The cosmic microwave background radiation (CMB) is not considered as direct evidence of variable light speed. The CMB is a form of electromagnetic radiation that permeates the universe and is a remnant of the early stages of the Big Bang. It is a crucial piece of evidence supporting the Big Bang theory and provides insight into the early universe.
The observed characteristics of the CMB, such as its uniformity and specific temperature distribution, are consistent with our understanding of the expanding universe and the predictions of the Big Bang theory. The fluctuations in the CMB temperature across the sky correspond to density variations in the early universe, which eventually led to the formation of galaxies and other cosmic structures.
The speed of light, as measured from the CMB, is consistent with the known value of the speed of light in a vacuum. The CMB photons travel through space and reach us at a constant speed, which aligns with the speed of light that we measure in laboratory experiments. This consistency supports the principle of cosmic isotropy and the constancy of the speed of light throughout the universe.
Variable light speed, as you mentioned, would imply that the speed of light has changed over time or is different in different regions of the universe. However, there is currently no compelling scientific evidence or observation that supports such a claim. The prevailing consensus in physics, based on extensive experimental data and theoretical models, is that the speed of light is constant in our universe.
It's worth noting that if there were variations in the speed of light, it would have significant implications for our understanding of fundamental physics and would require a substantial revision of our current theories. Scientists continue to conduct research and experiments to refine our understanding of the nature of the universe, including the fundamental constants of nature, but as of now, there is no evidence to suggest that the speed of light has varied significantly throughout the history of the universe.