what is red shift and how do we know about it?
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Say you have a tennis-ball gun. It first one tennis ball every second. You fire it at me. I will get hit by one tennis ball every second.
But what if you start walking away from me (or me from you – relativity says it doesn’t matter). Each tennis ball now has further to travel than the previous one. So it will take a bit longer to hit me. While you are still firing one a second, I will get hit by one at a slightly slower rate (or frequency). If I know how often you are shooting them, and how often they are hitting me I can figure out how fast you are moving.
The same happens with light (and sound). Light has a frequency associated with colour (for example red light is about 405-480 THz, blue being 580-675 THz, so 600,000,000,000,000 ‘things’ per second). So if the thing emitted the light is moving, like with the tennis balls, the frequency it emits will be different to the frequency received; the colour of the light will change – if the thing is moving away the frequency will get lower (fewer hits per second) – so more towards the red (or towards blue if the thing is getting closer).
In cosmology this can be really useful, as if we know what colours light received from a distant galaxy *should* be, we can compare what it is and figure out how fast it is moving away. And that can help us figure out how far away it is.
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