Time, clocks, parametric invariance and the Pioneer Anomaly
  In the context of a parametric theory (with the time being a dynamical variable) we consider the coupling between the quantum vacuum and the background gravitation that pervades the universe (unavoidable because of the universality of gravity). In our model the fourth Heisenberg relation introduces a possible source of discrepancy between the marches of atomic and gravitational clocks, which accelerate with respect to one another. This produces in its turn another discrepancy between the observations, performed with atomic time, and the theoretical analysis, which uses parametric astronomical time. Curiously, this approach turns out to be compatible with current physics; lacking a unified theory of quantum physics and gravitation, it cannot be discarded {\it a priori}. It happens that this phenomenon has the same footprint as the Pioneer Anomaly, what suggests a solution to this riddle. This is because the velocity of the Pioneer spaceship with respect to atomic time turns out to be slightly smaller that with astronomical time, so that that the apparent trajectory lags behind the real one. In 1998, after many unsuccessful efforts to account for this phenomenon, the discoverers suggested "the possibility that the origin of the anomalous signal is new physics". 
Antonio F. Ranada and Alfredo Tiemblo