Origin of the Blueshift in Signals from Pioneer 10 and 11
  A previous paper [1] introduced a quantum-mechanical theory of gravity, and showed it agrees with the standard experimental tests of general relativity. Doppler tracking signals returned by the Pioneer 10 and 11 space probes offer an additional test. Analysis by Anderson, Laing, Lau, Liu, Neito and Turyshev [2] finds a persistent blueshift, equivalent to an extra acceleration of the probes toward the Sun. While unexplained by general relativity or prevailing cosmology, it's shown this effect is predicted by the quantum-mechanical alternative. 
Kris Krogh