Pioneer acceleration and variation of light speed: experimental   situation
  The situation with respect to the experiments is presented of a recently proposed model that gives an explanation of the Pioneer anomalous acceleration $a_{\rm P}$. The model is based on an idea already discovered by Einstein in 1907: the light speed depends on the gravitational potential $\Phi$, so that it is larger the higher if $\Phi$. The potential due to all the mass and energy in the universe increases in time because of its expansion, which has the consequence that light must be slowly accelerating. Moreover it turns out that the observational effects of a universal adiabatic acceleration of light $a_\ell =a_{\rm P}$ and of an extra acceleration towards the   Sun $a_{\rm P}$ of a spaceship would be the same: a blue shift increasing linearly in time, precisely what was observed. The phenomenon would be due to a cosmological acceleration of the proper time of bodies with respect to the coordinate time. It is shown that it agrees with the experimental tests of special relativity and the weak equivalence principle if the cosmological variation of the fine structure constant is zero or very small, as it seems now. 
Antonio F. Ranada