The Kinematic Origin of the Cosmological Redshift

A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must instead be viewed in terms of the stretching of space. We argue that the most natural interpretation of the redshift is in fact as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that of coordinate freedom, more specifically the idea that it is always valid to choose a coordinate system that is locally Minkowskian. We show that, in any spacetime, an observed frequency shift can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon’s path. In the context of the expanding Universe, the kinematic interpretation corresponds to a family of comoving observers and hence seems to be the more natural one.