Physicists believe that the speed of light coming from space cannot be measured. This is not true, as can be seen from the description of a technologically undemanding measuring method. Why has this speed of light from space not yet been measured? The reason is that there is opposition to this measurement, because the measurement results do not confirm always the same speed of light.
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Booklet: The Light from Space
Article: How light from space is refract on an optical prism