When astronomers observe a supernova explosion..?

Question

in a distant galaxy, they see a sudden, simultaneous rise in visible light and other forms of electromagnetic radiation. Is this evidence to support the idea that the speed of light is independent of frequency?

Answer 1

Short answer - yes. All of the electromagnetic radiation arrives at the same time. The reference cites a 1999 paper that uses supernovae observations to show that the speed of light is independent of frequency to 1 part in 10^-21 (second paragraph).