General relativity and quintessence explain the Pioneer anomaly
  The anomalous time depending blueshift, the so-called "Pioneer anomaly", that was detected in the radio-metric data from Pioneer 10/11, Ulysses and Galileo spacecraft may not result from a real change of velocity. Rather, the Pioneer anomaly may be understood within the framework of general relativity as a time depending gravitational frequency shift accounting for the time dependence of the density of the dark energy when the latter is identified with quintessence. Thus, instead of being in conflict with Einstein equivalence principle, the main Pioneer anomaly appears merely as a new validation of general relativity in the weak field and low velocity limit. 
J.P. Mbelek