Thank you for your reply.
So, does the vacuum energy consist entirely in the particle-antiparticle pairs? In other words, is the energy of a given volume of vacuum zero when there are no pairs present in that volume? Or are the paricle-antiparticle pairs the result of a non-zero energy state which subsequently manifests itself as virtual particles? Can the energy volume of a vacuum be non-zero when no virtual particles are present?
AFAIK, the production of a virtual particle pair would require a relatively large amount of energy. Smaller amounts of energy uncertainty could be associated with the vaccuum, which would, I guess, manifest themselves in the curvature of space-time at that point in the vaccuum. Again, this isn't really my field, so, while this is the best of my understanding, I'm not sure I am right here. BTW, in my previous post, I had stated that I don't know what happens to the gamma photons from a virtual pair annihilation. After reading the link given by another poster on this thread, I realize that no annihilation actually occurs. The particles disappear after a short interval as the energy returns to zero.