermore, if the space is 0-hyperbolic, the boundaries agree, and as a
consequence there are no non-Busemann points on the boundary of such
spaces. These results have significance for the study of Lip-norms on group
C∗-algebras.
Keywords
hyperbolic space, hyperbolic group, Gromov boundary, Cayley
graph, group C∗-algebra, quantum metric space