Cosmology and Geometry (2) ...
It is also helpful in understanding an expanding Universe to appreciate what the Hubble
expansion does not mean.
What the Expansion Does Not Mean
First, the Hubble expansion does not mean that small objects, like you, are expanding with
space. (If you are expanding, you must look elsewhere for the cause!) You, and the Solar
System, and the galaxy, and galaxy clusters on various scales
are held together by forces that are strong enough to prevent being
carried along by the expansion. It is only on the very large scale (larger than superclusters)
that the forces of attraction are consistently overcome by the expansion of space. One uses
the adjective cosmological to denote scales and distances where the expansion of the
Universe dominates all other motion. Recall also that those motions that are not a consequence
of the overall expansion are called peculiar motions.
Second, the expansion of space exemplified by the Hubble law does not violate special
relativity, even though it implies that galaxies that are far enough apart recede from each
other at greater than light velocity. The light speed limit imposed by relativity applies to
measurable velocities.
It does not apply to a relative velocity generated by the expansion of
space, for in that case it is not possible to measure a velocity that exceeds that of light.
The following animation illustrates.
