  A generalisation to electrodynamics and Yang-Mills theory is presented that
permits computation of the speed of light. The model presented herewithin
indicates that the speed of light in vacuo is not a universal constant. This
may be relevant to the current debate in astronomy over large values of the
Hubble constant obtained through the use of the latest generation of ground and
space-based telescopes. An experiment is proposed based on Compton scattering
to measure deviations in the speed of light.
