where ρ is the energy density in the vacuum. Each vacuum bubble contains a factor of δ (k) zeroing the total k at each vertex, and when there are no external lines, this contains a factor of δ (0), because the momentum conservation is over-enforced. In finite volume, this factor can be identified as the total volume of space time. Dividing by the volume, the remaining integral for the vacuum bubble has an interpretation: it is a contribution to the energy density of the vacuum.