arXiv:0808.1081 (cross-list from physics.pop-ph) [ps, pdf, other] Title: The kinematic origin of the cosmological redshift Authors: Emory F. Bunn, David W. Hogg Comments: 14 pages. Submitted to Am. J. Phys Subjects: Popular Physics (physics.pop-ph); Astrophysics (astro-ph); General Relativity and Quantum Cosmology (gr-qc); Physics Education (physics.ed-ph) A common belief among cosmologists 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, specifically the idea that it is always valid to choose a coordinate system that is locally Minkowski. 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 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.