Equivalence Principle in Cosmology
  We analyse the Einstein equivalence principle (EEP) for a Hubble observer in
Friedmann-Lemaitre-Robertson-Walker spacetime. We show that the affine
structure of light cone in the FLRW spacetime should be treated locally in
terms of the optical metric which is not reduced to the Minkowski metric due to
the non-uniform parametrization of the local equations of light propagation
with the proper time of the observer's clock. The physical consequence of this
difference is that the Doppler shift of radio waves measured locally, is
affected by the Hubble expansion.
