English: A congruence of radially accelerating worldlines ξ^a with the same uniform proper acceleration 1/α generates the stretched future light cone of point p and describes a timelike hypersurface Σ, with unit outward-pointing normal n^a. The boundary of Σ consists of the two codimension-two surfaces ∂Σ(0) and ∂Σ(ε) given by the constant-time slices of Σ at t=0 and t=ε, respectively. The codimension-one spatial ball B is the filled in codimension-two surface ∂Σ.