We generalize Bestvina's notion of a $\mathcal{ Z}$-boundary for a group to that of a "coarse $\mathcal{ Z}$-boundary." We show that established theorems about $\mathcal{ Z}$-boundaries carry over nicely to the more general theory, and that some wished-for properties of $\mathcal{ Z}$-boundaries become theorems when applied to coarse $\mathcal{ Z}$-boundaries. Most notably, the property of admitting a coarse $\mathcal{ Z}$-boundary is a pure quasi-isometry invariant. In the process, we streamline both new and existing definitions by introducing the notion of a "model $\mathcal{ Z}$-geometry." In accordance with the existing theory, we also develop an equivariant version of the above -- that of a "coarse $E\mathcal{ Z}$-boundary."
Categories: math.GT