Light is an electro-magnetic wave and has Energy (E=h*f) Like any wave, the light wave looses some of this energy as it passes vast distances of space (due to interactions with free electrons, plasma, gas clouds, and so on). As it looses energy, its frequency automatically decreases- because E=h*f, where h= constant (of Planck) As its frequency decreases, light is shifted to red- because red light has the lowest frequency of the visible light spectrum. Therefore, Hubble's int