The Pinciple of Least "available" Action Of consequence to theory is whether or not that mu_0 epsilon_0 represents a mimimal, or null physical parameter like null curvature does. Per QED, modern physics says that the "physical vacuum" is full of quantum particles and uncertainties in values, and then there is a "bare vacuum" that has no filled quantum states and is completely empty. The question, I think, is... Does the "LOCAL" physical vacuum posess properties which mediate and regulate the direction and speed of an information carrying object, and do these properties fall off in gradient fashion, in a direction that results in a NEAR empty bare vacuum with distance from massive clusters? General Relativity derives that positive vacuum energy density will elicite an attractive gravitational "force" which will interact with massive objects, and vacuum energy density is already necessarily comparitively higher in the region of space immediately surrounding massive clusters, since real and virtual pair production is more likely to occur where there are concentrated levels of matter and radiation. The gravitational effect must serve to maintain a minimal field of posititive vacuum energy density immediately surrounding massive clusters. If General Relativity is correct, then vacuum energy cannot be evenly distributed throughout the universe and there is a region of gravitationally "attractive" positive vacuum energy density which surrounds massive clusters. We can measure the mass of the Milky Way by watching the orbits of distant objects that are bound to the galaxy and move around it. There is an inference that the Milky Way galaxy may contain an extended sphere of matter that is dark, that is not radiating light like normal stars do. Observations show that the dark matter in a galaxy surrounds the visible matter in a halo that is larger and more nearly spherical than the stars and gas that we see. The visible matter density is higher than the dark matter density near the centers of most galaxies, so the dark matter is not very important there. But it extends well beyond the stars and gas, so the outer parts of galaxies are essentially all dark matter. Concentrated levels of mass energy increase the likelihood of virtual pair production over an affected region of the vacuum and you can produce both, real and virtual particles, by directing two particle beams toward each other, indicating that both forms of energy are ultimately of the same nature. GR supports that the creation of a particle pair from the vacuum causes the surrounding region to tend to expand proportionally, due to the increased gravitational effect, as the boundary contracts due to increasing tension, while negative pressure increases in proportion to the "hole" that gets left behind. Of note: Concentrated regions of positive vacuum energy density enable the vacuum to expand without running away, and without any work being done, since a region of positve energy density is immediately counter balanced with the negative pressure component via the increased gravitational effect.