what is red shift and how do we know about it?
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Red-shift has to do with the wavelength of light.
Light, while moving at a constant speed in a vacuum, propogates (moves) from one place to another in a wave-like pattern (not literally in waves, that’s just how it is expressed in the electromagnetic spectrum, but since this is ELI5 I’m going to keep it simple). Those waves can be measured, which is called the wavelength. The number of times a full wavelength passes a fixed point is called the frequency. In short, the frequency/wavelength of light determines the color.
So you’ve got light coming from a source. As you move toward the light source, the waves are going to get to you a bit faster. This increases the frequency of the wave, which means the light is going to appear more blue. Move away from the source, and the waves take a little longer which lowers the frequency, which means the light is going to appear a little more red. Go faster toward it, even more blue. Go faster away from it, even more red. Stop moving, and the light appears to be it’s original color again.
That’s all red-shift and blue-shift means, really. There is an impressive amount of gloriously complicated answers to the original question (there’s an awful lot of math involved), but that’s the simplest way I can break it down for you.
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