  It is shown that a wave vector representing a light pulse in an adiabatically
evolving expanding space should develop, after a round trip (back and forth to
the emitter) a geometric phase for helicity states at a given fixed position
coordinate of this expanding space.In a section of the Hopf fibration of the
Poincare sphere that identifies a projection to the physically allowed states,
the evolution defines a parallel transported state that can be joined
continuously with the initial state by means of the associated
Berry-Pancharatnam connection. The connection allows to compute an anomaly in
the frequency for the vector modes in terms of the scale factor of the
space-time background being identical to the reported Pioneer Anomaly.
