What's the reason for it? Is there a mechanism already existing in physics to explain it, such as virtual particle energy density, to account for the anomalous acceleration? Will such an explanation solve the mystery of the acceleration of the Pioneer spacecraft? To know we've got to solve http://math.ucr.edu/home/baez/physics/Relativity/GR/cosConstant.html [Broken] . because, according to this web site, the vacuum density predicted by quantum mechanics is 120 orders of magnitude too high (a density of 1091g/cm3.