The Expansion of Space: Free Particle Motion and the Cosmological Redshift

Abstract

The meaning of the expansion of the universe, or the `expansion of space,' is explored using two phenomena: the motion of a test particle against a homogeneous background and the cosmological redshift. Contrary to some expectations, a particle removed from the Hubble flow never returns to it. The cosmological redshift is not an `expansion effect;' in special cases it can be separated into a kinematic (special relativistic) part and a static (gravitational redshift) part, but in general must be thought of as the effect on light rays of their propagation through curved space-time. Space as such does not affect matter by `expanding,' but only through its curvature.