  The global geometry of the universe is in principle as observable an
attribute as local curvature. Previous studies have established that if the
universe is wrapped into a flat hypertorus, the simplest compact space, then
the fundamental domain must be at least 0.4 times the diameter of the
observable universe. Despite a standard lore that the other five compact,
orientable flat spaces are more weakly constrained, we find the same bound
holds for all. Our analysis provides the first limits on compact cosmologies
built from the identifications of hexagonal prisms.
