The kinematic origin of the cosmological redshift

TL;DR

This paper argues that the cosmological redshift can be naturally interpreted as a Doppler shift rather than space stretching, emphasizing the relativity principle and the kinematic nature of the phenomenon in an expanding universe.

Contribution

It demonstrates that the cosmological redshift can be understood as a Doppler shift through a suitable choice of observers, challenging the common space-stretching interpretation.

Findings

Redshift can be interpreted as a Doppler shift with appropriate observers.

The kinematic interpretation aligns with the natural choice of comoving observers.

The space-stretching view obscures the relativity principle.

Abstract

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 be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is 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 it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime 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 is more natural.